Method and apparatus for color spectrophotometry

• US 3,874,799 A
• Filed: 06/01/1973
• Issued: 04/01/1975
• Est. Priority Date: 06/01/1973
• Status: Expired due to Term

• Current Assignee: David Isaacs, David L. Fried, George A. Ameer
• Original Assignee: David Isaacs, David L. Fried, George A. Ameer
• Inventors: Isaacs, David, Fried, David L., Ameer, George A.
• Primary Examiner(s): Wibert, Ronald L.
• Assistant Examiner(s): Evans, F. L.
• Application Number:
  US05/366,090
• Time in Patent Office: 669 Days
• Field of Search: 250/228,226 356/43,97,173,178,179,195,210,218,236
• US Class Current: 356/402
• CPC Class Codes: G01J 3/0202 Mechanical elements; Supports for optical elements
• CPC Class Codes: G01J 3/021 using plane or convex mirrors, parallel phase plates, or particular reflectors
• CPC Class Codes: G01J 3/0251 Colorimeters making use of an integrating sphere
• CPC Class Codes: G01J 3/2803 using photoelectric array detector
• CPC Class Codes: G01N 21/474 Details of optical heads therefor, e.g. using optical fibres
• CPC Class Codes: G01N 2201/065 Integrating spheres

Abstract

Reflectance measurements of a color sample are made by employing a diffusing sphere having target ports for a color sample, a white standard and a black body. A mirror is mounted within the sphere for pivotal motion about the axis of an exit port to sequentially reflect light from the several targets along a common optical path into a polychrometer which disperses the received light into its spectrum and projects the spectra upon a sensor surface. The sensor may be in the form of an array of light-sensitive elements or the face of an image tube that is scanned or sampled at selected wave lengths of points of the spectrum to be employed in the reflectance measurement. For each sample point, the intensity of light reflected from the black body is subtracted from the intensity of light reflected from the sample and is also subtracted from the intensity of light reflected from the white standard to thereby correct both the sample and white standard measurement for both electrical and other noise of the system, and for stray light within the optical path that impinges upon the sensor surface. The ratios of intensities of the sample to the standard provide reflectance values that may be employed in various color measurements, including calculations of tristimulus values and chromaticity coordinates.

Claims

1. The method of making color measurements comprising the steps of impinging light from a light source upon a standard and a sample, collecting light from said light source that is transmitted along at least one optical collection path from said standard and from said sample, generatiing first and second signals indicative of collected light transmitted from said standard and sample respectively, generating a noise signal indicative of stray light included in said collected light, said stray light including light in said optical collection path derived from said source, and differentially combining said noise signal with each of said first and second signals so as to yeild third and fourth signals indicative of light transmitted from said standard and sample respectively as corrected for the presence of stray light.

2. The method of claim 1 including the step of dispersing light from said source into a spectrum, said steps of generating signals comprising the steps of generating said signals at each of a number of points of said spectrum, and wherein said step of combining comprises combining said noise and said first and second signals at each of said points of said spectrum.

3. The method of claim 2 wherein said step of dispersing light comprises dispersing light transmitted from said standard and from said sample.

4. The method of claim 3 wherein said step of collecting comprises the step of illuminating a light-sensitive area with said dispersed light transmitted from at leAst one of said standard and sample, and wherein said step of generating said first and second signals at said points of said spectrum comprises the step of generating electrical signals indiciative of intensity of illumination of points on said area corresponding to said points of said spectrum.

5. The method of claim 4 said light-sesnitive area comprises at least part of an image tube having a readout beam, and wherein said step of generating electrical signals comprises sweeping said readout beam across said area from one side of said spectrum to the other.

6. The method of claim 4 wherein said light-sensitive area comprises an array of photo-sensitive elements each providing an electrical output indicative of received illumination, and wherein said step of generating electrical signals comprises sensing the electrical outputs of said photo-sensitive elements.

7. The method of claim 4 wherein said step of illuminating said light-sensitive area comprises the steps of sequentially illuminating the same area with dispersed light transmitted from each of (a) said sample, (b) said standard, and (c) a black body reference target that is illuminated by light from said light source, but not necessarily in the stated sequence, said step of generating said noise signal comprising generating electrical signals indicative of intensity of illumination of points on said area when the area is illuminated by dispersed light transmitted from said black body reference target.

8. The method of making measurements for use in color analysis comprising the steps of impinging light from a light source upon a target of which the color is used in said analysis, collecting light from said light source that is transmitted along a first optical collection path from said target, generating a target signal indicative of collected light transmitted from said target, said target including a noise component indicative of stray light in said optical path that is not derived from said target, generating a noise signal indicative of stray light included in said collected light, said stray light including light in said optical collection path derived from said source but which is not derived from said target, and differentially combining said noise signal with said target signal so as to yeild a corrected signal indicative of light transmitted from said target as corrected for the presence of stray light.

