US3825742A - Lamp unit with controlled-diffusion reflector and method of making the reflector - Google Patents

Abstract

For a lamp unit, a sagged glass reflector having a stippled, concave surface for providing controlled diffusion of light. The method of making the reflector comprises: providing a preheated concave mold having a uniform pattern of peened indentations in its surface and a plurality of vacuum drawing holes; placing a flat blank of glass across the opening of the mold; heating the glass to a plastic state; drawing a partial vacuum in the mold to force the plasticized glass to sag against the peened surface of the mold; cooling the glass to a rigid state; and applying a coating of reflective material on the concave surface of the sagmolded glass.

Description

United States Patent 1191 Levin 1 1 1451 July 23, 1974 LAMP UNIT WITH CONTROLLED-DIFFUSION REFLECTOR AND METHOD OF MAKING THE REFLECTOR [75] Inventor: Robert E. Levin, South Hamilton,

Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: Jan. 2, 1973 [21] Appl. No.: 319,321

52 us. c1..... 240/103 R, 65/106, 240/413511 240/4136 51 1111.0 ..F21v.7/06,F21v 7/10 [58] Field of Search 240/4135 R, 41.36, 103 R; f

[56] References Cited i' UNITED STATES PATENTS 2,702,411 2/1955 Winstead .i 65/106 7/1967 Elmer 240/103 R 5/1970 Dorman 240/103 R Primary Examiner-Samuel S. Matthews Assistant Examiner-Russell E. Adams, Jr. Attorney, Agent, or Firm-Edwardl J. Coleman [57] ABSTRACT For a lamp unit a sagged glass reflector having a stippled, concave surface for providing controlled diffusion of light/The method of making the reflector comprises: providing a preheated concave mold having a f uniform pattern of peened indentations in its surface and a plurality of vacuum drawing holes; placing a flat blank of glass across the opening of the mold; heating the glass to a plastic state; drawing a partial vacuum in the mold to force the plasticized glass to sag against the peened surface of the mold; cooling the glass to a rigid state; and applying a coating of reflective material on the concave surface of the sag-molded glass.

1 12 Claims, 6 Drawing Figures PATENIEUJummH 3.825742 sum 1 0F 2 FIG.3

LAMP unrrlwrrn CONTROLLED-DIFFUSION REFLECTOR AND METHOD OF MAKING THE it REFLECTOR BACKGRQUND OF THE INVENTION Reflectors have been formed in a multitude of manners, such as spinning, electr-forming, hydro-forming, explosion forming, stamping, etc., if the-material was properly workable; such as aluminum. Glass reflectors have been formed by blowing orgravity sagging into a mold or by pressing'between a two-part mold. Techniques such'as grinding andpolishing are not applicable to low cost, large volume'production of nonspherical elements.

For precise control of light, the reflector surface must be specular in nature. However,itis oftennecessary to smooth out irregularities in the light distribution by causing the spread of light about the specular direc tion from each point on the reflective surface within a well-defined cone. The common diffusiontechniques cause an uncontrolled diffusion over large angles, typi cally as much as-2rr steradians. The control desired can be achieved byforming small (with respect tothe total reflector) zones of greater or less curvature than the basic reflector at a multitude of adjacent points over the reflective surface. In the past this has been done by forming the negative of the small elements in a forming tool that mates with the reflective surface. Thus, this technique has been applied to the aforementioned metal forming techniques and to pressed glass components. t

. A controlled-diffusion glass reflector-is particularly useful when the heat must bereduced in the light beam.

A glass reflector with a cold dichroic mirror coating willreflect the visiblelight while passing a major portion of the infrared power through the reflector. This separation technique is not possible with metal reflectors, where the most that might be provided is for the pressed reflectors which were heavy, were rimmounted as opposed to base-andsocket mounted, and

relatively expensive. The engraving of the male tool exhibited wear that caused progressive changes in successive lamps and was difficult to replicate due to its fine structure.

SUMMARY OF THE INVENTION In view of the above-discussed disadvantages of the prior art, it is an object of the present invention to provide an improved controlled diffusion lighting device.

