WO2001009588A1

WO2001009588A1 - Method and apparatus to reduce reflectance errors due to non-uniform color development

Info

Publication number: WO2001009588A1
Application number: PCT/US2000/017811
Authority: WO
Inventors: Richard Riedel
Original assignee: Umm Electronics Inc.
Priority date: 1999-07-30
Filing date: 2000-06-28
Publication date: 2001-02-08
Also published as: AU6057600A

Abstract

A method and apparatus for decreasing measurement error in the colorimetric analysis of two-dimensional optical sample strips. In one embodiment the apparatus (60) includes a pair of photodetectors (24) with a light source (22) centered therebetween. An opaque shield around the light source (22) prevents direct illumination of the optical detectors (24), while a window (32) in the opaque shield allows illumination of the optical sample strip (42). Light is reflected from the strip (32) onto the two detectors (24). A microprocessor is operationally coupled to the photodetectors (24) to interrogate and compare their output signals. The microprocessor is adapted to calculate the optical uniformity of the optical sample (32) and map the variations of reflectance thereof.

Description

METHOD AND APPARATUS TO REDUCE REFLECTANCE ERRORS DUE TO NON-UNIFORM COLOR DEVELOPMENT

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field of optics, and more specifically to the use of geometrically arranged non-imaging photodetectors to reduce reflectance errors in the colorimetric analysis of optical samples having non-uniform color development.

BACKGROUND OF THE INVENTION The colorimetric analysis of optical samples is of great use in clinical chemistry and medical diagnostics. For example, color imagery is commonly used to measure changes in reagent strips or films. Commonly, reagent strips are constructed to develop a uniform color in response to a chemical reaction. The reagent strips ideally should develop a uniform color over the entire reaction area. This uniform color development allows the technician to use optical analysis without necessarily requiring imaging capability and in many cases, a single optical detector can be used to convert the respective light to an electrical signal. However, there are situations where uniform color development cannot be guaranteed, such non-uniformities being due to non-uniformities in the reaction matrix such as spots, uneven chemical coatings, relative orientation of the reaction matrix to the measurement optics, or the like. Because of the possibility of non- uniform color development, a need arises to develop an optical detection system that will minimize the errors associated with non-uniform color development. One approach would be to image the reaction area using digital imaging techniques. However, the use of digital imaging puts minimum limitations on the size and cost of the final instrument. Therefore, a need arises for a low cost method of colorimetric analysis minimizing reflectance errors due to non-uniform color development in the optical samples that is inexpensive to produce and to use and portable. The present invention satisfies this need.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for decreasing measurement error in the colorimetric analysis of two-dimensional optical sample strips. In one embodiment, a pair of photodetectors is positioned on a mounting board with a light source centered therebetween. An opaque shield is provided around the light source to prevent direct illumination of the optical detectors. The opaque shield includes a window allowing direct illumination of the optical sample strip. Light is reflected from the strip onto the two detectors. A microprocessor operationally coupled to the photodetectors interrogates and compares their output signals. The microprocessor calculates the optical uniformity of the optical sample and maps the variations of reflectance thereof.

One form of the present invention contemplates a combination, comprising: an optical sample; a first photodetector; a second photodetector; a light source substantially centered between the first photodetector and the second photodetector and adapted to illuminate the optical sample; and an opaque shield positioned between the light source and the photodetectors wherein the opaque shield is adapted to block the direct illumination of the photodetectors by the light source; wherein the optical sample is positioned to reflect illumination from the light source substantially equal to the first photodetector and the second photodetector. Another form of the present invention contemplates a combination, comprising: a housing; a mounting board positioned within the housing; a pair of photodetectors mounted on the mounting board; a light emitting photodiode substantially centered between the photodetectors; an opaque baffle column having a transparent top window positioned around the light emitting photodiode and adapted to prevent the direct illumination of the photodetectors; a strip sample holder formed in the housing and adapted to hold an optical sample strip positioned over the light emitting photodiode such that light from the photodiode illuminates the strip and is reflected to the photodetectors; a microprocessor mounted in the housing and operationally coupled to the photodetectors, wherein the microprocessor is adapted to measure and compare output signals from the photodetectors; and a display mounted in the housing and operationally coupled to the microprocessor.

