WO1995024620A1

WO1995024620A1 - Color meter

Info

Publication number: WO1995024620A1
Application number: PCT/US1995/002925
Authority: WO
Inventors: Josef Shimony
Original assignee: Cohen, Cindy
Priority date: 1994-03-08
Filing date: 1995-03-08
Publication date: 1995-09-14
Also published as: AU2098995A; IL108897A0

Abstract

A color meter including: a plurality of light sources (30, 32, 34) which generate a plurality of preselected, respective, bandwidths of light; a control unit (26) operative for activating said light sources (30, 32, 34) in accordance with a preselected sequence; an optical medium (18) associated with said light sources (30, 32, 34) which carries light generated by said light sources (30, 32, 34) along a preselected projection path onto a preselected sample region (12), and carries light reflected from said sample region (12) along a reflection path which at least partially overlaps said projection path; and an optical sensor (28) associated with said optical medium (18) which provides an output responsive to the intensity of light reflected from said sample region (12).

Description

COLOR METER

FIELD OF THE INVENTION

The present invention relates to color meters in general and, more particularly, to a compact color meter using tri-color illumination.

BACKGROUND OF THE INVENTION

Color meters, particularly reflectometers, are known in the art. Generally, color meters include apparatus for projecting a light beam of a preselected spectral range, typically a white light beam, onto the surface of a sample to be analyzed. The light reflected from the surface of sample is received by an optical receiver which includes spectral discrimination means, such as a prism or a diffraction element, and photodetectors which provide outputs in response to magnitudes of preselected spectral ranges of the reflected light. It is appreciated that such spectrometers require the use of an array of photodetectors and/or mechanical means for controlling the relative positions of the discrimination means and the photodetectors.

Alternatively, the spectral discrimination means includes a plurality of preselected color filters, typi¬ cally red, green and blue filters, which are used sequen¬ tially to admit light only in preselected, respective, spectral ranges. The color of the sample may be evaluated based on the relative magnitudes of the reflections in the preselected spectral ranges. Color measurement re¬ sults may have various forms, for example, standard scale readings of red green and blue (RGB) .

For many technical applications, such as for evalu¬ ating metals or painted surfaces, there is a need for a quick, reasonably accurate, simple and inexpensive method of evaluating color. Furthermore, for some applications, there is a need for a portable color meter which would be

SUBSUME SHEET RULE 26 suitable for outdoor use.

Unfortunately, existing color meters are either complex and expensive or extremely unreliable and/or inconvenient to use. Furthermore, the accuracy of known color meters is generally strongly dependent upon the reflectivity of the surface and/or the angle of the sampled surface and/or the distance to the sample.

SUMMARY OF THE INVENTION

The present invention seeks to provide a compact, inexpensive, reasonably accurate, preferably portable, color meter. A color meter constructed and operated according to a preferred embodiment of the present inven¬ tion provides quick and efficient evaluation of the color of a sample, even under rough working conditions such as for outdoor use. The present color meter is suitable, inter alia, for highly light-reflective surfaces.

There is thus provided, in accordance with a pre¬ ferred embodiment of the invention, a color meter includ¬ ing: a plurality of light sources which generate a plu¬ rality of preselected, respective, bandwidths of light; a control unit operative for activating the ϊight sources in accordance with a preselected sequence; an optical medium associated with the light sources which carries light generated by the light sources along a preselected projection path and onto a preselected sample region, and carries light reflected from the sample region along a reflection path which at least partially overlaps the projection path; and an optical sensor associated with the optical medium which provides an output responsive to the intensity of light reflected from the sample region.

Preferably, the color meter further includes a processor which determines preselected color characteris¬ tics of the sample based on the optical sensor output. In a preferred embodiment of the present invention, the optical medium includes an optical fiber having a first end associated with the plurality of light sources and a second end associated with the sample region. Preferably, the optical sensor is associated with the first end of the optical fiber.

Alternatively, in a preferred embodiment, the first end of the optical fiber is a split end including at least two branches, one branch associated with the opti¬ cal sensor and at least one other branch associated with the plurality of light sources. According to one varia¬ tion of this embodiment of the invention, the at least one branch associated with the plurality of light sources includes a single branch associated with all the light sources. According to another variation of this embodi¬ ment of the invention, the plurality of light sources includes a plurality of branches, each branch being associated with a respective one of the plurality of light sources.