Another object of the invention is to'providea reflector of glass or a similar material having an improved stippled surface overa substantial portion thereof for providing controlled diffusion of light.

A further object of the invention is to provide im proved means for making such a reflector.

In accordance with the invention, I have discovered that by sagging a plasticizable material, such as glass, into a concave mold having a stippled surface, and thereafter coating the concave surface of the sagged piece with a reflective material, a reflector can be provided which has controlled local zones of increased'or decreased curvature, as compared to the general reflector curve at the same points, for providing a conusing processes such as etching, sand-blasting, etc.,

infrared power to be absorbed by a dichroic coated metal mirror, raising local temperatures. However, this process generally is not considered functional at the present time.

As a specific example of an application of light diffusing reflectors andthe problems associated therewith, consider a reflectivecondensing system for- 135mm slide projector in which the reflector is integral to the lamp. Such a system includes: an objective lens, a film gate aperture, arelay lens, and a lamp assembly with a reflector. For a lamp in the 250 watt range with conventional projection optics of a 4 inch, f/3.5 objective and a relay lens of about 15 diopters paraxial, and with the normal spacelimitations, a reflector-condenser system requires controlled spreading of light within limited solid angles to'prevent localized nonuniformities on the screen. Previously, this was accomplished with trolled diffusion of reflected light. The sagging process comprises forcing a material, such as glass, in the plastic state against the stippled surface of a mold by gas pressure and/or gravity such that the mold is only in contact with the surface opposite the final reflective surface. Inthis manner the final reflective surface is uncontaminated by physical contact with toolsThe resulting stippled reflecting surface: forms a spread of light from each part of the reflector which is confined withinwell'defined spatial regions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective viw of a lamp unit having a sagged reflector according to the invention;

FIG. 2 is a front view of the lamp unit of FIG. 1; FIG. 3 is a simplified cross-section of a mold used in making the reflector of FIGS. 1 andv 2;

FIG. 4 isa partial section viewof the molded glass used in the reflector of FIGS. 1 and 2;

FIG. 5 is a magnified view of a region of the reflector cross-section of FIG. 4; and

FIG. 6- is a diagram illustrating the optical effect of a peen on a reflecting surface.

'DESCRIP'TIONOF PREFERRED EMBODIMENT A reflector according to the invention is formed by sagging a material, typically glass, heated beyond the softening point into a mold using gravity and/or a pressure differential between the surrounds and the region contained between the glass and the mold. The mold differs from the final reflective surface by the thickness of the glass with allowances for the variable glass thick ness'due to glass thickness due to glass flow during the forming. Small, local variations in curvature, hereinafter referred to as stippling, are provided over the mold surface. Generally, this stipplingtakes the form of concave spherical indentations on the mold surface, but

other shapessuch as spheroidal, ellipsodial, flat or nonconic may be used; also, convex elements may be used. The localzones, comprising the stippling, or peens, are

of specified size and shape as well as being arranged in some specified'orderly fashion such that the amount of localized spread can be controlled as well as permitting convenient replication of molds. The peens are conveniently specified in terms of the forming tool (e. g., for

a spherical peen, by the radius of the sphere) and the average spot diameter.

The formulation of the peens may vary over the reflecting surface depending on the degree of spread required for the light rays in a particular direction. The reflecting surface, which is ultimately coated with a reflective material such as aluminum, silver, an interference filter, etc., is free to tool marks and other irregularities commonly developed during intimate contact with forming tools.

A projection lamp unit employing a reflector according to the invention is shown in FIGS. 1 and 2. An ellipsoidal reflector is coated with a dichroic cold mirror on the interior concave surface 112 which carries the stippling, or peens, providing the optical control. Peens of greater curvature on the outer convex surface 14 are produced by direct transfer from a female mold. The lamp base 16 is used to position and support the lamp 18 in the reflector. Lamp 18 is typically a tungsten halogen lamp with an incandescent filament and is introduced through a hole 20 at the apex of the reflector. The lamp may be supported within or in immediate proximity to the volume defined by the reflector 12 and the aperture plane thereof.

