Compact spectrophotometer

• US 6,002,488 A
• Filed: 09/17/1996
• Issued: 12/14/1999
• Est. Priority Date: 09/14/1994
• Status: Expired due to Term

• Current Assignee: X-Rite Incorporated (Danaher Corporation)
• Original Assignee: X-Rite Incorporated (Danaher Corporation)
• Inventors: Kalinka, Gary T., Cargill, Mark A., Bowden, David R., Baker, Douglas V., Berg, Bernard J.
• Primary Examiner(s): HANTIS, KAREN
• Application Number:
  US08/714,969
• Time in Patent Office: 1,183 Days
• Field of Search: 356/445-448, 356/402-411, 356/300, 356/418, 356/73, 356/246, 356/326, 356/236, 356/419, 250/228, 315/311, 315/185 R, 315/185 S
• US Class Current: 356/418
• CPC Class Codes: G01J 2003/2866 Markers; Calibrating of scan
• CPC Class Codes: G01J 3/0251 Colorimeters making use of an integrating sphere
• CPC Class Codes: G01J 3/0272 Handheld
• CPC Class Codes: G01J 3/50 using electric radiation detectors
• CPC Class Codes: G01J 3/51 using colour filters
• CPC Class Codes: G01J 3/524 Calibration of colorimeters

Abstract

A portable spectrophotometer includes a rotating wheel provided with a plurality of filters having filter characteristics in the 400 to 700 nanometer wavelength range. The filters are moved between an optical conduit and a photoelectric sensor as the wheel is rotated. In one embodiment, a lamp housing contains three object illuminating lamps circumferentially spaced apart by 120° and projecting light onto the object sample at a 45° angle to the object sample. Light reflected from the object sample is conducted through the optical conduit and focused on the optical sensor by means of a focusing lens. A blocking filter is interposed between the lens and the wheel and serves to block light outside of the 400 to 700 nanometer wavelength range. A side sensor receives light from one of the three lamps through the filters as the wheel is rotated and provides output signals which are used as reference signals for the individual filters. In another embodiment, the spectrophotometer includes an integrating sphere with an aperture for conducting specular-included light reflected from the object and an aperture for conducting specular-excluded light from the object. The integrating sphere spectrophotometer also includes a rotatable filter wheel, with a plurality of filters having filter characteristics in the 400 to 700 nanometers wave length range. A side sensor is also provided with the integrating sphere spectrophotometer. As the wheel is rotated, specular-included and specular-excluded light from an object sample to be tested is projected simultaneously through apertures of the sphere. Light detected by a side sensor is also simultaneously detected through one of the filters of the filter wheel. The integrating sphere spectrophotometer also includes an arrangement for providing calibration and compensation of reflectance measurements.

Claims

1. Portable color measuring apparatus comprising:

a wheel having an axis of rotation;

an electric motor operably connected to the wheel to rotate the wheel about the axis of rotation at a constant speed through a plurality of revolutions;

an aperture disposed on one side of the wheel;

a photoelectric primary sensor disposed on an other side of the wheel, the aperture and the sensor defining a light path;

a plurality of light filters, each having predetermined wavelength characteristics, disposed on the wheel and spaced apart along a circumferentially extending line on the wheel, one of the filters being a low wavelength filter and another of the filters being a high wavelength filter, the circumferentially extending line intersecting the light path, whereby light from an object sample aligned with the aperture conducted in the light path is projected through individual ones of the filters onto the sensor as the wheel is rotated about its axis of rotation;

the primary sensor operative to generate electrical output signals representative of the intensity of light projected through each of the plurality of filters as the wheel is rotated at the constant speed; and

signal processing circuitry connected to the primary sensor and responsive to the electrical signals as the wheel is rotated through the plurality of revolutions for determining the relative position of the filters with respect to the primary sensor by detecting the occurrence of a significant difference in intensity between light passed through the low wavelength filter and the high wavelength.

2. The apparatus in accordance with claim 1 wherein the signal processing circuitry further includes means responsive to the electrical signals generated by the primary sensor for generating output signals representative of spectral information of light conducted in the light path.

