WO1999000623A1

WO1999000623A1 - Automotive lamp lens and lamp system utilizing diffractive optics and method for making the same

Info

Publication number: WO1999000623A1
Application number: PCT/US1998/013610
Authority: WO
Inventors: Jon K. Nisper
Original assignee: Donnelly Corporation
Priority date: 1997-06-30
Filing date: 1998-06-30
Publication date: 1999-01-07
Also published as: AU8276898A

Abstract

An automotive lens and lamp system utilizing diffractive optics includes an automotive lens which generates a predetermined light pattern when used in conjunction with an automotive light source and a reflector. The lens includes an automotive lens body optically-transmissive to light emitted by the light source and reflected from the reflector. A diffractive grating is formed on the surface of the lens body to phase-shift the wavefront of the light passing through the lens body. The diffractive grating comprises a non-holographic microstructure having a pattern predetermined to cause the light passing through the lens body to generate the predetermined light pattern.

Description

AUTOMOTTVE LAMP LENS AND

LAMP SYSTEM UTILIZING

DIFFRACπVE OPTICS AND

METHOD FOR MAKING THE SAME

FIELD OF THE INVENTION

The present invention relates to automotive lighting, and more particularly to automotive lighting systems utilizing diffractive optics in the lens, and methods for making the same.

BACKGROUND OF THE INVENTION

Automotive lighting systems must comply with government regulations regarding various characteristics such as illumination patterns, intensity, color, etc. For example, in the United States, the National Highway Transportation Safety Administration (NHTSA) is responsible for administering the Federal Motor Vehicle Safety Standards (FMVSS). FMVSS 108 describes the required illumination patterns for various automotive exterior lights. For example, a head lamp or turn signal must be focused in a particular way so that, within various angular regions of space at given distances in front of the bulb, the illumination intensity falls within specified ranges, either above a minimum, or below a maximum, or within a range.

In addition to compliance with various government standards, it is desirable that the lights have smooth illumination patterns, i.e., no abrupt changes in intensity of light other than those required by the government standards. It is also desirable to eliminate any occurrence of nonuniformity of spectral distribution or "rainbows". In addition to meeting required and desired photometric requirements, automotive exterior lights must meet minimum environmental, mechanical, spatial, stylist, aerodynamic, and weight requirements, all while keeping costs within acceptable ranges.

Typical automotive lighting assemblies, such as turn signals, head lamps, brake lights, or the like, πήnimally consist of a bulb, a bulb holder, a reflector, and a lens. The reflector is typically parabohc in shape and acts as a housing for the complete assembly, as well as redirecting and collimating light emitted by the bulb. The bulb holder is mounted to the reflector and positions the bulb at the focal point of the reflector, or intentionally shifted from the focal point in order to provide a divergent or convergent light pattern. Light from the bulb reflects off of the reflector and passes through the lens. The lens is typically injection molded from acrylic, polycarbonate, or other similar material.

The lenses in typical automotive exterior lighting elements are either conventional or projection in style. The conventional lens is characterized by a refractive lens that is typically segmented into regions which either have no optical power, or have optical power in the form of small refractive lenses (lenslets) or prisms. This arrangement may contain hundreds of lenslets and prisms and is sometimes referred to as "pillow optics" due to the lenslets' resemblance to small pillows. The lenslets and prisms are typically grouped according to type and function (aperture size, focal length, bending power, etc.) in order to reflect light into desired angular regions in compliance with the governmental standards.

The fabrication of a lens in an injection mold to produce such a conventional lens is a very tedious, time consuming, and expensive task. Each lenslet or prism cavity in the mold must be individually formed and polished, adding greatly to the time and expense involved in forming the mold. Even after the mold is produced, the injection molding process is extremely difficult The intricate pattern of lenslets and prisms create many flow restrictions in the mold that interrupt the flow of polymer as it fills the cavity in the mold, creating distortions. The flow restrictions also increase the likelihood that air will be trapped in the mold during the fabrication, which can result in a part that is operatively unacceptable or, at a minimum, cosmetically undesirable. Air vents can be incorporated into the mold, further increasing its cost but these do not always satisfactorily address the problem.

In addition to fabrication difficulties, conventional pillow optics arrangements generally do not make efficient use of the light generated by the bulb. At each transition between a lenslet and/or a prism is a draft surface to facilitate mold release. These draft surfaces can refract light at unpredictable or undesirable angles, resulting in wasted light or glare light

Projection-style lighting is becoming more and more prevalent in the automotive industry as an alternative to conventional pillow optics. Auto makers are using this technique in many applications, such as head lamps, turn signals, cornering lamps, brake lamps, fog lamps, and the like. The projection style terminology was first coined for head lamps in which- the bulb, reflector, and lens configuration resembled that of movie projector condenser systems. Indeed, a more correct terminology for such a lens is a condenser lens. As with the conventional lens, a projection style lens is also characterized by a refractive lens function. However, a projection style lens is designed to produce the desired photometric pattern with either a single smooth surface or a few segments (generally no more than 2-4). Because very few segments are used to produce the desired pattern, any small variation in the surface features can result in a lens which fails photometric requirements.

The reflector in a projection assembly is typically elliptical in shape and focuses the reflected light at a specific position. A bulb holder is mounted to the reflector housing and positions the bulb at a position either at the focus of the reflector, or intentionally shifted from the focus in order to provide a divergent or convergent light pattern.