A typical arrangement for the sagging process is illustrated in FIG-3, which shows a simplified cross section through the center of a mold 22 for sag-forming a stippled reflector according to the invention. The concave surface 24 of the mold, which mates with the outer reflector surface during sagging, is peened to provide the desired stippling configuration, represented by the spherical indentations 26. Preferably,this stippling is disposed in a non-random pattern; hence, the mold surface 24 contains a substantially uniform pattern of spherical indentations 26. To enable a vacuum to be drawn between the mold and the sagged glass, the stippled surface of the mold contains a plurality of small holes, in the order of 0.020 inch diameter, as illustrated by a typical hole 28. Relief 30 in the central portion of the mold produces additional thinning of the glass in that regionwhere the central hole 20 (FIG. 2) is to be formed in the reflector. The shape of this relief is not critical. Minor variations in the upper edge of the mold are dictated by the processes used to cut and finish the glass after molding.

A preferred method of making a stippled reflector according to the invention will now be described. First, the above described. mold is preheated to between l,l00 and 1,200C. Next, a flat glass blank 32 is placed across the opening of the preheated mold 22 with sufficient overlap to provide a seal thereabout. Typically the glass is about 0.070 inch thick and comprises ordinary soda lime glass having a mean coefficient of thermal expansion in the range of 87 to 93 X 10 per C between 0C and 300C. In lieu of glass, blank 32 may comprise other rigid material which may be heated to a plastic state for forming. The glass is then heated to a plastic state and allowed to gravity sag while simultaneously drawing a vacuum between the glass blank and the mold, via holes 28. In this manner, the plasticized glass is forced to sag against the stippled surface of the mold without tools, whereby the glass is formed to have a untooled concave surface with stippling thereon. The

creation of a vacuum assumes an intimate contact between the glass and the mold without the voids that may occur during a pure gravity sag. Typically, in a production process, the above steps are accomplished by carrying several molds on a conveyor belt through a furnace having a controlled temperature in the range from about 1,l00 to l,200C. The sag-molded glass is then permitted to cool to a rigid state.

FIG. 4 is a partial section view illustrating the thickness variation of the molded glass 34. The thin central portion 36 formed by relief 30 is subsequently cut off at about 38 to provide hole 20. A magnified view of one region 40 of the glass cross-section is shown schematically in FIG. 5. The outer, or convex, surface 42 is formed by direct contact with the mold, while the optically weaker stippling elements 26 are formed on the interior surface 44 by the plastic flow of the glass. A thin deposited film 46 of reflective material such as aluminum, is coated on the inner surface 44 of the molded glassas the light reflecting component The coating 46 may be a cold dichroic mirror coating for reflecting visible light from the incandescent filament of lamp 18 but passing infrared radiation back through the reflector.

A basic reflector shape (FIGS. 1 and 2) suitable for the 35 mm projector optics described hereinbefore is a prolate ellipsoid of major diameter 1.439 inch and minor diameter 0.992 inch. A female mold 22 made for forming this reflector allowed for an apex glass thickness of 0.024 inch and a' positive rate of change of thickness of 0.038 inch per inch as a function of axially measured distance from the apex. Thus, the mold was cut back from the desired ellipsoid by this amount. The specific values used are based on the type and original thickness of the flat glass blank used plus the temperature distribution and processing times and pressures of the sagging operation. Thirty-two vacuum drawing holes 28 were arbitrarily spaced about the mold surface to provide an equal vacuum. The degree of peening on the reflective surface is generally subjective since no specifications commonly exist for local uniformity in terms of the generally objectionable banding, spotting, and other striations that are controlled by peening. The uniformity between various parts of the projection screen as defined by corner-to-center ratio is not the "principal criterion for peening. The necessary amount of controlled diffusion provided by the peening was arrived at by a consensus of experts on the acceptability of projection systems. This is the normal method of determining acceptability. Allowing for the decrease in optical strength of a peen formed on the back surface and transferred to the front reflecting surface by the flow of glass, the required rear surface peens were produced by forming non-random peens on the mold of 0.625 inch sphericaldiameter and of mean circular diameter of 0.060 inch. This provides a non-random, substantially uniform pattern of concave spherical indentations 261 on the concave surface of the sagged glass.