3. The apparatus in accordance with claim 1 wherein the light path comprises a optical conduit having a plurality of circumferentially extending baffles.

4. The apparatus in accordance with claim 3 wherein the conduit comprises two joined semicircular molded halves, each half having a plurality of molded baffles which form the circumferentially extending baffles when the halves are joined to form the optical conduit.

5. The apparatus in accordance with claim 3 wherein the conduit comprises an optical lens.

6. The apparatus in accordance with claim 1 wherein the plurality of light filters have predetermined filter characteristics within a specified range of wavelengths and the optical path further comprises a blocking filter having filtering characteristics such that light at wavelengths outside the predetermined range is blocked from reaching the plurality of filters disposed on the wheel.

7. The apparatus in accordance with claim 1 and further comprising a light source comprising a plurality of spaced apart lamps each emitting light along a light projection line extending at a predetermined angle to a line extending through the light path and the aperture, whereby an object sample disposed in alignment with the aperture is illuminated and light reflected from the object sample is conducted through the light path and the filters onto the sensor.

8. The apparatus in accordance with claim 7 wherein the light source comprises three lamps circumferentially spaced apart from each other by 120 degrees.

9. The apparatus in accordance with claim 7 wherein the aperture has one side facing in the direction of the wheel and an other side facing away from the wheel and wherein the light projection lines intersect at a point disposed adjacent the other side of the aperture and removed from the aperture by a predetermined distance.

10. The apparatus in accordance with claim 7 wherein the filters are each spaced apart from adjacent filters by a predetermined distance, the apparatus further comprising a photoelectric side sensor disposed on the other side of the wheel and spaced apart from the primary sensor by a distance corresponding to an integral multiple of the predetermined distance by which the filters are spaced apart and wherein at least one of the lamps is positioned relative to the side sensor and the filters such that light from the at least one lamp is projected through the filters and onto the side sensor as the wheel is rotated about its axis of rotation.

11. The apparatus in accordance with claim 10 and further comprising a lamp support housing and wherein the lamps are disposed in the lamp support housing, the lamp support housing comprising a light channel extending from the at least one lamp in the direction of the side sensor.

12. The apparatus in accordance with claim 1 and further comprising a plurality of filter holders mounted on the wheel and each comprising a lower housing and a removable cover and wherein each of the filters is retained in one of the filter holders.

13. The apparatus in accordance with claim 12 wherein the filters are rectangularly shaped nonhygroscopic filters.

14. The apparatus in accordance with claim 2 and further comprising a power source and a voltage reference circuit and a plurality of lamps connected to the power source in a series circuit, the series circuit further comprising a voltage control transistor connected to the voltage reference circuit.

15. The apparatus in accordance with claim 2 wherein the circuitry further comprises an integrator circuit having an input terminal and an output terminal and a capacitor having opposite sides connected to the input terminal and output terminal, respectively, and a switch connected between the opposite sides of the capacitor, the switch operable between an first position allowing the capacitor the capacitor to be charged by the integrator and a second position discharging the capacitor.

16. The apparatus in accordance with claim 15 and further comprising a light source and a photoelectric side sensor disposed in a position relative to the light source to receive illumination from the light source through the plurality of light filters when the wheel is rotated about its axis of rotation, the apparatus further comprising a reference source of a predetermined voltage and a comparator circuit connected to the side sensor and the reference and generating a synchronization output pulse when a signal from the side sensor exceeds the predetermined voltage.

17. The apparatus in accordance with claim 16 and further comprising a control circuit responsive to the synchronization output pulse to control operation of the switches.

18. The apparatus in accordance with claim 1 and further comprising:

projecting a reference light beam along a reference light path through filters of the wheel onto a secondary sensor; and

generating a synchronization signal in response to the secondary sensor in synchronism with a light beam projected onto the primary sensor.