The condenser lens is used for focusing and is therefore generally positive in nature (planar/convex or concave/convex). The lens itself will typically be very close to the bulb and therefore require a relatively short focal length (typically in the range of 30 to 80 mm). In order to meet the government light pattern standards with such a focal length, the lens requires high light-bending power, typically in the range of F/0.5 to F/1.0. The lenses are therefore characterized by steep curvatures on the convex side. In contrast, the planar or concave side generally has a relatively shallow curvature. As a result as the diameter of the lens increases, so does the center thickness. A typical turn signal lens, for example, may be as much as 14-20 mm thick at its center and only 2-3 mm at its edge. A lens with an 80 mm diameter, for example, will generally be 14 mm thick at its center and only 3 mm thick at its edge. The center thickness of a projection head lamp can be over 20 mm and its perimeter thickness is still typically 4 mm or less.

The sharp change in thickness across the part presents a number of challenges, as does its large center thickness. Very long injection molding cycle times (typically 3 to 5 minutes) at high temperatures (within 5 degrees of the plastic deformation temperature) are required to produce an acceptable part. This results in a lens which is very expensive to fabricate.

As the lens cools during formation, it will shrink. Because the lens is so thick, and solidifies at its outer edges first, it is difficult to maintain the proper pressure in its interior. Thus, the lens is prone to sinking (warpage) during fabrication as it shrinks, which can be fatal to the photometric characteristics of the lens.

The lens will generally be surrounded by a flange by which the lens is mounted. Often, this flange will have to be thicker than the lens thickness at the perimeter. The fact that the mounting flange is often thicker than the perimeter of the refractive surface can also cause the lens to cool and solidify in an undesirable sequence. It can therefore be difficult to pack out (or pressurize) the mold during the injection molding cycle, which is important to insure accurate replication of the part shape. Often the optical performance of the lamp must be compromised in order to meet fabrication demands.

U.S. Patent No. 5,582,474 to Van Order et al. refers to the use of a holographic optical element (HOE) to diffuse light in an interior light of an automotive vehicle. However, the apphcation of HOE's to automotive lighting presents a number of problems. HOE's are optical elements that may be diffractive in nature but are recorded in a laboratory environment using lasers and optical components. HOE's, as such, are hmited in functionality to that which can be modeled in the laboratory using readily available optical components. They are also characterized by random patterns of sinusoidal surface relief features with no angular or stepped profiles. Furthermore, they suffer from all of the problems associated with a photographic method of fabrication, including non-uniformities in the recording beam intensity, laser speckle, and non-linearities (impurities) of the recording photoresist (or other recording medium). In addition, the resultant surface features are unpredictable and ill-defined in that the exact surface profile is not certain until after the photoresist is etched away. As a result, HOE's are generally not used for complex and general diffraction effects. For example, the use of an arbitrarily shaped array generator consisting of hundreds of diffractive orders would be virtually impossible to record in an HOE. These limitations of the HOE diffuser prevents the a priori knowledge of the exact surface relief of the HOE. Only after recording the HOE and then measuring its characteristics does one know the lateral dimensions and height at any point on the HOE surface. This therefore limits the ability to design for the effect of using the diffuser with a given bulb and reflector. It also prevents the optimization and tolerance analysis of the lamp as a system (bulb, reflector, and lens).

The Van Order patent refers to injection molding through the use of a metal master fabricated by transferring the HOE from a photoresist to the metal using conventional processes. Conventional processes of replicating holograms from photoresists involve the use of nickel electro-forms or electro-less nickel. This results in an injection mold insert which is soft, distorts easily, and wears rapidly.

Thus, there is a need in the automotive lighting art for a lens which is easier and less expensive to manufacture but which has predictable optical characteristics. There is also a need for such a lens which may be used with conventional lighting automotive systems. Lastly, there is a need for methods of fabricating such lenses.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the problems of the related art, as enumerated above, by providing an automotive lamp lens having a diffractive grating formed on the surface thereof to properly configure the light beam passing therethrough.

According to a first aspect of the present invention, a lens for generating a predetermined light pattern when used in conjunction with an automotive lamp which includes a reflector and a light source comprises a lens body positioned to intercept light emitted by the light source and light reflected by the reflector. A diffractive grating is formed on the surface £>f the lens body and includes a non-holographic microstructure (for example, a Diffractive Optical Element or "DOE") designed to redirect the intercepted light to generate the predetermined light pattern.

According to a second aspect of the present invention, an automotive lamp for generating a predetermined light pattern includes a housing having a reflector and a bulb holder. A light source is mounted to the bulb holder, and a lens is positioned to collect hght emitted by the hght source and light reflected by the reflector. The lens includes a surface with a diffractive grating formed thereon which has a non-holographic microstructure designed to redirect the collected hght to generate the predetermined hght pattern.

According to a further aspect of the present invention, a method of forming an automotive Ughting lens for collecting and redirecting light into a predetermined hght pattern comprises the steps of (i) defining a non-holographic microstructure designed to redirect the collected light to generate the predetermined hght pattern, (ii) converting the defined microstructure into at least one pattern corresponding to a negative of the defined microstructure, (iii) machining directly into a molding base element a pattern corresponding to the negative of the defined microstructure, and (iv) molding a plastic element in order to transfer the pattern from the molding base into a positive of the defined microstructure in the plastic element. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a plan view of a blazed diffractive.

Fig. 2A is a cross-sectional view of a blazed diffractive.

Fig. 2B is a cross section view of a multiphase level diffractive lens.