The functioning of the stippling, or peens, is illustrated in FIG. 6. Consider a pencil of light rays 48, incident on a smooth reflective surface 50. The reflected pencil 52 will be at the specular angle with virtually no change in divergence. If the source is nonuniform in luminance, such as coiled incandescent filament, these non-uniformities may image in a crude sense producing the-undesirable uneven illumination field. If a peen 54 is located at the point of reflection, then the reflected pencil will have significant divergence, say

through the solid angle 56. This tends to destroy any partial imaging by the system. Nevertheless, the incident pencil is still directed predominately in the specular angle direction. If any form of general diffusion were used, the reflected light would be spread through a large angle approaching 211'steradians thereby lessening or eliminating the directional control of the reflector. Accordingly, specularly reflecting stippling is disposed over the sagged glass reflecting surface to provide improved directional control for application such as floodlights and projection lamps.

In summary, the present invention relates to specular reflector of glass or similar materials formed in the plastic state by forcing against a mold by gravity and/or gas pressure, the surface in contact with the mold being opposite to'the reflective surface, and the reflective surface being coated with a reflective material such as aluminum. Further a degree of controlled spread of the light is provided by forming gross surface irregularities while maintaining the specular nature of thesurface at all local points. The reflector is generally used for the control of light either in conjunction with, or as an integral part of, a lamp or luminous source. There are specific advantages to the sagging techniques that make it advantageous in many situations. The final reflective surface is untouched by the forming tool; thus, if the original surface is of high quality, it is not degraded by contact with forming tools. Economic advan-.

tages exist for some sizes and shapes over other methods of production. Further, the use of sagged glass with stippling to provide a controlled diffusion reflector is particularly advantageous with respect to heat control and heat resistance. More specifically, glass permits the use of a dichroic coating for reducing heat in the light beam without causing the'localized heating of a metal reflector, and the sagging process permits the use of much thinner glass than is typically required for pressed glass reflectors. for example, the maximum thickness of the sagged glass reflector is normally much less than 0.1 inch,'whereas a pressed glass reflector is typically much thicker than 0.1 inch, for example, 0.18 inch. As thinner glass has a much higher thermal shock resistance, the sagged reflector may be formed of the less expensive, higher expansion glasses, such as ordinary soda lime glass. Due to the lower thermal shock resistance, the thicker pressed glass reflectors typically employ the more expensive, low expansion, borosilicate glasses to lower the stress levels due to heat.

Although the invention has been describedwith respect to specific emobdiments, it will be appreciated that modifications and changes may be made by those skilled in theart without departing from the true spirit and scope of the invention. i

What I claim is: i 1. A reflector for providing controlled diffusion of light comprising, a piece of material sagged to have a concave surface with a stippled effect over a substantial portion thereof, said sagged material thereby having a concave surface on one side thereof with a plurality of indentations therein comprising said stippled effect and protuberance being greater-than the curvature of the indentation corresponding thereto, and a coating of reflective material on said concave surface.

, ances comprise a substantially uniform pattern of convexities on the convex surface of said sagged material which respectively correspond to said indentations.

4. Av reflector accordin to claim 1 wherein said material is glass.

5. A reflector according to claim 4 wherein the maximum thickness of said piece of sagged glass is less than about 0.1 inch.

6. A reflector according to claim 5 wherein said piece of sagged glass has a mean coefficient of thermal expansion about in the rang of 87 to 93 X 10 per "C between 0 and 300C. I

7. A lamp unit for producing a beam of light with controlled diffusion comprising, a concave reflector having a stippled effect thereon, and a light source supported within or in immediate proximity to the volume defined by said reflector and the aperture plane thereof, said reflector comprising a piece of material sagged to have a concave surface with a stippled effect over a substantial portion thereof, said sagged material thereby having a concave surface on one side thereof with a plurality of indentations therein comprising said a convex surface on the opposite side thereof with a stippled effect and a convex surface on the opposite side thereof with a plurality of protuberances thereon respectively corresponding to said indentations, the

curvature of each protuberance being greater than the curvature of the indentation corresponding thereto, and a coating of reflective material on said concave surface. e

8. The lamp unit of claim 7 wherein said sagged material is glass, said reflector is ellipsoidal, and said light source is .an incandescent filament.