In a preferred embodiment of the invention, the plurality of light sources are located at mutually proxi¬ mal positions. The optical sensor is preferably located proximal to the light sources. '

Additionally, in a preferred embodiment of the invention, the optical fiber is coated with an inwardly reflective coating.

In a preferred embodiment of the invention, the optical medium further includes a diffuser associated with the second end of the optical fiber.

Further, in accordance with a preferred embodiment, the activation sequence includes activation of each of the light sources separately. Additionally or alterna¬ tively, the activation sequence includes activation of at least two of the light sources simultaneously.

In one, preferred, embodiment of the present inven¬ tion, the plurality of light sources includes at least three light sources and the plurality of bandwidths includes at least three, respective, bandwidths. Prefera¬ bly, the at least three bandwidths includes a red band¬ width, a green bandwidth and a blue bandwidth. According to this embodiment of the invention, the sequence in¬ cludes exclusive activation of a red light source, exclu¬ sive activation of a green light source, exclusive acti¬ vation of a blue light source and simultaneous activa¬ tion of red, green and blue light sources.

In a preferred embodiment of the invention, at least one of the plurality of bandwidths is centered at a wavelength selected from the group of 1,000 nm, 635 nm, 590 nm, 567 nm, 470 nm and 340 nm.

Additionally or alternatively, in a preferred embod¬ iment of the invention, the plurality of bandwidths includes at least one infrared bandwidth. Additionally or alternatively, in a preferred embodiment of the inven¬ tion, the plurality of bandwidths includes at least one ultraviolet bandwidth.

Further, in accordance with a preferred embodiment, the optical sensor is proximal to the light sources.

In a preferred embodiment of the invention, the processor determines the relative intensities of^* the plurality of bandwidths in the light reflected by the sample region.

When red, green and blue light bandwidths are used, as described above, the processor preferably determines the relative intensity of red, green and blue light in the light reflected from the sample region.

A color meter according to the present invention preferably further includes a display which provides a visual indication of the color characteristics determined by the processor.

In a preferred embodiment of the present invention, the plurality of light sources includes a plurality of light emitting diodes (LEDs). Preferably, during operation of the color meter, the optical medium is substantially in contact with the sample region.

In accordance with one preferred embodiment of the invention, the plurality of light sources includes three light emitting-diodes (LEDs) fixedly mounted in a common, transparent, carrier member. According to one preferred variation of this embodiment of the invention, the opti¬ cal sensor is also fixedly mounted in the transparent carrier member.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the following detailed description of preferred embodi¬ ments of the invention, taken in conjunction with the following drawings in which:

Fig. 1 is a schematic illustration of a color meter constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 2 is a schematic illustration, given partly in block diagram form, of the optics and circuitry of one preferred embodiment of the color meter of Fig. 1;

Fig. 3 is a schematic illustration, given partly in block diagram form, of the optics and circuitry of anoth¬ er preferred embodiment of the color meter of Fig. 1;

Fig. 4 is a schematic illustration, given partly in block diagram form, of the optics and circuitry of yet another preferred embodiment of the color meter of Fig.

1;

Fig. 5 is a schematic, close-up, illustration of part of a portion of the color meter of Fig. 1 operative- ly engaging a surface to be measured; and

Fig. 6 is a schematic cross sectional illustration of a tri-color light source and sensor arrangement useful for the operation of the color meter of Fig. 2 in accord¬ ance with a preferred embodiment of the present inven¬ tion. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to Fig. 1, which schematically illustrates a color meter 10 constructed and operative in accordance with a preferred embodiment of the present invention, in operative engagement with a sample region 14 of a sample 12. Color meter 10 includes a housing 16 and an optical medium 17 having one end 36 thereof in operative engagement with sample region 14. A display 20 and a user interface 22, both described in greater detail below, are preferably mounted in housing 16.

Reference is now made also to Figs. 2 - 4, which illustrate in more detail two alternative, preferred, embodiments of color meter 10. In the embodiment of Fig. 2, optical medium 17 includes a single optical fiber 18, having a first end 24 and a second end 36. In the embodi¬ ment of Fig. 3, optical medium 17 includes a split-ended optical fiber 48 having first-end branches 42 and 44. In the embodiment of Fig. 4, optical medium 17 includes a split-ended optical fiber 78 having first-end branches 72, 73, 74 and 75. Fibers 18, 48 or 78 may be formed of any light-transmissive material known in the art, such as quartz or any transparent plastic material.