Fig. 2C is a partial cross section view of a phase zone plate diffractive lens.

Fig. 2D is a cross section of a linear blaze diffractive.

Fig. 3A is a plan view of a single phase level, three- dimensional repeating unit cell.

Fig. 3B is a cross section view of a single phase level, three- dimensional repeating unit cell.

Fig. 4A is a plan view of a multiple phase level, three- dimensional repeating unit cell.

Fig. 4B is a cross section view of a multiple phase level, three- dimensional repeating unit cell.

Fig. 5 is a partial perspective view of a blazed linear grating.

Fig. 6 is a cross section view of a generalized sinusoidal surface relief grating.

Fig. 7 is a schematic cross section of a polynomial kinoform compared to a conventional lens.

Fig. 8 is a cross-sectional view of "conventional style" turn signal lens containing a diffractive surface. Fig. 9 is a cross-sectional view of a hybrid "projection style" turn signal lens containing a diffractive on the inner surface.

Fig. 10A is a schematic profile of a reflector and projector style turn signal lens.

Fig. 10B is a hght distribution pattern of the turn signal profiled in Fig. 10A

Fig. 11A is a schematic profile of a reflector and turn signal replacement lens with diffractive component.

Fig. 1 IB is a hght distribution pattern of the turn signal profiled in Fig. 11A

Fig. 12A is a plot of diffraction efficiency against wavelength for two illustrative grating sizes.

Figs. 13A and 13B are top and side schematic cross-sectional views of a conventional projection-style head lamp.

Figs. 13C is a light distribution pattern of the head lamp shown in Figs. 13A and 13B.

Figs. 14A and 14B are top and side schematic cross-sectional views of a projection-style head lamp utilizing a diffractive optical element lens according to the present invention.

Fig. 14B is a hght distribution pattern of the head lamp shown in Figs. 14A and 14B. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 1. Introduction

The present invention relates to an automotive hght such as a turn signal, which includes a hght source and a lens having at least one non- holographic diffractive microstructure therein. A non-holographic diffractive microstructure is one that is not developed in the laboratory using optical equipment such as the photographic equipment associated with the preparation of holograms. Such a microstructure is, instead, a mathematically predetermined diffractive structure for phase-shifting the wavefront of the hght passing therethrough. The non-holographic diffractive microstructure is digitally designed and predefined, usually with computer assistance, and includes a number of non-sinusoidal surface features, again in distinction to holographic surface features which are sinusoidal. Thus, non-holographic diffractive microstructures can be characterized with mathematical equations, which permits them to be directly machined with numerically-controlled devices such as diamond-turning machines. Examples of such non-holographic diffractive microstructures include, inter alia, blazed diffraction gratings, Kinoforms, Damann gratings, and other diffractive microstructures including rectilinear surface features. DOE's are thus non-holographic diffractive microstructures since they are digitally predefined.

The non-holographic diffractive surface is made up of one or more defined grating structures. Because the grating structures are mathematically defined, as discussed in more detail below, they take various forms. A plan view of an ideal kinoform or "blazed" diffractive can be seen in Fig. 1, and a cross section is shown in Fig. 2A Such a kinoform- DOE has essentially 100% diffraction efficiency in a single diffractive order. A multiple-level diffractive lens is shown in cross section in Fig. 2B. Such a stair-case approximation to a kinoform will have diffraction efficiency dependent on the number of "levels" or steps, and will approach 99% for 16 levels, for example. Fig. 2C shows a single-level, or phase zone plate, diffractive in partial cross section. This diffractive lens focuses hght into multiple diffractive orders. Fig. 2D shows a cross section of a linear blazed DOE. A linear blazed DOE is an approximation to a true kinoform and is useful for fabrication purposes. Its diffraction efficiency is dependent on the approximation to the polynomial form of the kinoform. Often it is found to be 99% efficient.

These surface relief features can be employed in various patterns to achieve varying effects. Single and multiple phase level, three- dimensional repeating unit cells (commonly referred to as a Damann grating) are shown in Figs. 3A and 4A, respectively, and are shown in cross section in Figs. 3B and 4B, respectively. The single level repeating unit cell grating will diffract a single wavelength of light into a two-dimensional pattern of orders symmetric about the zeroeth order (the zeroeth order being that light which passes through the grating undiffracted). The multiple level repeating unit cell grating can create a diffraction pattern with little or no symmetry. Blazed or single or multiple level, two- dimensional linear gratings, which diffract light into orders aligned along a single axis, are also available. A blazed hnear grating is shown in Fig. 5. A digitally-simulated sinusoidal surface relief grating is shown in cross section in Fig. 6. While such a generalized sinusoidal surface relief grating may in certain circumstances function similar to an HOE, it differs in that it can be digitally manipulated during design, greatly enhancing its flexibility and the predictability of the optical prescription.

The present invention improves on standard automotive lighting arrangements by use of one of these non-holographic diffractive structures to reduce the overall size and complexity of the part, eliminating features which cause flow constrictions in molding, and reducing undesirable center-to-edge thickness variations. The result is a more easily fabricated and less expensive lens. In addition, the lens arrangement of the present invention further provides color correction, color selectivity, defined uniformity, reduced penumbra, greater control, increased efficiency, and simplified tooling, as discussed below.

