WO2006045168A1

WO2006045168A1 - Incoherent light interferometer for the measurement of internal and external cylindrical and nearly cylindrical surfaces

Info

Publication number: WO2006045168A1
Application number: PCT/BR2005/000211
Authority: WO
Inventors: Armando ALBERTAZZI GONÇALVES JR.
Original assignee: Photonita Ltda
Priority date: 2004-10-26
Filing date: 2005-10-03
Publication date: 2006-05-04
Also published as: BRMU8402588U

Abstract

Incoheret light interferometer for the measurement of internal and external cylindrical ad nearly cylindrical surfaces is an optical system to measure shapes and shape errors of cylindrical or nearly cylindrical surfaces (6) by the use of the principle of incoherent light interferometry, using a conical mirror (5) which reflects radially the collimated light in order to reach the surface to be measured on a nearly orthogonal incidence. A conical mirror (5) with external specular surface is used to measure cylindrical or nearly cylindrical internal surfaces (6), while a conical mirror (5) with internal specular surface is used to measure cylindrical or nearly cylindrical external surfaces (6). In both cases a special computer program detects the position of maximum contrast of the interference pattern in each measured pixel at the image distorted by the conical mirror and numerically maps it in order to numerically reconstruct it to represent the measured cylindrical shape.

Description

INCOHDERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF CYLINDRICAL AND NEARLY CYLINDRICAL; INTERNAL AND EXTERNAL SURFACES.

This concerns the physical modification of the incoherent light interferometer adding the capability of measuring cylindrical and nearly cylindrical internal and external surfaces by inclusion of a conic mirror capable of deflecting collimated light incident upon its surface into radial direction, collect the light reflected by the measured surface and direct it towards the camera.

Interferometry with incoherent light is an optical technology which has been used for tens of years for the measurement of the shape of surfaces. Its measuring principle is already of public domain. The surface to be measured is disposed on one of the arms of an interferometer and a reference surface, in the form of a plane mirror of good quality, is positioned on the other arm. Usually, a halide lamp or a LED (light emitting diode) is used as incoherent light source. The small coherence length of the light is used as comparator to determine the differences, point by point, between the reference surface and the measured surface. The interference patterns are visible when the optical path length is smaller than the coherence length of the used light. The condition for maximum visibility of the interference patterns is the vanishing optical path length difference. The surface to be measured or, alternatively, the reference surface, is displaced in a straight movement controlled by a computer, which acquires the images of the interference patterns at the same time, and determines the position of maximum contrast of the interference pattern. The set of positions of maximum contrast, determined individually for each pixel of the image, forms a cloud of points which represents the measured surface.

The typical application of the incoherent light interferometer, used for industrial applications and in research laboratories, is the measurement of surfaces in Cartesian coordinates The measurement of cylindrical surfaces is not performed with this type of devices At most, limited sections of cylindrical surfaces can be measured with existing incoherent light interferometers

The measurement of cylindrical surfaces by means of interferometry can be performed with the technique of grazing incidence interferometry In this case, coherent and collimated laser light is deflected by a diffractive optical element and meets the cylindrical surface to be measured at a grazing angle A second diffractive optical element receives the light reflected by the cylindrical surface and combines it with the not reflected light This result forms interference fringes, which are used to determine the surface shape error, related to a mathematically perfect cylinder

The present invention report describes a modification of the incoherent light interferometer, which makes it capable of measuring cylindrical and nearly cylindrical internal and external surfaces For this purpose, a conical mirror is introduced capable of deflecting the collimated light incident upon its surface into radial direction, and collect the light reflected by the surface to be measured and direct it towards the camera

Figure 1 shows the basic configuration and the measurement principle for external cylindrical surfaces

Figure 2 shows the configuration and the measurement principle for external cylindrical surfaces with larger dimensions. Figure 3 shows the basic configuration and the measurement principle for the measurement of internal cylindrical and nearly cylindrical surfaces.

Figure 1 illustrates the configuration and operation principle of the interferometer.

The incoherent light which originates from source (1) is collimated by the collimation optics(2) and is incident on the beam splitter (3) A fraction of the light is reflected to the upper part of the figure, passes through the attenuation filter (7A), is incident on the plane reference mirror (4), is reflected back, passes once again through the attenuation filter and the beam splitter (3) and is incident on the optics (8A and 8B) which transforms it into an image captured by the camera (10) The other fraction of the light collimated by the optics (2) passes through beam splitter(3) and the neutral density filter (7B) and is incident on the 45° conical mirror(5), where it is directed to radial incidence on the external cylindrical (or nearly cylindrical) surface(6) to be measured, which is located in the center of the conical mirror The light reflected by this surface is reflected again by the conical mirror (5) and is transformed into parallel light again, passes through the neutral density filter (7B) and is incident on the beam splitter (3) where it is deflected to the lower part of the figure and is captured by the optics (8 A and 8B) and forms an image in the camera (10) An aperture(9) with appropriate diameter filters the image and fits it for the detection of interference patterns by the camera

