WO1994025849A1

WO1994025849A1 - Device for the measurement of the reflectance at a point on a surface

Info

Publication number: WO1994025849A1
Application number: PCT/EP1994/001305
Authority: WO
Inventors: Thomas Morgenstern; Jost SCHÜLER; Lutz Papenkordt
Original assignee: Jenoptik Gmbh
Priority date: 1993-04-30
Filing date: 1994-04-26
Publication date: 1994-11-10
Also published as: DE4314219A1; EP0648327A1; JPH07508590A

Abstract

The invention concerns a device for the measurement of the reflectance of small areas of solid, liquids or gases in several regions of the spectrum (visible, near IR, medium IR), it being possible to determine the surface distribution of the spectral reflectance and/or perceived colour. The invention proposes that special concentrators are disposed behind the individual radiation sources, thus making it possible, without the need for sophisticated design or adjustment measures and with only low radiation losses, to concentrate the radiation on a very small area of the surface under examination.

Description

description

Arrangement for the punctual measurement of the reflectance of surfaces

The invention relates to an arrangement for measuring the reflectance of small areas of solid and liquid bodies in several spectral ranges (in the VTS, near and middle IR), whereby the areal distribution of the spectral reflectance and / or the body color is possible.

The invention can be used in a wide variety of sectors of industry and manufacturing, in order to ensure that the colors of products are the same, to detect color deviations or to match colors. The latter plays e.g. also play a major role in the manufacture of dentures, where it is important to match the color of the denture as closely as possible to the natural tooth color.

A large number of arrangements for determining the spectral reflectance of bodies are known from the prior art, which generally work with a plurality of spectrally selective radiation sources which can be controlled sequentially and a receiver. The optical part of these arrangements differs essentially in the supply of the emitted radiation to the surface to be tested.

In the method described in DE-OS 41 20749 and the device described in DD-PS 99439, the surface to be examined is directly irradiated, so that the size of the "punctiform" areas to be measured is determined by the radiation cone of the

LED and the necessary distance between the object to be measured and the radiation source is determined and therefore cannot be chosen to be sufficiently small. Due to the divergence of the radiation, very high light intensities are required to obtain accurate measurement results with strongly absorbing surfaces. In addition, the overexposure accuracy of the various LEDs on the surface to be measured is inadequate due to the angular scatter in the radiation characteristics of the LEDs.

DE-PS 36 26 373 describes a device in which the radiation from the individual radiation sources is guided via two filter units onto a lens arrangement which focuses the radiation onto the sample to be examined. This beam guidance requires a high adjustment effort and a high mechanical stability. The achievable area resolution is just as in the abovementioned writings the emission characteristics of the LEDs determine and limit. In addition, the radiation intensity is reduced by the filter units and limited by the opening of the imaging system.

From DE-OS 30 38 786 a device for measuring the color of the gums is known, in which the light of a light source via a flexible

Optical fiber bundle is directed to the area to be examined. With a small distance between the light exit surface and the surface to be examined and the additional lenticular grinding of this exit surface, it is possible to concentrate the light on a very small measurement spot. The problem with such an arrangement is the coupling of sufficient radiation energy into it

Light guide bundle.

The object of the invention is to provide an arrangement for the punctiform measurement of the reflectance in different spectral ranges, in which the radiation of several radiation sources with different radiation characteristics is concentrated with as little means and little loss of radiation energy as possible on a small section of the area to be examined.

The object is achieved with an arrangement for the punctiform measurement of the remission of surfaces of solid or liquid bodies in different spectral ranges with at least two radiation sources of different spectral ranges and a receiver which is sensitive at least for the selected different spectral ranges, the angular position being different for incident and reflected light and on the one hand corresponds to the surface normal of the surface and on the other hand to an angle which is substantially different from the surface normal and parallel to the surface, solved in that the radiation sources are followed by a concentrator which has the shape of a

Has truncated cone and consists of a higher refractive light-transparent material, encased by a lower refractive material, so that the radiation is concentrated essentially by total reflection on a small section of the surface.

The concentrator advantageously consists of any transparent material with a lacquer layer produced by dipping or spraying on as a lower-refractive coating. In a further advantageous embodiment, the concentrator consists of glass, in particular quartz glass, the lower-refractive coating of which is produced by tapping the glass. When using divergent LEDs, half the cone angle of the quartz glass concentrator is preferably about one fifth of the numerical aperture of an optical waveguide made of the same material. The concentrator is preferably glued to the window of the LED used, the adhesive layer being the refractive index of window material of the LED and that of the

Concentrator is adjusted. The radiation sources are preferably arranged as separate LEDs around the receiver arranged in the direction of the surface normal at a uniform angular distance from one another and at approximately 45 ° to the surface normal. To compensate for the divergence of the radiation sources in the form of LEDs, the light exit surfaces of the concentrators are expediently designed spherically.