A plurality of light emitting diodes (LEDs), prefer¬ ably at least three LEDs 30, 32 and 34, are optically associated with first end 24 of optical fiber 18, in the embodiment of Fig. 2, or with branch 42 of split fiber 48, in the embodiment of Fig. 3. In the embodiment of Fig. 4, LEDs 30, 32 and 34 are associated with branches 72, 73 and 75, respectively, of split fiber 78. LEDs 30, 32 and 34 are driven by a driver unit 26 which preferably communicates with a processor 40, as described below.

An optical sensor 28 is optically associated with first end 24 of fiber 18, in the embodiment of Fig. 2, or with branch 44 of fiber 48, in the embodiment of Fig. 3, or with branch 74 of fiber 78, in the embodiment of Fig. 4. In the embodiment of Fig. 2, optical sensor 28 is

SUBSTITUTE SHEET(RULE26} preferably provided with a horn shield 29 which prevents the light generated by LEDs 30, 32 and 34 from projecting directly onto sensor 28, thereby avoiding entry of unde- sired, possibly intense, noise.

In a preferred embodiment of the present invention, LEDs 30, 32, and 34 generate three preselected, respec¬ tive, bandwidths of light. The three bandwidth of light preferably include a red light bandwidth, a green light bandwidth and a blue light bandwidth, respectively, collectively referred to as RGB. More specifically, the light bandwidths generated by LEDs 30, 32 and 34 may be selected in accordance with the CIE 1931 2DEG STANDARD OBSERVER FUNCTION, as known in the art. It should be appreciated, however, that the selection of light band¬ widths may dependent on the application of the present color meter. Alternatively, at least one of LEDs 30, 32 and 34 generates an infrared light bandwidth. Alterna¬ tively, at least one of LEDs 30, 32 and 34 generates an ultraviolet light bandwidth.

For example, bandwidths centered at the wavelengths of 1,000 nm, 635 nm, 590 nm, 567 nm, 470 nm and 340 nm have been found particularly suitable for color measure¬ ments of gold samples. Specifically, it has been found that typical differences in color between different gold samples are enhanced when at least some of the above listed wavelengths are used. Thus, various combinations of these wavelengths may be selected when the color meter is to be used for the purpose of assaying gold. It is appreciated, however, that various other combinations of these and other wavelengths may be used in accordance with various color measurement applications.

It should be understood that the use of three wave¬ lengths emitted by LEDs 30, 32 and 34, as described above, is merely an example. In other preferred embodi¬ ments of the invention, the number of LEDs may vary in accordance with specific requirements, such as the spec-

EET RW E 26 tral ranges to be measured and the desired measurement accuracy.

It should be appreciated that LEDs 30, 32 and 34 may be replaced by other suitable light sources and that the number of light sources used may vary in accordance with specific requirements. It should be further appreciated that the above-described bandwidths may, alternatively, be produced by using appropriate color filters in con¬ junction with wide-band light sources, such as white light sources.

The light generated by LEDs 30, 32 and 34 is carried along optical fiber 18, in Fig. 2, or split fiber 48, in Fig. 3, or split fiber 78, in Fig. 4, and projected through second end 36 onto sample region 14 of sample 12. A diffuser element 38, which is preferably mounted on second end 36, ensures homogeneous projection of light on sample region 14. Diffuser element 38 may include a holographic diffuser, for example the LSD-TC Holographic Diffuser which is available from Physical Optics, a U.S. company of Torrance, California.

Reference is now made to Fig. 5 which schematically illustrates a close-up of second end 36 of fiber 18, 48 or 78 in operative engagement with sample region 14.

During operation of color meter 10, second end 36 of fiber 18, 48 or 78 is placed juxtaposed or, more prefera¬ bly, in contact with sample region 14, such that most of the light reflected from region 14 is recovered through second end 36 of fiber 18, 48 or 78. Diffuser element 38 ensures homogeneous recovery of the light reflected by sample region 14. Additionally, in a preferred embodi¬ ment, fiber 18, 48 or 78 is coated with an inwardly reflecting coating 19 which prevents energy losses at the surface of fiber 18, 48 or 78. It should be appreci¬ ated that such an arrangement ensures efficient recovery of the light reflected from sample region 14. This is a substantial improvement over prior art systems in which the percentage of reflected light picked-up by optical sensors is generally lower.