9. The lamp unit according to claim 8 wherein said stippled effect is disposed in a ordered pattern over a substantial portion of said sagged glass.

10. A lamp unitaccording to claim 9 wherein said stippled effect comprises a substantially uniform pattern of concave spherical indentations in the concave surfaceof said sagged glass, and said plurality of protuberances comprise a substantially uniform pattern of convexities on the convex surface of said sagged glass which respectively correspond to said indentations.

11. A lamp unit according to claim 8 wherein said piece of sagged glass has a maximum thickness of less than about 0.1 inch and a mean coefficient of thermal expansion about in the range of 87 to 93 X 10 per C between 0 and 300C.

12. A lamp unit according to claim 8 wherein said coating of reflective material is a cold dichroic mirror coating for reflecting visible light from said incandescent filament but passing a substantial portion of the infrared radiation therefrom through said reflector.

Claims (12)

1. A reflector for providing controlled diffusion of light comprising, a piece of material sagged to have a concave surface with a stippled effect over a substantial portion thereof, said sagged material thereby having a concave surface on one side thereof with a plurality of indentations therein comprising said stippled effect and a convex surface on the opposite side thereof with a plurality of protuberances thereon respectively corresponding to said indentations, the curvature of each protuberance being greater than the curvature of the indentation corresponding thereto, and a coating of reflective material on said concave surface.

2. A reflector according to claim 1 wherein said stippled effect is disposed in a ordered pattern over a substantial portion of said sagged material.

3. A reflector according to claim 2 wherein said stippled effect comprises a substantially uniform pattern of concave spherical indentations in the concave surface of said sagged material, and said plurality of protuberances comprise a substantially uniform pattern of convexities on the convex surface of said sagged material which respectively correspond to said indentations.

4. A reflector accordin to claim 1 wherein said material is glass.

5. A reflector according to claim 4 wherein the maximum thickness of said piece of sagged glass is less than about 0.1 inch.

6. A reflector according to claim 5 wherein said piece of sagged glass has a mean coefficient of thermal expansion about in the rang of 87 to 93 X 10 7 per *C between 0* and 300*C.

7. A lamp unit for producing a beam of light with controlled diffusion comprising, a concave reflector having a stippled effect thereon, and a light source supported within or in immediate proximity to the volume defined by said reflector and the aperture plane thereof, said reflector comprising a piece of material sagged to haVe a concave surface with a stippled effect over a substantial portion thereof, said sagged material thereby having a concave surface on one side thereof with a plurality of indentations therein comprising said stippled effect and a convex surface on the opposite side thereof with a plurality of protuberances thereon respectively corresponding to said indentations, the curvature of each protuberance being greater than the curvature of the indentation corresponding thereto, and a coating of reflective material on said concave surface.

8. The lamp unit of claim 7 wherein said sagged material is glass, said reflector is ellipsoidal, and said light source is an incandescent filament.

9. The lamp unit according to claim 8 wherein said stippled effect is disposed in a ordered pattern over a substantial portion of said sagged glass.

10. A lamp unit according to claim 9 wherein said stippled effect comprises a substantially uniform pattern of concave spherical indentations in the concave surface of said sagged glass, and said plurality of protuberances comprise a substantially uniform pattern of convexities on the convex surface of said sagged glass which respectively correspond to said indentations.

11. A lamp unit according to claim 8 wherein said piece of sagged glass has a maximum thickness of less than about 0.1 inch and a mean coefficient of thermal expansion about in the range of 87 to 93 X 10 7 per *C between 0* and 300*C.

12. A lamp unit according to claim 8 wherein said coating of reflective material is a cold dichroic mirror coating for reflecting visible light from said incandescent filament but passing a substantial portion of the infrared radiation therefrom through said reflector.

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