Diffractive optics is an emerging technology wherein an optical element (e.g., a lens) includes a mathematically-defined phase surface rehef pattern, such as those discussed above, for effecting modulation of an optical wavefront passing therethrough. Thus, not only is the hght beam refracted by the lens element it is also diffracted by the surface rehef pattern to produce an image from an incident wavefront. A binary optical element (BOE) may be defined as a diffractive optical element having 2ⁿ levels of phase which is a stepped approximation (such as shown in Fig. 2B) of an ideal surface profile of a kinoform lens (shown earher with reference to Fig. 2A). Binary optics refers to the dual-level (high-low) nature of the phase-relief pattern used to control the phase, - amplitude, and polarization of an optical wavefront. It is also possible to fabricate DOE's with an arbitrary number of phase levels.

The kinoform lens such as shown in Fig. 1, like a Fresnel lens, has a discontinuous thickness (or refractive index) profile. However, the operation of a kinoform relies on its interference of hght from different zones, i.e., diffraction mechanisms (optical path difference at the discontinuities is an integral number of the wavelength), while a Fresnel lens bends rays of light by the refraction mechanism (optical path difference at the discontinuities is not carefully controlled). Kinoforms are sometimes referred to as "micro-Fresnel lenses." However, the performance of the kinoform can be diffraction-limited, while that of the Fresnel lens is not diffraction-limited.

Fig. 7 is a schematic depiction of a polynomial kinoform showing a corresponding conventional lens in dashed line. The corresponding linear kinoform depicted in Fig. 2D has slightly lower efficiency. A corresponding DOE or BOE (such as shown in Fig. 2B) may have higher or lower efficiency than the linear kinoform depending on the number of levels used. For example, a four level binary has an efficiency (scalar diffraction theory) of approximately 81.1%, whereas a 16-level binary has an efficiency of approximately 98.7%. The single level phase zone plate shown in Fig. 2C will split approximately 40% of the light into each of the plus and minus one orders, with the remaining 20% being divided among the zeroeth order and orders having a magnitude greater than one. Figure

2B depicts a four-level non-holographic, binary microstructure.

The properties of DOE's can be exploited to carry out a variety of tasks such as dispersion compensation, thermal compensation, beam steering, optical multiplexing, hght wave modulation, optical interconnecting of a variety of hght signals, collimating, light wave redistributing, etc. These different functions may be achieved by varying the location and size of the array of phase gratings on the surface of the lens.

One of skill in this field can readily understand that by varying the location, size, and pattern of the microstructure of the DOE, a single lens can be produced which comprises a single plastic or moldable material that is able to optically function in a manner which normally requires multiple materials with different refractive indices. Such a plastic DOE lens system can be thermally and/ or chromatically corrected, as well as provide an optimized balance of other optical aberrations. The concept thus combines three technologies: diffractive optics; conventional optical design; and the process of reproducing diffractive optical elements in plastic or moldable materials.

Chromatic and thermal correction may be accomplished by appropriately designing the height width, and pattern of the diffractive surface features. For example, as the wavelength of the incident light changes, the refractive index of the DOE material will also change. However, the diffractive structure can be designed to cancel out this effect so that changes in the wavelength in the incident light do not produce a change in the color of the hght emerging from the DOE. Likewise, as the temperature of the DOE changes, the refractive index of the DOE material will also change. Again, the diffractive structure of the DOE may be designed to cancel out the changes in refractive index which occur with temperature changes. As used in the specification the term "diffraction" includes the phase shifting of the incident hght wavefront

A major difference between a refractive optical surface and a diffractive optical surface in ray tracing analysis is that a refractive surface is governed by Snell's law: n₁sinΘ₁=n₂sinθ₂ where n is the refractive index of the material, and a diffractive optical surface is, in scalar terms, subject to the grating equation: λ =2dsinΘ, where λ = the wavelength of the incident light beam. d= grating periodicity

Θ= diffractive angle.

The grating height h is determined according to the formula h= λ (n-1)

Depending upon the particular effect desired, the feature size will be some multiple of h, where the multiple will determine the phase shift in units of wavelength. In practice, this multiple will generally be between 1/20 and

200 times the wavelength of hght. Typical values include 1/4, 1/2, 3/4, 1, 2, 3, etc. In general, for plastic materials with visible hght this translates into feature heights on the order of micrometers.

The lateral dimensions or spacing of the features, as well as the periodicity, is determined differently for different diffractive patterns, as will be discussed in more detail below. Typically, the lateral dimensions will range from a fraction to about a thousand times the wavelength of hght. For plastic materials with visible hght, the range is generally between about one-half to about one hundred micrometers. It should be noted that the present invention apphes equally as well to hght outside the visible spectrum, i.e., infrared and ultraviolet light

In refractive analysis, when a collimated beam of light impinges on a curved surface of a lens, the plane wavefront is transformed according to Snell's law into a spherical wavefront. Using a diffractive structure, the wavefront is transformed as determined by the grating equation.

2. The Embodiments

For clarity, this discussion will center on the application of the lighting arrangement of the present invention to an automotive turn signal and to an automotive head lamp, but applies equally to other automotive lighting fixtures, such as fog lamps, brake lights, etc. Fig. 8 shows a turn signal according to the present invention, designed to replace a turn signal with the "conventional" or "pillow" type of refractive lens, and includes a bulb 2, a bulb holder 4, a reflector 6a, and a lens 8a. The assembly may include a separate housing, or this function may be served by the reflector 6a. The bulb holder 4 may be a separate part or integral to the reflector housing. The bulb 2 is chosen to produce sufficient hght to meet the appropriate government regulations.