Still another form of the present invention contemplates a method, comprising the steps of: providing a first photodetector, a second photodetector, and a light source positioned therebetween; providing an optical sample positioned to receive light from the light source; shielding the photodetectors from direct exposure to the light source; shining light from the light source onto the optical sample; reflecting the light shining on the optical sample substantially equal onto the first photodetector and the second photodetector; and calculating the difference in reflected light shining on the photodetectors.

One object of the present invention is to provide an improved optical detection system. Related objects and advantages of the present invention will be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the preferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the embodiment of FIG. 1. FIG. 3 is a top elevational view of FIG. 1.

FIG. 4 is a sectional view of FIG. 3 taken along the line FF\

FIG. 5 is a sectional view of FIG. 3 taken along the line GG\

FIG. 6 is a sectional view of FIG. 3 taken along the line HH\

FIG. 7a is a top elevational view of a second embodiment of the present invention.

FIG. 7b is a side sectional view of FIG. 7a taken along the line DD\ FIG. 7c is a side sectional view of FIG. 7a taken along the line EE'.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. FIGS. 1 and 2 illustrate one preferred embodiment of a colorimetric analysis apparatus 10, having a housing 12 containing a mounting board 14 and an optical assembly 20. Optical assembly 20 includes a plurality of light sources 22 and a plurality of photo detector pairs 24. Each light source 22 is associated with a respective photodetector pair 24. Each photodetector pair 24 includes a first photodetector 26 and a second photodetector of 28.

Preferentially, the light sources 22 are light emitting diodes (LEDs) and the photodetectors 26, 28 are photodiodes. More preferentially, the photodetectors 26, 28 are photodiodes adapted to have peak sensitivity to the frequencies emitted by the respective light sources 22. Each respective light source 22 is surrounded by an opaque shield 30 positioned to prevent light source 22 from shining directly onto the photodetectors 26, 28. Opaque shield 30 also includes a top window portion 32 positioned. Optical assembly 20 may be protected by a transparent shield 36. In one form, window portion 32 is separate and distinct from transparent shield 36. In an alternate form, window portion 32 and transparent shield 36 are integral.

Housing 12 also contains a sample strip holder 40 formed to present a flat optical sample strip 42 to optical assembly 20. Sample holder 40 is adapted to hold sample strip 42 substantially perpendicularly to optical assembly 20. Optical sample strip 42 may be placed in strip holder 40 such that each light source 22 shines a beam of light through window 32 in opaque shield 30 directly onto optical sample strip 42. Strip holder 40 may be optically transparent, or may be formed of an opaque material having a plurality of transparent windows 43 positioned such that light shining from a light source 24 may reflect from sample 42 onto detector pair 24. The light beam is reflected substantially equally from sample strip 42 onto each respective photodetector 26, 28 of photodetector pair 24.

Colorimetry analysis apparatus 10 also includes a cover portion 44 that protects optical assembly 20 and, if present, sample strip 42. Cover portion 44 also acts as a gray standard for the optical assembly 20. Cover portion 44 contains one or more springs 45 to apply pressure to the sample strip 42 to keep it flat in relation to the optical assembly 20. Colorimetry analysis assembly 10 further includes a microprocessor 46, preferentially in the form of a printed circuit board. Microprocessor 46 is operationally coupled to the output of detector pair 24. The output of microprocessor 46 is operationally coupled to a visual display 48, preferentially a liquid crystal display (LCD). Colorimetry analysis assembly 10 also contains a power source assembly 50 including a battery 52 and connectors 54 operationally coupling battery 52 to optical assembly 20, microprocessor 46, and display 48.

