GB2219870A

GB2219870A - Optical reflector for use in imaging a fingerprint

Info

Publication number: GB2219870A
Application number: GB8814486A
Authority: GB
Inventors: Peter Leonard Hurricks; Derek Anthony Melleney
Original assignee: De la Rue Systems Ltd
Current assignee: De la Rue Systems Ltd
Priority date: 1988-06-17
Filing date: 1988-06-17
Publication date: 1989-12-20
Also published as: GB8814486D0; FR2633063A1

Abstract

An optical device (1) for forming an image of a fingerprint comprises a transparent substrate (2), on one side of which is provided a pressure sensitive assembly comprises a transparent resilient layer (3), and a reflective layer (4) comprising a number of spaced discrete portions (15) to prevent cracking and loss of adhesion of the reflective layer (4) to the resilient layer (3), layer (4) reflecting optical radiation (8) which passes through the substrate. The reflection characteristics of the reflective layer (4) change in response to the application of pressure of ridges (7) of the finger (6) on the pressure sensitive layer in the region of applied pressure. Preferably, the reflective layer (4) is covered by a protective layer (5) to prevent damage to the reflective layer (4). Layer (4) is preferably produced by photoetching. <IMAGE>

Description

Optical Device for use in Imaging Apparatus The invention relates to an optical device for use in imaging apparatus for imaging the surface of an object, and in particular for imaging a fingerprint.

Up to the present time, a number of systems have been proposed for imaging the unique configuration of peaks and valleys exhibited by a finger of an individual.

Traditionally this has been done by an ink impression of a finger. However, recently more sophisticated techniques have been developed which rely on optical imaging and provide an identification image which is more detailed and accurate than the ink impression techniques.

With these systems, the finger of the individual is placed against the rear face of a transparent substrate and an image of the finger which is present on the back surface of the block is formed through the front surface of the substrate and is projected onto a receiver or processing system. The receiving equipment may be constructed in the form of a screen, photographic camera or an array of photoelectric cells.

In one imaging system described in French Patent Application No. 2,407,530 a transparent plate has a transparent resilient layer attached to the back surface of the plate on which is deposited a metallic coating.

The optical radiation passes through the plate and the resilient layer and is reflected from the metallic coating back through the resilient layer and the substrate. When a finger is placed on the coating the ridges on the finger deflect the coating and the resilient layer only where the ridges contact the coating and this produces a change in the intensity of the light reflected from the metal coating at these areas.

However, we have found that when the coating and resilient layer are deformed by the pressure of the finger, the coating layer will tend to crack and possibly lose its adhesion to the resilient layer.

In accordance with the present invention, an optical device for use in imaging apparatus comprises a substrate, on one side of which is provided a pressure sensitive assembly comprising a resilient layer, the substrate and the resilient layer being transparent at optical wavelengths, and a reflective layer provided on the resilient layer to reflect optical radiation passing through the substrate whereby, in response to the application of pressure on the pressure sensitive layer sufficient to deform the resilient layer, the reflection characteristics of the reflective layer change in the region of applied pressure, wherein the reflective layer comprises a number of spaced portions.

By having a discontinuous reflective layer, stresses exerted on the layer are greatly reduced and therefore the reflective layer is less likely to lose its adhesion to the resilient layer.

Preferably, the reflective layer is covered by a protective resilient layer to prevent the reflective layer from being damaged.

Typically, the substrate would be in the form of a flat plate however it could also be in the form of a prism with the resilient layer being provided on the hypotenuse or any other face of the prism.

Preferably, the discrete portions should be as small as possible in order to minimise the stresses on each portion.

In order to minimise the amount of information lost, the spaces between the discrete portions should also be kept as small as possible. If the size of the spaces are significantly less than the smallest feature which is to be detected, then the amount of information lost will be negligible.

Conveniently, the reflective layer comprises a reflective coating. In this case, the discrete portions of the reflective layer could be formed by initially depositing the reflective coating as a continuous layer and then splitting this continuous layer into discrete portions or zones by a photo etching process.

Alternatively it is possible that the reflective coating could be deposited onto a deeply etched glass master plate and then stripped off using the resilient layer.

The optical device would be useful not only for fingerprints, but also for any surface which has a low contrast structure.

The term "optical" is intended to include not only optical wavelengths but also infra-red wavelengths and ultra-violet wavelengths.

An example of an optical device for use in imaging apparatus will now be described with reference to the accompanying drawings in which: Figure 1 is a side view of the optical device in use; and Figure 2 is a plan view of the optical device in Figure 1 on a larger scale showing the arrangement of the discrete zones.