The lens 8a may employ a refractive in combination with a diffractive, which need only contain a few diffractive elements instead of hundreds of refractive features. The exterior surface 10 of the lens 8a may be the outermost surface of the assembly, depending on whether a protective or aerodynamic cover is used. If the exterior surface 10 of the lens 8a is exposed, and if no abrasion resistant coating is used, then it is preferred that the exterior surface be purely refractive in nature. If an abrasion resistant coating is used, then the diffractive may be on the outer most surface of the assembly. In that case, the diffractive should be designed to compensate for the refractive index of the abrasion resistant coating. Preferably, the diffractive is on the inner surface 12 of the lens in order to protect the diffractive structure from dirt, dust fluids, and the like.

The dimensions of the exterior surface 10 of the lens may be dictated by the styling of the vehicle and may therefore be of any shape, or the shape may be dictated by other considerations. If the exterior surface is curved and refractive in nature, and the inner surface 12 contains a diffractive structure, the overall lens 8a retains refractive power in combination with diffractive power. The diffractive structure may be placed on a planar (flat) surface or segment of the inner surface, or it may be placed on a curved surface or segment of the surface. In the case of the conventional-type turn signal, the diffractive can be used to replace any or all of the lenslets and prisms of the conventional lamp. This facilitates injection molding, eliminating the complications involved in molding pillow optics, and significantly lowers the cost of the part

In addition to simply facilitating production while achieving the same light-bending and focusing functions of the refractive pillow optics, the diffractive can be designed to produce an illumination pattern which is superior to the conventional lens in uniformity, color, penumbra, and any of the other figures of merit understood by one of skill in the field. Further, the simple ehmination of the lenslets and prisms also eliminates the draft surfaces therebetween, thus eliminating the associated unpredictable light refraction. A diffractive surface can also be employed in the reflector itself. This arrangement can be used to direct the reflected light in such a way as to produce greater uniformity and efficiency.

Fig. 9 shows an embodiment of the present invention which relates to a projection type automotive turn signal including a bulb 2, a bulb holder 4, a reflector 6b, and a lens 8b. The optical requirements of the lens 8b are defined by the light pattern produced by the bulb/reflector combination and the pattern required by the applicable government regulations. It is instructive to imagine a refractive lens with surface characteristics sufficient to meet these optical requirements. A diffractive structure is used to achieve any or all of the optical power required, permitting the lens thickness to be greatly reduced. With the diffractive, there is no longer a need for the center-to-edge thickness variation of a purely refractive lens. Because the thickness of the lens can be substantiaUy uniform, the lens can be injection-molded with much less difficulty and expense than refractive projection lenses.

The exterior surface 10 of the lens can be characterized by a few segments in order to beam-steer and intensity-shape, and can retain some refractive characteristics. However, any or all of the refractive optical power can be removed and replaced by the diffractive in order to permit the lens to be more uniform in thickness, facilitating injection molding. The diffractive structure is preferably disposed on the inner surface, which is typically convex in nature, but may be planar or concave.

In addition to simply assuming the optical characteristics of the refractive while facilitating fabrication, the "projection style" lens of the present invention (as with the "conventional style" lens of the present invention) can also be designed to produce a superior illumination pattern, with improved compliance with government regulations, uniformity, color, penumbra, and any of the other figures of merit understood by one of ordinary skill in the field. Again, a diffractive surface can be included in the reflector itself in order to direct the refracted light in such a way as to produce greater uniformity and efficiency.

For example, diffractive and refractive power can be used in combination to achieve color correction. Plastics have a positive Abbe number, which is a quantification of the dispersive effects of a material on light passing therethrough. The larger, in absolute terms, the Abbe number, the less dispersive the material. For example, acrylic has an Abbe number of 35, and polycarbonate has an Abbe number of 65. A diffraction grating, on the other hand, has a relatively small, negative Abbe number. For example, in the visible region, the Abbe number of a diffractive grating is effectively -3.45. This means that a diffractive will disperse hght in a manner inverse to plastic refractives (hence the negative number) as well as much more dramatically (hence the smaller absolute value). A refractive will focus blue hght (400mm) somewhat closer than red light (700mm), while a diffractive will focus red very much closer than blue. Therefore, a diffractive can be used to correct the dispersion of a refractive, and vice versa, so that the different colors are focused at the same spot. The same phenomena can be used to compensate for the penumbra that is caused by edge rolloff due to sharp edges in many lighting structures.

3. Design Of The Lens

The design of the diffraction grating itself in any of the embodiments will differ depending upon the government-prescribed illumination pattern and the desired effects. The outer curvature of the lens can be chosen based on styling considerations (or in order to retain sufficient refraction) and the diffractive designed to cooperate with the remaining refractive power to achieve the desired illumination pattern. A later example discusses the design of a rotationally symmetric diffractive with a single order which can be used to "act like" a refractive, i.e., replace all or a portion of the power of the refractive, in a projection-type assembly. The design of the diffractive can be achieved through mathematical modeling, using any of several techniques depending upon the level of sophistication required. These techniques include scalar analysis, rigorous coupled wave theory, parabasal ray tracing, vector analysis, and the beam propagation method. Commercially available codes can be employed to carry out the diffractive design analysis. Some of these codes include "Code V" or "Light Tools", both from Optical Research Associates (Pasadena, CA), "ASAP" from Breault Research Organization (Tucson, AZ), "Diffract" from MM Research (Tucson, AZ), or "Grating Solver" from Grating Solver Development Corporation (Allen, TX). Examples of this mathematical analysis are discussed in more depth in U.S. Patent No. 5,218,471 to Swanson et al., which is incorporated herein by reference.