Part of the light which reenters second end 36 is carried along fiber 18, 48 or 78 to first end 24, branch 44 or branch 74, respectively, where the light is re¬ ceived by optical sensor 28. As known in the art, sensor 28 provides an output responsive to the intensity of light received thereby. The output of sensor 28 is re¬ ceived by processor 40.

In a preferred embodiment of the invention, driver 26, which is preferably controlled by a driver control circuit in processor 40, activates LEDs 30, 32 and 34 in accordance with a preselected, preferably periodic, sequence dictated by processor 40. A preferred sequence includes exclusive operation of each of LEDs 30, 32 and 34, during three respective time intervals, simultaneous activation of all three LEDs during a separate time interval and a no-light time interval in which none of the LEDs are operative. For example, a red light inter¬ val, followed by a green light interval, followed by a blue light interval, followed by a white light (i.e. red, green and blue) interval, followed by a no-light inter¬ val.

In a preferred embodiment of the invention, the inputs from sensor 28 are sorted by time-sorting circuit¬ ry of processor 40 into a number of groups, wherein each group corresponds to one of the time intervals in the sequence described above. For example, a red light group, a green light group, a blue light group, a white light group and a no-light group. Synchronization between the driver control circuit and the time-sorting circuitry is preferably provided by synchronization circuitry in processor 40, ensuring correct sorting of the inputs from sensor 28.

Color characteristics of sample region 14 are deter¬ mined by processor 40 based on the relative magnitudes of - li ¬

the sorted inputs. For example, the relative magnitudes of the red, green and blue inputs indicate the color composition of sample region 14. The white input indi¬ cates the overall reflectivity of sample region 14. The no-light input, which generally indicates noise, is preferably subtracted from each of the other inputs for calibration. Based on the above-described inputs, proc¬ essor 40 preferably calculates the values of standard parameters, such as "L", "a" and "b", corresponding to the color and reflectivity of sample region 14.

The color characteristics of sample region 14, for example the above-mentioned "L", "a" and "b" values, are preferably displayed, in any suitable form, preferably in digital form, on display 20 which may include a liquid crystal display.

It should be appreciated that color meter 10 may be adapted, by appropriate calculation circuitry, to provide more specific color analysis, as required in particular applications. For example, color meter 10 may be used for assaying precious metals, such as gold, or for analyzing painted surfaces, such as car bodies. In such cases, color analysis results displayed on display 20 should be adapted for the particular context, for example Kcirat- ratings of gold samples. Selection between different color analysis options is preferably made by a user using user interface 22. For certain applications, external data may also be entered to processor 40 through user interface 22.

In a preferred embodiment of the invention, LEDs 30, 32 and 34 are located in mutually proximal positions. In the preferred embodiments of Figs. 2 and 3, LEDs 30, 32 and 34 may be cast together in one integrated LED unit, allowing more convenient adaptation of the LEDs for attachment to first end 24 of fiber 18 or to branch 42 of fiber 48. To allow separate operation of LEDs 30, 32 and 34, the LEDs in such units are separately addressable.

SUBSTITUTE SHEET BULE 26 Such integrated units are available, for example, from LEDTRONICS, U.S.A., under the trade name of RGB LEDs.

Reference is now made to Fig. 6 which schematically illustrates a tri-color light source and sensor unit 50, particularly useful for the embodiment of Fig. 2. Unit 50 is an integrated unit including red, green and blue (RGB) light sources 30, 32 and 34 and optical sensor 28 with horn shield 29, all fixedly mounted in a common, transparent, carrier member 52. It should be appreciated that such an arrangement is particularly suitable for attachment to first end 24 of optical fiber 18.

Reference is now made particularly to Fig. 4. In the preferred embodiment of Fig. 4, LEDs 30, 32 and 34 and sensor 28 are preferably all mounted on a common support plate 60. Support plate 60 is preferably formed of a highly heat conductive material, such as metal, which is preferably associated with a temperature control device 62. It should be appreciated that control of the tempera¬ ture of LEDs 30, 32 and 34 and of sensor 28 improves the accuracy of these elements and, thus, improves the color measurement accuracy of color meter 10.