9. The method of claim 8 wherein said step of generating a noise signal comprises the steps of impinging light from said light source upon a black body, collecting light from said light source that is transmitted along a second optical collection path from said black body and concomitantly collecting light from said second optical path that is not derived from said black body.

10. The method of claim 9 wherein said first and second optical collection paths are substantially entirely the same path, and including means for alternately transmitting light along said path from said black body and from said target.

11. The method of claim 9 wherein said steps of generating target and noise signals comprise the steps of alternately illuminating a light-sensitive area with light collected from said target and spectrally dispersed, and with light collected from said black body and spectrally dispersed, generating and storing electrical target signals indicative of intensity of illumination of points of said area when illuminated by light from said target, generating and storing electrical noise signals indicative of intensity of illumination of points of siad area when illuminated by light from said black body, said step of combining comprising subtracting said electrical noise signals from said electrical target signals at each of said points of said area.

12. The method of claim 9 including the steps of impinging light from said source upon a white standard, collecting light from said light source that Is transmitted from said white standard along a third optical collection path, generating a standard signal indicative of light collected from said white standard and including a noise component indicative of stray light in said third optical path that is not derived from said white standard, said step of combining comprising subtracting said noise signal from each of said standard signal and said target signal.

13. The method of claim 12 including the steps of generating a signal indicative of the ratio of (a) the combined noise and target signals to (b) the combined noise and standard signals.

14. The method of claim 13 wherein said first, second and third optical collection paths are substantially entirely the same path, and including means for alternately transmitting light along said path from said target, from said stadard, and from said black body, but not necessarily in the recited order.

15. Measuring apparatus comprising a target, a black body, a source of radiation, means for illuminating both said target and black body with radiation from said source, whereby radiation is transmitted from said illuminated target and black body along a path, and means for differentially combining radiation in said path so as to compensate for stray radiation in said path.

16. Measuring apparatus comprising a source of radiant energy, a plurality of tagets including a first target and a black body positioned to be illuminated by energy from said source, sensing means for generating a signal indicative of received radiant energy, means for directing energy from said targets to said sensing means, and means for differentially combining signals generated by said sensing means in response to receipt of energy directed from said black body and from said first target.

17. The measuring apparatus of claim 16 including a second target positioned to be illuminated by energy from said source, said means for directing energy including means for directing energy from said second target to said sensing means, said means for combining including means for differentially combining signals generated by said sensing means in response to receipt of energy directed from said black body and from said second target.

18. The apparatus of claim 17 wherein said means for directing energy comprises means for providing an energy collecting path common to transmission of energy from said targets to said sensing means, said path being nominally free of radiation that does not derive from said source.

19. The apparatus of claim 18 including means interposed in said path between said targets and said sensing means for dispersing energy.

20. The apparatus of claim 18 wherein said path includes a mirror, and means for effecting relative motion between said mirror and targets to sequentially reflect along said path energy received by the mirror from respective ones of said targets.

21. The apparatus of claim 20 wherein said path includes means interposed between said mirror and sensing means for illuminating said sensing means with a spectrum of energy reflected from said mirror along said path.

22. The apparatus of claim 21 wherein said sensing means comprises an energy-sensitive surface for receiving the spectra of energy reflected from said targets, and means for generating electrical signals indicative of intensity of energy received at different points on said surface.

23. The apparatus of claim 22 wherein said means for differentially combining signals comprises means for storing a set of noise signals indicative of intensity of energy received from said black body at a number of points on said surface, means for generating a set of first taget signals indicative of intensity of energy received from said first target at said points on said surface, means for generating a set of second target signals indicative of intensity of energy received from said second target at said points on said surface, and means for subtracting said noise signals from said first target signals and from said second target signals.

24. The apparatus of claim 23 including means for generating a set of reflectance signals indicative of the ratios of (a) the differences between said first target signals and said noise signals at said points, and (b) the differences between said second target signals and said noise signals at said points.

25. Measuring apparatus comprising a diffusing cavity having an exit port and first and second target ports, means for mounting a first target to said cavity at said first target port for illumination by energy reflected within said cavity, means for mounting a second target to said cavity at said second target port for illumination by energy reflected within said cavity, a radiant energy source for illuminating the interior of said cavity with radiant energy, and a mirror mounted within said cavity for reflecting through said exit port energy from said target ports.