Figure 1 shows an optical device 1 comprising a substrate 2 which is optically transparent (such as a glass plate) and a resilient layer 3 attached to the substrate 2 which is also optically transparent. A reflective coating 4 is deposited on the top surface of the resilient layer 3. The reflective coating 4 may be formed by depositing the coating initially as a continuous layer and then dividing the continuous layer by a photo etching process into discrete portions 15 on the resilient layer 3, as shown in Figure 2. In order to obtain the greatest amount of information from the imaging apparatus, the size of each space 16 between the discrete portions 15 should be significantly smaller than the smallest feature to be detected on the object.

Typically the size of the spaces would be 5corm.

In order to minimise the stresses on the discrete portions 15 it is desirable for each discrete portion to be as small as possible. Typically they would be squares with the sides having a length of 50ism.

A protective resilient layer 5 covers the reflective coating 4 and prevents the coating 4 from being damaged.

This protective layer 5 would typically be of the same material as the resilient layer 3. In some applications it may not be necessary to incorporate a protective layer 5 as the likelihood of damaging the reflective coating 4 would be minimal.

In operation, a finger 6 is placed on the device 1, as shown in Figure 1 and the ridges 7 of the skin deflect the protective layer 5, the reflective coating 4 and the resilient layer 3, as shown in Figure 1. When an input beam 8 illuminates the device 1 it is transmitted through the substrate 2 and the resilient layer 3, and is reflected by the reflective coating 4 and passes back through the resilient layer 3 and the substrate 2 and out of the device 1 to sensing apparatus (not shown).

Where the reflective coating 4 is below a valley 11 on the surface of the finger 6 it is not deformed by the finger 6. Thefore, an input beam 8 is reflected at an angle equal to the angle of incidence and it passes out of the device 1 as an output beam 9, as shown in Figure 1. The output beam 9 is then detected by the sensing apparatus.

However, if the reflective coating 4 has been deformed by a ridge 7 of the finger 6, as shown in Figure 1, then the input beam 8 is reflected in different directions (scattered) as output beams 10. Hence the output beams 10 from these areas are not detected by the sensing apparatus or the intensity of the beam received at the sensing apparatus is significantly reduced. Thus, the sensing apparatus detects a pattern of light and dark areas corresponding to the valleys 11 and ridges 7, respectively, of the finger 6.

In order for the optical device 1 to perform efficiently it is desirable for the resilient layer 3 to have a refractive index as close as possible to that of the substrate 2, in order to reduce reflections at the interface between the substrate 2 and the resilient layer 3.

Typically the resilient layer 3 could be polyurethane or plastisized polyvinyl chloride.

In an alternative embodiment of the invention the sensing apparatus could detect the light scattered from the reflective layer 4 where it has been deformed by the ridges 7 instead of the areas where there has been no deformation. In this embodiment the light areas detected by the sensing apparatus would correspond to the ridges 7 and the dark areas would correspond to the valleys 11.

Claims

1. An optical device for use in imaging apparatus comprising a substrate, on one side of which is provided a pressure sensitive assembly comprising a resilient layer, the substrate and the resilient layer being transparent at optical wavelengths, and a reflective layer provided on the resilient layer to reflect optical radiation passing through the substrate whereby, in response to the application of pressure on the pressure sensitive layer sufficient to deform the resilient layer, the reflection characteristics of the reflective layer change in the region of applied pressure, wherein the reflective layer comprises a number of spaced portions.

2. An optical device according to claim 1, further comprising a protective resilient layer covering the reflective layer.

3. An optical device according to claim 1 or claim 2, wherein the substrate is a flat plate.

4, An optical device according to any of the preceding claims, wherein the discrete portions are in the shape of squares.

5. An optical device according to claim 4, wherein the sides of the discrete portions have a length of substantially 50am.

6. An optical device according to any of the preceding claims, wherein the size of the space between each discrete portion is substantially 5Fm.

7. An optical device according to any of the preceding claims, wherein the reflective layer comprises a reflective coating.

8. An optical device according to claim 7, wherein the reflective layer is formed by initially depositing the reflective coating as a continuous layer; and the discrete portions are subsequert'y formed by removing party of the continuous layer by a photo retching process.

9. An optical device according to claim ?, wherein the reflective layer is formed by initially depositing the reflective coating onto an etched glass master plate; placing the resilient layer onto the reflective coating and separating the resilient layer from the glass master plate so that the reflective coating remains adhered to the resilient layer in the areas where the glass master plate has not been etched.

10. An optical device according to any of the preceding claims, wherein the reflective layer reflects radiation in the visible spectrum.

11. An optical device substantially as hereinbefore described with reference to the accompanying drawings.