19. Color measuring apparatus comprising:

a rotatable filter wheel having an axis of rotation and a plurality of light filters disposed on the wheel and spaced apart along a circumferentially extending line on the wheel, each of the filters passing light at a predetermined wavelength, one of the filters being a low wavelength filter and the other of the filters being a high wavelength filter;

first and second apertures;

the filter wheel disposed relative to the first and second apertures are aligned with the circumferentially extending line and light conducted through the first and second apertures is projected through the filters of the filter wheel;

an electric motor operably connected to the filter wheel to rotate the filter wheel at a constant speed through a plurality of revolutions;

first and second light sensors for sensing light conducted through the first and second apertures, respectively, as the filter wheel is rotated at the constant speed, the light sensors operative to generate electrical signals representative of the intensity of light received by the sensors through each of the filters as the wheel is rotated at the constant speed, the first aperture and the first sensor defining a first light path, the second aperture and the second sensor defining a second light path; and

signal processing circuitry means connected to the light sensors and responsive to the electrical signals as the wheel is rotated through the plurality of revolutions for determining the relative position of the filters with respect to the sensors by detecting the occurrence of a significant difference in intensity between light passed through the low wavelength filter and the high wavelength filter.

20. The apparatus in accordance with claim 19 and further comprising a circuit board and wherein the light sensors comprise photo-electric sensors disposed on the circuit board and in alignment with the first and second apertures.

21. The apparatus in accordance with claim 20 light along the first and second light paths is simultaneously projected through individual ones of the filters of the filter wheel and onto individual ones of the photo-electric sensors.

22. The apparatus in accordance with claim 21 and further comprising light shield means for reducing the exposure of the filters to extraneous light.

23. The apparatus in accordance with claim 22 and further comprising a mounting plate supporting the electric motor the mounting plate comprising apertures disposed in alignment with the first and second apertures.

24. The apparatus in accordance with claim 23 wherein the light shield comprises a side wall extending upwardly from the mounting plate.

25. The apparatus in accordance with claim 23 and further comprising aperture extensions extending between the first and second apertures and the apertures in the mounting plate, whereby the aperture extensions serve to reduce the entry of extraneous light into the apertures in the mounting plate.

26. The apparatus in accordance with claim 19 and further comprising calibration and compensation means for compensating color measurements of the object to be tested for variables associated with mechanical, electrical and optical components of the apparatus.

27. The apparatus in accordance with claim 26, wherein the calibration and compensation means comprises means for adjusting signal strength measurements for amplifier and motor offsets.

28. The apparatus in accordance with claim 26, wherein the calibration and compensation means comprises means for calculating scale factors associated with specular-included and specular-excluded reflectance values for given wavelength intervals, based on reference sample measurements and measurements in the absence of a projected light source.

29. The apparatus in accordance with claim 26, wherein the calibration and compensation means comprises means for calculating specular-included reflectance values and specular-excluded reflectance values as a function of actual calculated scale values, actual reflectance measurements, amplifier offsets and motor offsets.

30. A method of measuring color of an object sample comprising:

projecting light from a light source onto the object sample;

directing light reflected from the object sample toward a photoelectric sensor generating electrical output signals representative of the intensity of light received by the sensor;

interposing a filter wheel having an axis of rotation and a plurality of light filters between the object sample and the photoelectric sensor, the filters being disposed on the wheel along a circumferentially extending line in alignment with the photoelectric sensor, each filter passing light at a predefined wavelength, one of the filters being a low wavelength filter and the another of the filters being a high wavelength filter;

rotating the filter wheel about the axis of rotation at a constant speed through a plurality of revolutions;

reading the electrical output signals corresponding to the light passed through each of the filters as the filter wheel is rotated at the constant speed through the plurality of revolutions; and

determining the relative position of the filters with respect to the photoelectric sensor by detecting the occurrence of a significant difference in intensity between light passed through the low wavelength filter and the high wavelength filter.

31. The method in accordance with claim 30 and further comprising the step of storing optical data related to light detected by the sensor in a storage device in a time period in which one of the filters is in alignment with the sensor.

32. The method in accordance with claim 30 and further comprising the step of projecting light from the light source through the filters onto another photoelectric sensor and generating a synchronization signal in synchronism with the rotation of the wheel.