FIG. 3 illustrates the colorimetry analysis assembly 10 with the cover 44 removed to show optical assembly 20. Optical assembly 20 includes a first linear optical array 60, a second linear optical array 62, and a bar code reader array 64.

FIG. 4 illustrates first linear optical array 60 in sectional detail. First linear optical array comprises a plurality of LEDs 22 alternating with a plurality of photodiodes 24. Each LED 22 is substantially centered between a photodiode pair 24. Preferentially, LEDs 22 alternate with photodetectors 24, with a straight line running through each member 22, 26, 28 of first linear optical array. Alternately, LEDs 22 may be linearly oriented, with each LED 22 centered between a photodetector pair 24, wherein a line drawn through the photodetector pair 24 and the LED 22 is substantially perpendicular to a line drawn through the LEDs 22. Light emitted from an LED 22 shines through window 32, optional transparent shield 36, and sample holder 40 onto optical sample 42 and is reflected therefrom onto photodiode pair 24. Since optical sample 42 is flat and oriented substantially perpendicular to first linear optical array 60, if sample strip 42 is optically homogenous the light from LED 22 is reflected substantially equally onto photodiodes 26 and 28 of photodiode pair 24. FIG. 5 illustrates second linear optical array 62 in sectional detail. Second linear optical array includes a plurality of linearly positioned LEDs 22, with each LED positioned between a pair of photodiodes 24. Each LED 22 is substantially centered between a photodiode pair 24. Preferentially, LEDs 22 are linearly oriented, with each LED 22 centered between a photodetector pair 24, wherein a line drawn through the photodetector pair 24 and the LED 22 is substantially perpendicular to a line drawn through the LEDs 22. Alternately, LEDs 22 may alternate with photodetectors 24, with a straight line running through each member 22, 26, 28 of second linear optical array. Light emitted from an LED 24 shines through window 32, optional transparent shield 36, and sample holder 40 onto optical sample 42 and is reflected therefrom onto photodiode pair 24. Since optical sample 42 is flat and oriented substantially perpendicular to second linear optical array 60, if sample strip 42 is optically homogenous the light from LED 22 is reflected substantially equally onto photodiodes 26 and 28 of photodiode pair 24. FIG. 6 illustrates bar code reader array 64 in sectional detail. Bar code reader array includes a plurality of illuminators 70 and a plurality of associated detectors 72. Illuminators 70 are arranged linearly and are positioned to directly illuminate an identifying bar code (not shown) printed on sample strip 42. A plurality of detectors 72 is provided with each detector 72 offset from a respective associated illuminator 70. Opaque structure 74 is provided to prevent illuminators 70 from directly illuminating detectors 72. Illuminators 70 and detectors 72 may also be protected by transparent shield 36. Light from illuminator 70 shines through optional transparent shield 36 and sample holder 40 and is reflected from sample strip 42 onto detector 72.

FIGs. 7a, 7b and 7c illustrate colorimetric analysis apparatus 10 with a sample strip 42 operationally engaged in sample holder 40. Sample strip 42 is held in place by closed spring- loaded cover 44. Preferably, the sample strip 42 is held stationary while it is read by the optical assembly 20. In another form, a motor 80 may be used to move the sample strip 42 over the optical assembly 20. Alternately, the sample strip 42 may be moved over the optical assembly 20 by hand.

In operation, a light beam is shined from light source 22 onto optical sample strip 42 and reflected onto photodiodes 26 and 28, respectively positioned substantially symmetrically about light source 22. Photodiodes 26 and 28 are at all times optically shielded from direct illumination from light source 22. Each detector 26, 28 generates an electrical signal proportional to the intensity of the light incident thereupon. Since each detector 26, 28 is optically shielded from light sources other than light reflected from optical sample 42, the electrical signal generated from each photodetector 26, 28 is proportional to the reflection thereupon of the light from light source 22 reflected thereto by optical sample strip 42. The electrical impulses generated from photodiodes 26, 28 are sent to microprocessor 46. Microprocessor 46 calculates the intensity of the signal from each photodetector 26, 28, and thus the intensity of the reflected light illuminating falling thereon. Microprocessor 46 may then perform qualitative and quantitative comparisons of the light intensity striking each photodiode 26, 28 and, given the known geometry of the detector system 20, may determine the color uniformity of the optical sample 42 by calculating the difference in intensity between photodetectors 26 and 28. If the optical sample were moved along the axis perpendicular to the line between the two detectors 26, 28, the color uniformity of the two dimensional strip 42 surface could be calculated and mapped by microprocessor 46.