In order to realize a particularly small measuring head, it is generally favorable to arrange the radiation sources at any point away from the surface and to glue optical fibers to the light exit surface of the concentrators, the ends of the optical fibers being around those in the direction of the surface normal

Receivers are arranged at an even angular distance and at approximately 45 ° to the surface normal. In this case, the radiation divergence is expediently reduced by spherical design of the light exit surfaces of the optical fibers. A fundamentally different advantageous design of the arrangement according to the invention is achieved by using a three-color LED, in that only a single concentrator follows the LED (as a combination of three radiation sources) and the three-color LED and concentrator are arranged in the direction of the surface normals and a fiber bundle arranged concentrically thereto of fiber optic cables (LWL), which transmits reflected light at an angle of about 45 ° to the receiver, the

The colors of the three-color LED can be switched on in series.

There are two advantageous variants of the arrangement of the optical fibers for receiving the remission light. On the one hand, the fiber optic cables (with the end surface perpendicular to the fiber optic axis) can easily be arranged at an angle of approx. 45 °. On the other hand, however, it is often advantageous if the measuring head with its light entry and exit surfaces has a uniform, smooth surface and the fiber optic cables are therefore beveled at their ends. In order to direct the light remitted at an angle of 45 ° to the receiver, the angle between the optical fiber and the surface normal of the body surface must be reduced based on the refractive index ratio LWL air (here for quartz glass / air). Due to the refraction of light at the transition from air

Quartz glass results in an angular position of approximately 30 ° to the surface normal for the FO. It is furthermore expedient to rigidly connect the concentrator to the three-color LED by means of an adhesive layer, the adhesive layer in turn adapting the refractive indices of the LED window and the concentrator to one another. The light exit surface of the concentrator is advantageously also spherically shaped. The basic idea lies in the consideration, the divergent radiation and different

Concentrate the radiation characteristics of several radiation sources, in particular LEDs, with simple, totally reflective truncated cones made of transparent material in order to have sufficient radiation intensity on specific areas of a body surface for remission measurements. Known principles of dark field lighting are used, which are also known as

0745 ° - measurement geometry are known, and adapted and modified according to the special features of the point remission measurement according to the invention. With the arrangements according to the invention, it is possible to align the remission measurements on the smallest areas of a body surface and to utilize the radiation from divergent light sources of low power for precise remission measurements with low radiation losses.

The reflectance measurements for color matching can be used particularly advantageously in a wide variety of industrial and commercial sectors.

Above all, the flat and compact design of a special design of the

The invention proves to be advantageous and suitable for use in dentistry (manufacture of dentures) and forensic medicine (e.g. determining the age of hematomas), since the critical surfaces are easy to keep sterile.

The invention will be explained in more detail below using two exemplary embodiments. The drawings show:

Fig. 1 shows the schematic diagram of an arrangement according to the invention in a

(incomplete) side view

2 shows the top view of FIG. 1

Fig. 3 shows the schematic diagram of a further arrangement according to the invention in

Side view

4 shows the top view of FIG. 3 The arrangement according to the invention for measuring the reflectance of surfaces 1 of solid and liquid bodies preferably contains in its basic structure a 0745 ° measurement geometry from radiation sources 3 of different spectral ranges, wherein according to the invention a radiation concentrator 2 is arranged downstream of each radiation source 3 to collect its diverging light. The concentrator 2 is made of transparent material and is in the form of a truncated cone, the outer surface of which reflects the radiation totally and at the same time concentrates.

An arrangement according to the invention - as shown schematically in FIG. 1 - shows the 0745 ° structure typical for reflectance measurements, with the receiver 5 in here

Direction of the surface normal of surface 1 and the radiation sources 3 are arranged in a 45 ° position to the surface normal. Only one of the radiation sources 3, which form modules 4 when expanded by concentrators 2, is shown in FIG. 1. Depending on the number of spectral ranges used, such assemblies 4 are distributed at a uniform angular distance around the surface normal on which the receiver 5 is arranged.

In the example (as the top view from FIG. 2 shows) three different colored LEDs are to be assumed. A red LED 3J (for example TLS 181 P with _p = 635 nm from Toshiba, Japan) is advantageous, a green one

LED 3.2 (e.g. TLGP 181 P with λ _p = 555 nm from the same manufacturer) and a blue LED 3.3 (e.g. E 470 with λ _p = 470 nm from Optosys, chip manufacturer Cree, USA) are used. Each of the red, green and blue LEDs 3.1 to 3.3 forms together with the concentrator 2 one of the assemblies 4, which in this case are arranged at 120 ° around the receiver 5, as shown in FIG. 2 as a top view.