It will be appreciated by persons skilled in the art that the present invention is not limited to what" has been thus far described. Rather, the scope of the present invention is limited only by the following claims:

Claims

C L A I M S

1. A color meter including: a plurality of light sources which generate a plu¬ rality of preselected, respective, bandwidths of light; a control unit operative for activating said light sources in accordance with a preselected sequence; an optical medium associated with said light sources which carries light generated by said light sources along a preselected projection path onto a preselected sample region, and carries light reflected from said sample region along a reflection path which at least partially overlaps said projection path; and an optical sensor associated with said optical medium which provides an output responsive to the inten¬ sity of light reflected from said sample region.

2. A color meter according to claim 1 and further comprising a processor which determines preselected color characteristics of the sample based on the optical sensor output.

3. A color meter according to claim 1 or claim 2 where¬ in said optical medium comprises an optical fiber having a first end associated with the plurality of light sources and a second end associated with the sample region.

4. A color meter according to claim 3 wherein said sensor is associated with the first end of said optical fiber.

5. A color meter according to any of claims 1 - 4 wherein said plurality of light source^'s are located at mutually proximal positions.

6. A color meter according to claim 5 wherein said optical sensor is proximal to said light sources.

7. A color meter according to any of claims 3 - 6 wherein the first end of said optical fiber is a split end comprising at least two branches, one branch associ¬ ated with said optical sensor and at least one other branch associated with said plurality of light sources.

8. A color meter according to claim 7 wherein the at least one branch associated with the plurality of light sources comprises a single branch associated with all the light sources.

9. A color meter according to claim 7 wherein the at least one branch associated with the plurality of light sources comprises a plurality of branches, each branch being associated with a respective one of the plurality of light sources.

10. A color meter according to any of claims 3 - 9 wherein said optical fiber is coated with an inwardly reflective coating. ^*

11. A color meter according to any of claims 3 - 10 wherein said optical medium further comprises a diffuser associated with the second end of said optical fiber.

12. A color meter according to any of the preceding claims wherein said activation sequence includes activa¬ tion of each of said light sources separately.

13. A color meter according to any of the preceding claims wherein said activation sequence includes activa¬ tion of at least two of said light sources simultaneous¬ ly.

14. A color meter according to any of the preceding claims wherein said plurality of light sources comprises at least three light sources and wherein said plurality of bandwidths comprises at least three, respective, bandwidths.

15. A color meter according to claim 14 wherein said at least three bandwidths comprises a red bandwidth, a green bandwidth and a blue bandwidth.

16. A color meter according to claim 15 wherein said sequence includes exclusive activation of a red light source, exclusive activation of a green light source, exclusive activation of a blue light source and simulta¬ neous activation of red, green and blue light sources.

17. A color meter according to any of the preceding claims wherein at least one of the plurality of band¬ widths is centered at a wavelength selected from the group of l,000nm, 635nm, 590nm, 567nm, 470nm and 340nm.

18. A color meter according to any of claims 2 ■* 17 wherein said processor determines the relative intensi¬ ties of the plurality of bandwidths reflected from the sample region by comparing the magnitudes of the corre¬ sponding sensor outputs .

19. A color meter according to claim 15 or claim 16 wherein said processor determines the relative intensi¬ ties of the red, green and blue light reflected from the sample region.

20. A color meter according to any of the preceding claims wherein said plurality of bandwidths comprises at least one infrared bandwidth.

21. A color meter according to any of the preceding claims wherein said plurality of bandwidths comprises at least one ultraviolet bandwidth.

22. A color meter according to any of claims 2 - 21 and further comprising a display which provides a visual indication of the color characteristics determined by said processor.

23. A color meter according to any of the preceding claims wherein said plurality of light sources comprises a plurality of light-emitting diodes (LEDs).

24. A color meter according to any of the preceding claims wherein, during operation, said optical medium is substantially in contact with said sample region.

25. A color meter according to any of the preceding claims wherein said plurality of light sources comprises three light emitting diodes (LEDs) fixedly mounted in a common, transparent, carrier -member. *

26. A color meter according to claim 25 wherein said optical sensor is fixedly mounted in said transparent carrier member.