26. The apparatus of claim 25 wherein said second target comprises a black body, and including means for differentially combining energy reflected through said exit port from said first and second target ports.

27. The apparatus of claim 25 including means for moving said mirror between first and second positions in which it reflects energy from said first and second target ports respectively.

28. The apparatus of claim 27 wherein said mirror is mounted for pivotal motion about an axis aligned with said exit port and wherein said target ports are positioned to reflect light from one or the other thereof to said mirror and through said exit port when said mirror is pivoted to said first or second positions thereof.

29. The apparatus of claim 28 wherein said energy source provides optical energy, wherein a light reflecting sample is mounted at said first target port as said first target, and wherein a black body is mounted at said second target port as said second target.

30. The apparatus of claim 25 wherein said cavity is formed with a third target port, and including means for mounting a third target to said cavity at said third port for illumination by energy reflected within said cavity, said mirror being mounted to reflect energy from said third port through said exit port.

31. The apparatus of claim 30 wherein targets mounted at said first, second and third target ports comprise, respectively, a sample, a black body and a white standard.

32. The apparatus of claim 30 including an energy-sensitive device positioned to receive energy from said exit port, means for positioning said mirror to reflect energy through said exit port from said first, second and third target ports in a predetermined sequence, and means responsive to said device for subtracting energy received thereby from said second target port from energy received thereby from said first and third target ports.

33. The apparatus of claim 32 including an energy dispersing grating for transmitting to said energy-sensitive device spectra of energy projected from said exit port.

34. The apparatus of claim 33 wherein targets mounted at said first, second and third target ports comprise, respectively, a sample, a black body and a white standard.

35. The apparatus of claim 33 wherein said energy-sensitive device comprises an image tube having a photo-sensitive surface for receiving said spectra and having a scanning beam providing an output electrical signal, said tube including means for moving said scanning beam in a first direction across the length of each of said spectra and in a second direction in a plurality of scans across the width of each of said spectra, and means for storing the output electrical signal of said scanning beam at a number of said scans.

36. The apparatus of claim 35 including means for causing said scanning beam to repetitively move across the length of each of said spectra a number of times before the next spectrum is received by said pHoto-sensitive surface.

37. The apparatus of claim 25 wherein said cavity comprises an inner spherical surface of a hollow body and includes an input port positioned to receive energy from said source, said input port being axially aligned with said exit port, and means for blocking direct transmission of energy from said input port to said exit port.

38. The apparatus of claim 37 including a mirror support mounted to and extending at least partly within said body for pivotal motion about the axis of said port, said mirror being fixed to said support and having a reflective surface lying in a plane that extends at an angle relative to the axis of said exit port.

39. The apparatus of claim 38 wherein said means for blocking direct transmission comprises a diffuser reflecting body on said support between said mirror and said input port, whereby energy transmitted through said input port from said source is reflected from said reflecting body to said spherical surface before impinging upon said targets.

40. The apparatus of claim 39 including a polychrometer having an entrance slit positioned to receive energy reflected from said mirror through said exit port, and means at said exit port for variably focusing exiting energy upon said polychrometer.

41. The apparatus of claim 40 wherein said polychrometer includes an energy dispersing grating, a spectrum exit port, and means for directing energy from said grating through said spectrum exit port, and further including an energy-sensitive surface positioned to receive energy from said spectrum exit port, means for scanning said sensitive surface to generate electrical signals indicative of intensity of received energy t selected points of said surface, a plurality of storage devices, addressing means for storing said signals in different one of said storage devices, drive means for sequentially pivoting said support and mirror to first and second positions for respectively reflecting through said cavity exit port energy from said first target and from said second target, and means for synchronizing said addressing means with said drive means so that signals stored in one of said devices represent energy reflected from said first target and signals stores in another of said devices represent energy reflected from said second target.

42. In a radiant energy measuring system, the method of compensating for noise including stray energy entering the system and adversely affecting the measurement, said method comprising the steps of a. illuminating a target and a black body from an energy source, b. employing said system to measure enery received when the system is directed at said illuminated target, c. employing said system to measure energy received when the system is directed at said illuminated black body, and d. differentially combining measurements of energy received in steps (b) and (c).

43. The method of claim 42 wherein each of the measurements of steps (b) and (c) include the making of a plurality of measurements at different wavelengths within a band of wavelengths.

44. The method of making measurements for color comprising the steps of differentially comparing light received from a color sample, over its spectrum, with light received from each of a white standard and a black body over corresponding spectra.

45. The method of claim 43 wherein said source is a polychromatic lamp, and wherein said measuring system makes color measurements within said band.