The area 1 to be examined is successively irradiated by the red, green and blue LEDs 3.1, 3.2 and 3.3 at an angle of essentially 45 ° in a pulsed manner. The respective downstream concentrator 2 detects the radiation emitted by the LED and concentrates it on a narrowly limited section of the area to be examined 1. The size of this section does not depend on the different radiation characteristics of the LEDs, but is dependent on parameters of the concentrators 2 ( Cone angle, refractive index ratio between the higher refractive truncated cone and the lower refractive cladding) and the distance between the light exit surface of the concentrator 2 and surface 1. For the

Refractive index ratio applies to the generally known relationship for optical waveguides (LWL) with respect to half the aperture angle sin u = Yn _κ - n _M ² where n _κ and n _{M are} the refractive indices of the core and the cladding. The refractive index ratio of the concentrators 2 is therefore similar to that of conventional optical fibers, with approximately 1.45 and n _{M being} approximately 1.43. In terms of material, the concentrators 2 essentially match the fiber optics from Ensign-Bickford Optics Company (USA). Because of the divergence of the emerging light that also occurs at the light exit of the concentrators 2 and that is somewhat larger than conventional optical fibers, the distance to the surface 1 must be chosen to be small. A conflicting condition is the fact that the distance must always remain so large that the light cones of all radiation sources 3, starting from the light exit of the concentrators 2, illuminate the area 1 detected by the receiver 5 as completely as possible. Since the receiver 5 advantageously receives the remitted light via an optical fiber 6, which is embedded in a metal cannula 7 to avoid the incidence of extraneous light, the distance from the surface 1 can hardly be less than 2 mm. The distance is realized with respect to the metal cannula 7, while the entry surface of the fiber optic cable 6 is slightly shifted back by the

Avoid incidence of radiation not remitted on surface 1. For a concentrator 2, which concentrates an LED with a 3 mm diameter on an exit surface of 1.4 mm, the result is that the half cone angle is approximately 2.5 °, a length of less than 2 cm. The half cone angle should not be more than 10% larger than a fifth of the numerical aperture. Because of the relatively short length of the concentrators 2 and the advantageous 45 ° position with respect to the surface normal to the surface 1, it is advantageous in the interest of a compact, slim design of the arrangement according to the invention not to arrange the light sources 3 directly in the vicinity of the surface 1, but instead to bring their light to the appropriate position via fiber optics (not shown in this configuration). The light exit surfaces of the concentrators 2 are advantageously glued to the end faces of the optical fiber. Those of the HCG-MO365T-10 type from Ensign-Bickford Optics Company (USA) are expediently used as optical fibers.

Instead of the LEDs used in the example, special semiconductor arrangements or incandescent lamp filter arrangements can also be used as radiation sources 3. The concentrators 2 are then to be adapted accordingly with regard to their light entry surface and length.

FIG. 3 shows an arrangement according to the invention which is fundamentally different, which is changed compared to the first example in the position of radiation sources 3 and receiver 5. Here the radiation sources 3 are arranged in the direction of the surface normal of the surface 1. A three-color LED 8 (for example of the type CMS 124 from ELCOS GmbH Pfaffenhofen, Germany), the color segments in turn being driven in a pulsed, serial manner. The three-color LED 8 is in turn advantageously coupled via an adhesive layer 9 to the concentrator 2, which is designed in the same way as in the first example. The coupling is carried out in such a way that the window material of the three-color LED 8 via the adhesive layer

9 is adapted to the material of the concentrator 2 with respect to its refractive indices (index matching). Because of the divergence of light occurring after the light exit surface of the concentrator 2, a small distance from the surface 1 is required, as in the first example. As a result of the vertical position of the three-color LED 8 with the upstream concentrator 2, a distance of 1 to 1.6 mm from the surface 1 can be achieved if the reception of the remission light is appropriately organized. In this example - as shown in FIG. 3 in connection with FIG. 4 (as a plan view) - the remitted light is received by means of a receiver 5 via a fiber bundle composed of a plurality of optical fibers 6 which is concentric, even around the radiation sources arranged along the surface normal. Concentrator unit is distributed. For this purpose, step index fibers of the type HCG-M 0200 T-10 (numerical aperture: 0.22, manufacturer: Ensign-Bickford Optics Company, USA) are advantageously used. The FO 6 are advantageously arranged so that they receive the remitted light at an angle of preferably 45 ° with respect to the surface normal. Two constructive solutions are possible for this.

