WO2006011065A1

WO2006011065A1 - Method of microtopographic inspection of surfaces of transparent objects by optical triangulation

Info

Publication number: WO2006011065A1
Application number: PCT/IB2005/052164
Authority: WO
Inventors: Manuel Filipe Pereira Da Cunha Martins Costa
Original assignee: Universidade Do Minho
Priority date: 2004-07-20
Filing date: 2005-06-29
Publication date: 2006-02-02
Also published as: WO2006011065B1; EP1769219A1

Abstract

A method that allows the use of systems based on optical triangulation in the perfilometric and microtopographical measurement of surfaces of transparent materials by the application, on the surface opposing to that one to measure, of a layer of coupling gel with refractive index approaching the one of the material to be measured, being that this layer may have non parallel faces specially when inspecting thin pieces.

Description

METHOD OF MICROTOPOGRAPHIC INSPECTION OF SURFACES OF TRANSPARENT OBJECTS BY OPTICAL TRI-

ANGULATION

Field of the invention

[1] The invention refers to a method that allows the use of optical triangulation based systems of perfilometric and microtopographical inspection of surfaces of transparent materials. For that we propose the application, on the opposing surface to that one to measure, of a layer, eventually of non parallel faces, of coupling gel that, by reducing the difference of refractive indexes in this diopter, reduces the reflection of light of this surface allowing the effective measurement of the face to be inspected.

Background of the invention

[2] The rugometric evaluation and the microtopographical inspection of surfaces of transparent materials are basic necessities in a wide variety of situations in the industry and ID: evaluation of the quality of finishing of the surface of glass blades, lenses, optic prisms, wedges and other optical components; topographical inspection and measurement of curvature radius of lenses, including contact lenses; rugometric and microtopographical evaluation of thin films, blades, lamellas and other types of parts of glass or other transparent materials.

[3] Inspection systems based on different types of interferometry are commercially available at high costs being applicable, successfully, to several of the situations enumerated above in different ranges and or conditions.

[4] However systems of dimensional metrology based on optic triangulation are, nowadays, recognized as the most robust, versatile and efficient, but also economic ones. Being or tending to be of more common use in rugometric or microtopographical evaluation of surfaces. The possibility of its application to the inspection of transparent materials is however, inherently, considerably problematic.

[5] Among the patents related to surface inspection systems by optical triangulation, for example, among others, patents DE4334060, EP00890822 and US5880846, US6040910 and JP1998000363095, only this last one discloses the possibility of ap¬ plication to the inspection of surfaces of transparent samples without however demonstrating this possibility or reporting to the limits of application or to the possible resolutions. In the literature there are some references to the application of systems based on triangulation to the inspection of surfaces of transparent materials (some of which published by the author of this patent) being that the important limitations of this metrological process are pointed out. Patent DE4213601 approaches the particular question of the measurement of thicknesses of transparent materials describing the involved errors. [6] Some patents or patent applications, such as US5002357 and WO9107677A1 report the use of coupling gel to eliminate losses of light in the connection between transparent materials, in particular between optical fibres. The author has no knowledge of any communication or patent where is presented any method of the type of the one reported here for the inspection of surfaces of transparent materials by sys terns based on optical triangulation.

Brief description of the drawings [7] The description that follows makes reference to the attached drawings. In the drawings: [8] Figure 1 represents an illustration of the general geometry of measurement by optical triangulation; and [9] Figure 2 represents a schematic representation of the reflection of light in the surfaces of a transparent material.

Detailed description of the invention

[10] The invention proposal consists of a method that allows the efficient use of di¬ mensional metrology systems based on optic triangulation, in its different approaches, (also being able to be applicable to systems based on focus sensing) in the rugometric evaluation and microtopographical inspection of surfaces of materials and transparent parts (or even translucent).

[11] Whatever the triangulation geometry used, the methods of dimensional measurement by optical triangulation are based on the localization of the light reflected by the surface to be measured (figure 1). A light beam is projected onto the surface to measure under a certain angle, h . From another direction, making a determined angle, q , with the surface, an opto-electronic system locates the position where, in the surface, the reflection of light occurs. The registered position y¹ read by the opto¬ electronic system corresponds to an unevenness Z of the surface:

[12] l'siιi ( ;/ ♦ tf) v - Λ. cos ;/ * ( ~.v)cot( 7/ ♦ β) where M=f/F is the magnification of the optical observation system, being f and f its focal distances, h is the angle of incidence and q the observation angle. [13] In the case of transparent materials (specially the polished ones), the amount of light reflected in the diopters, surfaces of separation between the material and the en¬ vironment where it is placed (generally air), or even between zones with different index of refraction inside the material, is reduced. Being this amount a function of the angle of incidence and the relation of indexes of refraction in the diopter. Furthermore at all diopters there will be reflection of the incident light being, in general, difficult or even impossible to distinguish between the light reflected in different diopters thus precluding the accomplishment of any dimensional measurement on it.

[14] For most of the situations of interest it is only in the external diopters of the two faces of the transparent piece that appreciable reflections occur. Part of the light is reflected in the surface of the face to measure whereas the part of the light that is refracted on it and traverse the material will be partially reflected in the opposing face.

[15] If that amount of reflected light is significant, it will mask, making it indistin¬ guishable of the light reflected in the surface to measure.

[16] The considered method comprises the elimination of this effect by the reduction of the difference of indexes of refraction in the second diopter. Such is achieved by the application, on the opposing surface to that one to measure, of a layer of a transparent gel with a refractive index as close as possible of that of the material in cause. The gel we suggest to employ is of common use in the coupling between optical fibres to reduce losses of light in the connections and is known as matching or coupling gel. This type of gel is sufficiently homogeneous transparent and stable. It shows a good adhesion to glass and polymeric materials, being not invasive and easy to be removed.

[17] As to be able to use an as thin as possible layer of gel or whenever the thickness of the part to measure is very reduced, this layer can be of non parallel faces making that the light reflected in the second gel layer surface (the free one), goes off the zone of measurement of the sensor (Figure 2). As it can be seen in figure 2 the reflection of light in the surfaces of a transparent material (1.), to which was applied a layer of non parallel faces of coupling gel (2), is made outwards the zone of measurement of the sensor.

Claims

[1] 1. Method of microtopographical inspection of the surface of transparent materials by optical triangulation of surfaces of transparent materials, char¬ acterized by the reduction of the difference of the refractive indexes at the diopter of the second face of the part to be measured, by means of application, on the surface opposing to that one to measure, of a layer of coupling gel of refractive index similar to that of the material to measure, being that this layer will have non parallel faces whenever the thickness of the material to be measured is small.

2. Method of microtopographical inspection of the surface of transparent materials, in accordance with claim 1, characterized by the fact that, the coupling gel is homogeneous transparent and stable and shows good adhesion to glass and polymeric materials.