The surface shape is measured with the aid of a computer program The position of the reference mirror (4), assembled on a micropositioner plate(l l), can be controlled by way of a computer program which performs an incremental scan of a certain programmable range The interference pattern captured by tlie camera is processed by a special algorithm that detects and interpolates the position of the micropositioner plate, which results in maximum contrast of the interference patterns for each pixel captured by the camera These positions are mapped through a dedicated algorithm, which calculates the rays corresponding to each image pixel and numerically reconstructs and displays the measured external cylindrical surface The reconstruction process requires a geometrical mapping which transforms numerically the image, distorted by the reflection at the conical mirror, into a three-dimensional, cylindrical surface

The maximum measurable diameter and height of the surface are limited by the dimensions of the conical mirror The dimensions of the conical mirror, on its turn, are limited by the size of the collimation optics and, basically by the size of the beam splitter cube. In order to enlarge the size of the conical mirror, and consequently the external cylindrical surface to be measured, the alternative configuration shown in figure 2 is used. In this configuration,- the light originating from the source (1) is divided by the beam splitter (3) before collimation by the optics (12A) in the reference arm, and by the optics (12B) in the interferometer's arm which contains the object to be measured. This modification permits the use of^" collimation optics, filters and reference and conical mirrors with dimensions larger than the beam splitter's, allowing the measurement of larger external cylindrical and nearly cylindrical surfaces. The remaining elements of the interferometer basically have the same functions as before.

For the measurement of internal cylindrical and nearly cylindrical surfaces- the configuration in figure 3 is used. Only the interferometer's arm with the surface to be measured was modified here: an internal conical mirror(13) of 45° is used to redirect the collimated light radially so that it is incident on the internal cylindrical or nearly cylindrical surface to be measured (14). The light reflected from said surface(14) is again incident on the surface of the conical mirror(13), passes through the neutral filter (7B) and is reflected by the beam splitter (3) towards the optics (8 A and 8B), and is captured by the camera in form of an image distorted from the reflection on the conical mirror. The mentioned computer program also controls the micropositioner plate(l l) which modifies the position of the reference mirror (4) in an incremental scanning movement along a programmable range. The interference pattern captured by the camera is processed by a special algorithm which detects and interpolates the position of the micropositioner plate, which results in maximum contrast of the interference patterns for each pixel captured by the camera. These positions are mapped through a dedicated algorithm which calculates the corresponding rays to each image's pixel and numerically reconstructs and displays the measured internal cylindrical surface. The reconstruction process requires a geometrical mapping which transforms numerically the image, distorted by the reflection on the conical mirror, into a three-dimensional cylindrical surface.

An alternative configuration of this interferometer comprises assembling the conical mirror and the measured object, instead of the reference mirror, on the micropositioner plate. In this case, the position of the set conical mirror - measured object would be subject to the scanning movement which produces the effect resulting in the position of maximum contrast of the interference pattern.

Claims

1 - INCOHERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF INTERNAL AND EXTERNAL_T CYLINDRICAL AND NEARLY CYLINDRICAL SURFACES,, which uses the process of incoherent light interferometry as optical technique for the shape measurement of surfaces, characterized by the use of a conical mirror with specular surfaces which directs radially part of the collimated and parallel light towards the to be measured surface of cylindrical (or nearly cylindrical) shape, which is reflected back by the measured surface(6) and again by the conical mirror (5) that is anew transformed into parallel and interferes with the other part of the collimated light that passes through filter (7A), is incident on and reflected by the plane reference mirror (4) and again passes through filter (7A);

2 - INCOHERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF INTERNAL AND EXTERNAL, CYLINDRICAL AND NEARLY CYLINDRICAL SURFACES according to claim I₃ characterized by the cylindrical surface to be measured be positioned, related to the conical mirror, so that their axes, the mirror's(5) and the cylinder's(ό), be practically coincident.

3 - INCOHERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF EXTERNAL, CYLINDRICAL AND NEARLY CYLINDRICAL SURFACES according to claim 1, characterized by having the conical mirror(5) an inner specular surface, which directs the light radially towards the surface to be measured surface; and the light incident under 45° on the conical mirror (5) be directed to be radially incident on the external side(6) of the cylindrical (or nearly cylindrical) surface.

4 - INCOHERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF INTERNAL, CYLINDRICAL AND NEARLY CYLINDRICAL SURFACES according to claim 1, characterized by having the conical mirror(13) an e>cternal specular surface, which directs the light radially towards the surface to be measured, and the light incident under 45° on the conical mirror (5) is directed to be radially incident on the internal side(6) of the cylindrical (or nearly cylindrical) surface 5 - INCOHERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF INTERNAL AND EXTERNAL, CYLINDRICAL ANT> NEARLY CYLINDRICAL SURFACES according to claim 1, characterized by the positioning of the collimation optics (12A and 12B) be behind the beam splitter (3), 6 - INCOHERENT LIGHT INTERFEROMETER FOR THE MEASUREMENT OF INTERNAL AND EXTERNAL, CYLINDRICAL AND NEARLY CYLINDRICAL SURFACES according to claim 1, characterized by comprising a computer program which controls a micropositioner plate(l l) and positions the reference mirror(4), and which detects the position of maximum contrast of the interference pattern at each measured pixel of the image distorted by the conical mirror and numerically maps it so as to numerically reconstruct, and represent the shape of the measured cylindrical surface