The above-discussed mathematical techniques result in a mathematical model of the diffractive pattern. In the case of a rotationally symmetric diffractive, the mathematical description will take the form of the phase equation:

Φ(r)=Ar²+Br ⁴+Cr ⁶... where Φ(r) is the phase of the diffractive structure as defined on the surface. This equation defines a rotationally symmetric diffractive structure where r is the radial distance on the lens from the vertex.

In the case of a repeating unit diffractive, the description will be a two-dimensional matrix of phase descriptors. The physical description of the diffractive structure for a rotationally symmetric lens is defined by the Fresnel Zone Equation and the phase step height. The Fresnel Zone Equation defining the ring radii is: r_m = SQRT((2mfλ/n+ (mλ/n)²) where r_m is the m-th zone radius such that 0(r_m)=2jnn

A ring is located at each radius for which the phase is found to be an integer value. The phase step height as defined and stated above is determined by Θ(r_m)=2πm and is equal to λ/(n-l).

A more advanced application would be to closely balance the optical power of the refractive portion of the lens with the diffractive portion of the lens in such a way as to provide color correction, as mentioned earlier. This is possible because the optical power is linearly proportional to the wavelength. For a refractive surface, the optical power is a second order effect to the wavelength. In other words:

These values are generally solved iteratively with the aid of a computer. The approach is similar for a repeating unit diffractive, but requires a greater number of computations because of the multiple cells,- These approaches can be better understood with reference to the examples below.

By designing the diffractive mathematically, it is possible to achieve a high degree of control over the optical characteristics of the resultant lamp. The diffractive mask which results is a defined or pseudorandom phase mask, in contrast to a random phase masks achieved with HOE's recorded in a laboratory environment using optical components. In the defined phase mask, it is possible to determine with high precision the phase effect of the physical elements at each point on the mask before the lens itself is created. This permits the lens to be modeled in combination with the bulb and reflector, and the design to be tolerance-analyzed and optimized before a lens is ever actually fabricated. In addition, it is possible to customize the patterns with much more precision and flexibility than can be done with laser interferometry. Furthermore, it is possible to create diffractive structures which are not sinusoidal in profile.

Example 1: Projection Turn Signal Lens

A schematic profile of typical projection style turn signal is shown in Fig. 10A and is characterized by a prescription that includes a reflector that is parabolic in shape with a base radius of 45.6mm. The irmer surface of the lens is concave and spherical in shape with a radius of curvature of 31.1mm. The outer surface of the lens is convex and elliptical with semi-axis lengths of 53.08mm (horizontal or "x"), 44.79mm (vertical or "y"), and 58.62mm (along the axis extending forward of the vehicle or "z?). The lens has a center thickness of 14mm. Fig. 10B shows a schematic hght distribution pattern of this assembly.

A diffractive replacement according to the present invention for such a turn signal can be rotationally symmetric, as discussed earlier. The profile of the resultant design is shown schematically in Figure 11A The reflector outside shape is unchanged. The inner surface of the lens retains its spherical base radius of 31.1mm. The outer surface is defined by an ellipse with semi-axis lengths of 58.08mm in x, 58.79mm in y, and 58.62mm in z. The decreased refractive optical power in the outer surface has been replaced with diffractive power, which can be added to the inner surface and described in terms of the phase equation described above. For the case at hand, where we are only concerned with removing refractive power, the coefficients derived with the aid of the ASAP software are:

A=6.667xl0^"3 B=-2.85xl0^"7

For a polycarbonate material designed for use in the visible region, the refractive index is approximately 1.586 at 633nm wavelength. This will result in a physical step height of 1.08 m for each diffractive ring, of which there are 3,041 in this example. Fig. 11B shows a schematic light distribution pattern for this replacement lens. Example 2: Patterned Turn Signal Lens

As discussed earlier, the unit cell diffractive patterns can be used to diffract a single wavelength of hght into a two-dimensional pattern, which can be either symmetric or asymmetric depending upon the number of levels employed. The unit cells can be configured to permit light to pass to any or all of the affected orders, effectively creating a dot matrix-like pattern in which the size and repetition spacing of the unit cells will dictate spread of the "dots." If hght of multiple wavelengths is used, then the dispersive characteristics of the diffractive can be manipulated to blur the distinctions between neighboring orders in the pattern, producing a smoother hght pattern.

This patterning concept can be applied in various ways in automotive lighting. One application is to create a manufacturer's logo or other design in a brake or tail light. Another is to take advantage of the control afforded to custom design a head lamp or turn signal pattern within the applicable government specifications.

Example 3: Color Selecting Turn Signal Lens

Diffraction efficiency can be defined as the percentage of incident hght that is diffracted into a desired order. As discussed, the feature size h of the diffractive can be manipulated (by multiplying h by a different factor) in order to dictate the phase shift in units of wavelength. For a given aspect ratio, the diffraction efficiency will be high across the entire visible spectrum if this factor is held at 1 (i.e., h=ho). As this factor is increased, meaning the diffraction grating will be deeper, different wavelengths will be affected differently. At higher factors, certain wavelengths will continue to be diffracted efficiently, while others will not This phenomenon is illustrated in Figure 12, which plots a typical diffraction efficiency against wavelength for the size factors of 1 and 4 (i.e., h=h₀ and h=4h,₎). This characteristic can be employed to achieve greater flexibility with a single hght source. For example, diffractive gratings can be employed which pass white hght for a cornering lamp and which pass amber hght for a turn signal from the same hght source. In any situation in which colored light is needed (i.e., park, turn, or brake signals), the necessary wavelengths can be directed outwardly while the remaining light can be diffracted elsewhere, where it can be collected and distributed as desired.