The reflectance of an optical strip sample 46 may be measured with a minimum of error arising from non-uniform color development and/or surface optical defects by shining light from optical source 22 onto sample strip 42 positioned perpendicular to source 22. Light is then reflected from sample strip 42 onto photodiodes 26 and 28 positioned substantially symmetrically about light source 22. Photodiodes 26, 28 are optically shielded from illumination from any source other than reflection from sample strip 42.

If strip 42 has an optical defect, such as a spot, surface chemistry gradient, physical iπegularity, or optical shadow arising from irregular positioning relative to the light source 22 and/or detectors 26, 28, the intensity of light reflected from strip 42 may be non-uniform. For each individual detector 26, 28, the measured reflectance depends strongly on the position of any optical defect in the illuminated region of the strip 42. Measurement error of optical strip 42 is minimized for the region of central illumination by taking an average of the signals from both photodiodes 26, 28. The average of the optical signals corresponding to the centrally illuminated region is almost a constant, independent of optical defects. If the defect is on the edge of the illuminated region, measurement eπor is less effectively reduced by taking the average of the two photodiodes 26, 28.

The error compensation arising from microprocessor 46 averaging the signals from the photodetector pair 24 becomes less dramatic as the optical defect increases in distance from the plane passing perpendicularly through the axis between the photodetector pair 24. To better compensate for defects lying away from this plane, another photodetector pair 24 may be positioned along an axis perpendicular to the one between the first photodetector pair 24. If we think of the first photodetector pair 24 as compensating for an optical dipole moment arising from the optical defect, the addition of the second photodetector pair 24 allows for compensation for a quadrupolar moment likewise arising from the presence of the optical defect. Additional detector pairs could be used to compensate for higher- order multipole moments, logically culminating in the imaging of the illuminated area onto an area detector having hundreds of thousands of detector pairs (such as a CCD) and using DSP techniques to eliminate errors due to optical defects thereon.

Occasionally, practicality dictates that optical sample strip 42 contain more than one area of interest. Optical strip 42 may be moved over photodetector pair 24 with the optical analysis being performed while strip 42 is in motion. Due to the motion of strip 42 relative to light source 22, the region of central illumination is not fixed and optical defects may move into and out of the illuminated region. Moreover, the reflectance measurement may be influenced by variations in the reflectance strip 42 arising from inconsistencies in strip 42 and/or its placement relative to light source 22 and photodetectors 26, 28. Again, averaging of the signals from photodetectors 26, 28 minimizes error contributions from optical defects moving through the illuminated region.

The reflectance measurement may be influenced by variation in the angular position of the strip 42 relative to the photodetectors 26, 28. This is because the strip 42 will in general emit light according to the Lambertian cosine law, causing the apparent brightness of the strip 42 to depend upon its angular orientation. Again, averaging the signals from the photodetectors 26, 28 minimizes the eπor contributions arising from angular variation of the sample strip 42.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.

Claims

ClaimsWhat is claimed is:

1. An optical system, comprising: an optical sample; a first photodetector; a second photodetector; a light source substantially centered between the first photodetector and the second photodetector and adapted to illuminate the optical sample; and an opaque shield positioned between the light source and the photodetectors wherein the opaque shield is adapted to block the direct illumination of the photodetectors by the light source; wherein the optical sample is positioned to reflect illumination from the light source substantially equally to the first photodetector and the second photodetector.