On the one hand, the ends of the FO 6 are adjusted under the selected angle setting (e.g. exactly 45 °) and cast in advantageously. The end faces of the FO 6 remain perpendicular to the fiber axis and practically protrude from the composite surface. This variant is not explicitly shown in the drawing. Fig. 3 illustrates the other variant in which the refractive index of quartz glass and the often advantageous fact that the surface of a sensor should be as smooth as possible are taken into account. Here the fiber optic cables 6 are adjusted at an angle of 30 ° to the surface normal of surface 1 (e.g. by embedding them in a casting resin). The ends of the LWL 6 have bevels with the same angle of 30 °, so that the sensor has a flat, smooth surface parallel to the surface 1. The angular dimension of 30 ° results from the selection of an optical fiber 6 made of quartz glass and a receiving angle for the reflected light of 45 ° and for this reason does not limit the scope of the teaching according to the invention as a fixed angle. It is important that preferably the light remitted at 45 ° is broken into the fiber optic cable 6 almost parallel to the fiber axis due to the refractive index jump at the slanted light entry surface of the fiber optic cable 6.

An additional advantage is that the distance between the sensor and the surface can be reduced from 1 to almost 1 mm, so that with a light exit surface the Concentrator 2 of 1.4 mm a measured area 1 of about 2 mm in diameter is realized. In addition, the smooth sensor surface fulfills the requirement of medical technology that it can be kept sterile particularly easily.

Claims

1. Arrangement for the punctiform measurement of the reflectance of surfaces (1) of solid or liquid bodies in different spectral ranges with at least two radiation sources (3) of different spectral ranges and a receiver (5) which is sensitive at least for the selected different spectral ranges, the angular position for radiated and remitted light is different and corresponds on the one hand to the surface normal of the surface (1) and on the other hand to an angle which is substantially different from the surface normal and surface parallel, characterized in that the radiation sources (3) are followed by a concentrator (2) which adjusts the Has the shape of a truncated cone and consists of a higher refractive light-transparent material, encased by a lower refractive material, so that the radiation is concentrated essentially by total reflection on a small section of the surface (1).

2. Arrangement according to claim 1, characterized in that the concentrator (2) is any transparent material with a lacquer layer produced by dipping or spraying as a lower-refractive sheathing, the transparent material being as little larger as possible

Has a refractive index to air.

3. Arrangement according to claim 1, characterized in that the concentrator (2) consists of glass, in particular quartz glass, the lower refractive coating being produced by tapping the glass.

4. Arrangement according to claim 3, characterized in that the concentrator (2) when using conventional quartz light guide material and using divergent radiation sources (3) in the form of LED a half cone angle of about one fifth of the numerical aperture of the same

Has material of existing optical fiber.

5. Arrangement according to claim 4, characterized in that the concentrator (2) is glued to the window of the LED, the refractive index of window material of the LED and that of the LED by means of the adhesive layer (9)

Concentrator (2) is adapted.

6. Arrangement according to claim 4, characterized in that the radiation sources (3) as separate LEDs (3J, 3.2, 3.3) are arranged around the receiver arranged in the direction of the surface normal (5) at a uniform angular distance from one another and at approximately 45 ° to the surface normal .

7. Arrangement according to claim 6, characterized in that the light exit surfaces of the concentrators (2) are designed spherically.

8. Arrangement according to claim 4, characterized in that the radiation sources (3) are arranged at any point away from the surface and glued to the light exit surface of the concentrators (2) optical fibers, the ends of the optical fibers being located in the direction of the surface normal Receiver (5) are arranged at a uniform angular distance and at approximately 45 ° to the surface normal.

9. Arrangement according to claim 8, characterized in that the light exit surfaces of the optical fibers are designed spherically.

10. The arrangement according to claim 4, characterized in that the different radiation sources (3) are combined in a three-color LED (8), which is followed by a single concentrator (2), with three-color LED (8) and concentrator (2) are arranged in the direction of the surface normal to the surface (1), and a concentrically arranged fiber bundle of optical waveguides (6) is provided for receiving and supplying the remitted light on the receiver (5), the colors of the three-color LED (8) being switchable in series are.

11. The arrangement according to claim 10, characterized in that the ends of the optical waveguide (6) are arranged at an angle of approximately 45 ° to the surface normal of the surface (1).

12. The arrangement according to claim 10, characterized in that the ends of the optical fibers (6) are arranged at an angle of approximately 30 ° to the surface normal of the surface (1) and the end faces are chamfered by the same angle, the optical fibers (6) consist of quartz glass and their end faces in the same plane as that mediated by the concentrator (2)

The light emission of the three-color LED (8) is parallel to the surface (1).

13. The arrangement according to claim 10, characterized in that the concentrator (2) via an adhesive layer (9) with the three-color LED (8) is connected, wherein the refractive indices from the window of the three-color LED (8 ) and the concentrator (2) are adapted to one another.

14. Arrangement according to claim 10, characterized in that the light exit surface of the concentrator (2) is spherically shaped.