Example 4: Head Lamp Lens To Compensate For Bulb Filament Orientation

As discussed, a diffractive structure can focus light into several

"orders" or locations in space. This aspect can be used to compensate for the fact that a bulb filament is not a perfect point source, but rather is extended spatially. In automotive applications, bulb filaments are generally hnear or helical or the hke, and oriented either transverse (9004, etc.) or axial (9007) to the reflector, the lens and the front of the vehicle. A diffractive designed to have the various orders (...-1, 0,+ l...) focused at different points along the length of the filament can provide greater control and efficiency over the pattern of distribution of hght, regardless of the orientation. Alternatively, one or several of the orders can be used to control first incident light and the others can control hght from the reflector. Further, diffractive power can be used to direct hght from either the reflector or first incident and refractive power the other (or vice versa).

Example 5: Head Lamp Lens To Compensate For High Intensity Discharge Bulb

A high intensity discharge bulb outputs very intense light, and therefore a single bulb can be used in combination with fiber optics to provide light to several assembhes in an automobile, especially if the light is properly harnessed. However, in a discharge bulb (ex., standard gas discharge dl, d2, d2-l, etc.), the discharge region of the bulb is characterized in that various colors of hght emanate from different regions of space (which are oriented much like layers of an onion). A purely refractive lens will have difficulty compensating for this effect resulting in a head lamp pattern with significant color distortion, shift, "rainbows", etc. A diffractive (with or without a refractive portion) can be designed to compensate for this fact by having focal points (surfaces) that correlate to the various chromatic discharge regions.

Example 6: Projection Style Head Lamp

A profile of a conventional projection style head lamp is shown in Figs. 13A (top view) and 13B (side view), and is characterized by a prescription that includes a reflector that is parabolic in shape with a base radius of 45.6mm. The reflector has been tilted 2.5 degrees both horizontally, and vertically to direct the light into the fourth (lower right) quadrant A bulb shield is inserted between the bulb and lens and acts to create a cutoff in intensity of the pattern above the horizon (zero degrees vertical). Alternatively, it is recognized that the bulb shield could be eliminated and the same effect produced by the reflector and/or lens. The inner surface of the lens is concave and spherical in shape with a radius of curvature of 31.1mm. The outer surface of the lens is convex and elliptical with semi-axis lengths of 51.89mm (horizontal or V), 44.79mm (vertical or "y"), and 58.62mm (along the axis extending forward of the vehicle or "z"). The lens has a center thickness of 20mm. Fig. 13C shows the hght distribution pattern of this assembly.

According to the present invention, a diffractive replacement for such a head lamp can be rotationally symmetric, as discussed above. The resultant design is shown in Figs. 14A (top view) and 14B (side view). The reflector outside shape and angle is unchanged. The bulb shield is unchanged. The inner surface of the lens retains its spherical base radius of 31.1mm. The outer surface is defined by an ellipse with semi-axis lengths of 58.08mm in x, 58.79mm in y, and 56.62mm in z. The decreased refractive optical power in the outer surface has been replaced with diffractive power, which can be molded to the inner surface and characterized by the polynomial phase equation described above. For the case at hand, where we are only concerned with removing refractive power, the derived coefficients are: A=6.667xl0³ B=-2.85xl0^"7

For a polycarbonate material designed for use in the visible region, the refractive index is approximately 1.586 at 633nm wavelength. This will result in a physical step height of 0.938μm for each diffractive ring, of which there 20,720 rings in this example. The smallest ring size is 1 micron. Fig. 14C shows a hght distribution pattern for this replacement lens.

4. Mold Fabrication

The mold from which diffractive is formed can itself be fabricated by diamond turning ion-milling, reactive ion etching, or the like. Diamond-turning is described in U.S. Patent No. 5,589,983 to Meyers et al., which is incorporated herein by reference. Diamond-turning is particularly advantageous in preparing a mold for a rotationally symmetric diffractive grating, which, as discussed earlier, is well suited for applications in which the goal is to replace refractive power with diffractive power in order to simplify mold geometry. Lithography techniques are described in U.S. Patent No. 5,538,674 to Nisper et al., which is also incorporated by reference. It is also possible to fabricate the mold using a nickel electroform, but this method suffers from the limitations described earlier.

During molding, as the plastic is injected into the mold, a "skin layer" against the surface of the mold forms and solidifies almost immediately. Because the surface features are small compared to the thickness of this skin layer, the diffractive features are rephcated with a -high degree of accuracy as the skin layer forms. Also, even if sinking in the mold does occur, while the refractive power of the lens will likely be affected, the resultant deformations are so gradual in comparison to the diffractive features, that the diffractive power is usually unaffected.

5. Conclusion

Preferably, the diffractive structures used in automotive lenses according to the present invention have surface features with blazed or rectilinear shapes which may or may not be combined with conventional sinusoidal features. For example, blazed feature surfaces can be combined with sinusoidal and rectilinear surfaces in a diffractive.