2. The system of claim 1 wherein the optical sample is positioned perpendicular to a line drawn through the first photodetector, the light source, and the second photodetector.

3. The system of claim 1 wherein the optical sample is a flat strip.

4. The system of claim 1 wherein the first photodetector and the second photodetector are non-imaging

5. The system of claim 1 further comprising a transparent window covering the light source and the photodetectors.

6. The system of claim 1 further comprising a third photodetector and a fourth photodetector positioned linearly on either side of the light source, wherein a line drawn through the third photodetector, the light source, and the fourth photodetector is perpendicular to a line drawn through the first photodetector, the light source, and the second photodetector.

7. The system of claim 6 wherein the photodetectors are non-imaging.

8. The system of claim 1 further comprising: a microprocessor operationally coupled to the first photodetector and the second photodetector; and a display operationally coupled to the microprocessor; wherein the microprocessor is adapted to measure signals sent therefrom and calculate differences therein; and wherein the microprocessor is adapted to send a signal to the display coπesponding to the calculated differences.

9. The system of claim 1 wherein the first photodetector and the second photodetector are elements of an area detector array comprising a plurality of detector elements.

10. The optical system of claim 1 further comprising: a microprocessor operationally coupled to the first photodetector and the second photodetector; a display operationally coupled to the microprocessor; and a transparent window covering the light source and the photodetectors; wherein the first photodetector and the second photodetector are non-imaging; wherein the optical sample is a flat strip positioned perpendicular to a line drawn through the first photodetector, the light source, and the second photodetector; wherein the microprocessor is adapted to measure signals sent therefrom; wherein the microprocessor is further adapted to calculate differences in the signals; and wherein the microprocessor is adapted to send a signal to the display coπesponding to the calculated differences.

11. An optical meter comprising: a housing; a mounting board positioned within the housing; a pair of photodetectors mounted on the mounting board; a light emitting photodiode substantially centered between the photodetectors; an opaque baffle column having a transparent top window positioned around the light emitting photodiode and adapted to prevent the direct illumination of the photodetectors; a strip sample holder formed in the housing and adapted to hold an optical sample strip positioned over the light emitting photodiode such that light from the photodiode illuminates the strip and is reflected to the photodetectors; a microprocessor mounted in the housing and operationally coupled to the photodetectors, wherein the microprocessor is adapted to measure and compare output signals from the photodetectors; and a display mounted in the housing and operationally coupled to the microprocessor.

12. The optical meter of claim 11 wherein the strip sample holder is further adapted to move the optical sample strip.

13. The optical meter of claim 12 wherein the strip sample holder is further adapted to move the optical sample strip back and forth over the light emitting photodiode.

14. The optical meter of claim 11 wherein the microprocessor is further adapted to determine the uniformity of the sample strip.

15. The optical meter of claim 11 wherein the microprocessor is further adapted to map variations in the reflectance of the sample strip relative to position.

16. A method for reducing eπor in the colorimetric analysis of an optical sample, comprising the steps of: a) providing a first photodetector, a second photodetector, and a light source positioned therebetween; b) providing an optical sample positioned to receive light from the light source; c) shielding the photodetectors from direct exposure to the light source; d) shining light from the light source onto the optical sample; e) reflecting the light shining on the optical sample substantially equally onto the first photodetector and the second photodetector; and f) calculating the difference in reflected light shining on the photodetectors.

17. The method of claim 16 further comprising the step of: after step f), determining the color uniformity of the optical sample.

18. The method of claim 16 further comprising the steps of: after step f), inteπogating the photodetectors; and comparing the outputs of the photodetectors.

19. The method of claim 16 further comprising the steps of: after step e), moving the optical sample along a line peφendicular to the an axis line drawn between the photodetectors in the plane defined by the photodetectors and the light source; and after step f), mapping variations in reflectance relative to position on the optical sample.

20. The method of claim 19 further comprising the step of: coπecting for angular mispositioning of the strip by averaging the signals form the photodetectors.