As discussed above, multiple diffractive patterns in one or more lenses may be utilized to accomplish different beam-steering and color selection functions. For example, a turn signal lens may have multiple diffractive patterns, side-by-side in one lens to direct the light beam in the forward and side directions of the automobile, and/or to direct different colors of hght in the different directions.

Another important advantage of the present invention lies in the ability to optimize the combination of bulb, reflector, and lens to minimize the bulb power, reflector surfaces, and lens dimensions required to perform the same functions achieved by existing bulb-reflector-lens technology. That is, the use of diffractives allows the automotive lighting designer to select lower-power bulbs and smaller reflectors with thinner diffractive lenses to achieve the same optical performance as the larger _ structures used at present. Thus, the use of diffractives permits the automotive lens designer more flexibility in matching optical requirements with physical structure.

Another alternative available with the use of diffractives is to provide diffractive surfaces on both the front and back of the lens. These multiple diffractive layers provide additional flexibility for the lighting designer to shape the hght beam, conect for color and thermal variations in the refractive index of the lens, and perform the other optical functions discussed above. In fact, if diffractive surfaces are provided on the front and rear faces of a curved surface, the lighting designer has four surfaces (two diffractive and two refractive) in a single body to use in managing the hght from the bulb.

While the lenses described above are used with incandescent and high energy discharge lamps, other light sources may be profitably utilized. For example, LED, ultraviolet, infrared, etc. light sources may be used. Since diffractive surfaces are designed based on the wavelength of light passing therethrough, the automotive lighting designer may adapt different diffractive surfaces to different light sources to steer and shape and color-select the hght beam passing through the diffractive lens.

Another advantageous application of diffractive lenses in automotive hghting is an anti-reflective diffractive surface placed on the inside of the automotive lens. The anti-reflective surface will reduce the reflection of energy back into assembly and therefore prevent heat buildup inside of the lens package, thus prolonging bulb life. -

An alternative example of beam steering is the application of a diffractive lens over a reflector which holds a number of bulbs, for example, a hnear anay of bulbs. By conectly selecting the diffractive patterns in the lens, the hght passing through the lens can be merged together into a smooth, rectangular hght pattern. This may find use in tail lights, brake lights, and decorative lights in the automotive field.

It can readily be seen that diffractive lenses can be applied to a wide variety of automotive uses such as head lamps, turn signals, tail lamps, back-up lights, cornering lamps, sidemarker lamps, task lighting, interior lights, instrument panel displays, decorative lights, etc. The use of diffratives enables the automotive light designer to become a lighting architect able to direct light, focus light, color light, shape light, blend hght, and conect for hghting abercations, to satisfy governmental regulations and to enhance the aesthetics of the automobile.

Thus, what has been described is a product and method for production thereof whereby automotive lighting may be made smaller, lighter, less expensive and provide greater flexibility to the automotive lighting designer.

The individual components shown in outline or designated by blocks in the drawings are well-known to the artisan in the diffractive imaging arts, and their specific construction and operation are not critical to the operation or best mode for carrying out the invention. While the present invention has been described with respect to what is presently considered to be the prefened embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent anangements included within the spirit and scope of the appended claims. The scope of the followmg claims is to be accorded the broadest reasonable interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims

WHAT IS CLAIMED IS:

1. An automotive lens for generating a predetermined light pattern when used in conjunction with an automotive lamp which includes a reflector and a hght source, the lens comprising: an automotive lens body optically-transmissive to hght emitted by the light source and hght reflected by the reflector; and a diffractive grating formed on a surface of the lens body for phase-shifting the wavefront of the hght passing through the lens body, the diffractive grating comprising a non-holographic microstructure having a pattern predetermined to cause the hght passing through the lens body to generate the predetermined light pattern.

2. The lens of claim 1, further comprising a refractive surface disposed on an opposite surface of said lens body from said diffractive grating.

The lens of claim 1, wherein the lens comprises molded plastic.

4. The lens of claim 1, wherein the diffractive grating is rotationally symmetric.

5. The lens of claim 1, wherein the diffractive grating is nonrotationaUy symmetric.

6. The lens of claim 1, wherein the diffractive grating is

blazed.

7. The lens of claim 1, wherein the diffractive grating has multiple phase levels.

8. The lens of claim 1, wherein the diffractive grating comprises a three dimensional repeating unit cell.

9. The lens of claim 7, wherein each unit cell has multiple phase levels.

10. The lens of claim 1, wherein the diffractive grating includes phase zone plates.

11. An automotive lamp assembly for generating a predetermined hght pattern, comprising: a housing including a reflector and a bulb holder; a hght source mounted to the bulb holder; and a lens optically transmissive to hght emitted by the light source and light reflected by the reflector, the lens including a surface with a diffractive grating formed thereon for phase-shifting the wavefront of the hght passing through the lens, the diffractive grating comprising a non- holographic microstructure having a predefined pattern to cause the light passing through the lens to generate the predetermined hght pattern.

12. The assembly of claim 11, wherein the reflector includes a diffractive surface.

13. A method of forming an automotive lens for collecting and redirecting hght into a predetermined hght pattern, comprising the steps of: defining a non-holographic diffractive microstructure having a pattern to cause the collected light to generate the predetermined light pattern; converting the defined microstructure into at least one pattern conesponding to a negative of the defined phase structure; machining directly into a molding base the pattern conesponding to the negative of the defined phase structure; and molding a plastic element in order to transfer the pattern from the molding base into a positive phase structure in a plastic element.