WO2001028224A2 - Systeme d'imagerie colorimetrique - Google Patents

Systeme d'imagerie colorimetrique Download PDF

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Publication number
WO2001028224A2
WO2001028224A2 PCT/US2000/028465 US0028465W WO0128224A2 WO 2001028224 A2 WO2001028224 A2 WO 2001028224A2 US 0028465 W US0028465 W US 0028465W WO 0128224 A2 WO0128224 A2 WO 0128224A2
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Prior art keywords
filter
detector
colorimetric
scene
response
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PCT/US2000/028465
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English (en)
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WO2001028224A3 (fr
WO2001028224A9 (fr
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Peter J. Miller
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Cambridge Research & Instrumentation Inc.
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Publication of WO2001028224A2 publication Critical patent/WO2001028224A2/fr
Publication of WO2001028224A3 publication Critical patent/WO2001028224A3/fr
Publication of WO2001028224A9 publication Critical patent/WO2001028224A9/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/12—Generating the spectrum; Monochromators
    • G01J3/26—Generating the spectrum; Monochromators using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
    • H04N23/80—Camera processing pipelines; Components thereof
    • H04N23/84—Camera processing pipelines; Components thereof for processing colour signals
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/2823—Imaging spectrometer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters
    • G01J3/50—Measurement of colour; Colour measuring devices, e.g. colorimeters using electric radiation detectors
    • G01J3/51—Measurement of colour; Colour measuring devices, e.g. colorimeters using electric radiation detectors using colour filters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/46—Colour picture communication systems
    • H04N1/48—Picture signal generators
    • H04N1/482—Picture signal generators using the same detector device sequentially for different colour components
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/12—Generating the spectrum; Monochromators
    • G01J2003/1282—Spectrum tailoring
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters
    • G01J2003/467—Colour computing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters
    • G01J3/462—Computing operations in or between colour spaces; Colour management systems
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters
    • G01J3/465—Measurement of colour; Colour measuring devices, e.g. colorimeters taking into account the colour perception of the eye; using tristimulus detection
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N2209/00—Details of colour television systems
    • H04N2209/04—Picture signal generators
    • H04N2209/041—Picture signal generators using solid-state devices
    • H04N2209/042—Picture signal generators using solid-state devices having a single pick-up sensor
    • H04N2209/043—Picture signal generators using solid-state devices having a single pick-up sensor using an alternating colour separation filter, e.g. colour wheel or colour LCD

Definitions

  • the invention pertains to electronic color image acquisition, as well as digital image storage, retrieval and transmission.
  • Digital images are normally captured in RGB color space, meaning that each pixel or point in the image is characterized by three values indicating the amount of red, green, and blue light present at that point.
  • the filters used partition the light energy into three bands according to wavelength, so the shortest wavelengths are recorded as blue, the middle range as green, and the longest range as red.
  • the blue range is 400 - 490 nm
  • green is 490 - 580 nm
  • red is 590 - 660 nm.
  • the overall signal levels of red, green, and blue are adjusted to achieve a white balance.
  • color-correction matrix is applied to increase color separation, normally in the form of a non-diagonal 3x3 matrix that is multiplied by the raw [R, G, B] data for each pixel (expressed as a column vector) , to produce the improved [R, G, B] output data for that pixel.
  • the definition of the three primaries is not universal amongst all manufacturers and all types of equipment.
  • the color basis used to acquire images is not colorimetric in nature. This means it is possible to have two objects in a scene which appear to have a different visual hue or brightness from one another, yet an RGB camera would record the objects as having the same RGB reading. Conversely, one can have two objects which present the same visual appearance to the eye, yet which would be recorded as having different RGB color values.
  • the color error can be quantified for a given camera using measures such as the difference in L*a*b units between the color as recorded, and the true color of the object.
  • measures such as the difference in L*a*b units between the color as recorded, and the true color of the object.
  • a recent paper determined the relative spectral response of a Kodak DCS-200 and DCS-420 camera, from which the color error may be determined for objects with various color spectra. In some cases, errors up to 20 L*a*b units are found, where 1 color unit represents the limit of human perceptibility.
  • the use of a transformation matrix presumes that the original color representation weighted the various color components in the same fashion that the human eye does, or in some linear algebraic transform of this fashion. Since the RGB color image is ambiguous in the sense that the camera or scanner used to record it has a color response that is not the same as the color perception response of the eye, the color error in the original color space can be increased when transforming to another color space.
  • Another problem relates to the fact that the color error of different cameras is not the same, and further depends on the hues being captured. If the transformation matrix is optimized for a certain range of colors or wavelengths, other colors will not be transformed well. Similarly, a transformation that works adequately for one camera (with its attendant color error properties) may not work well for another camera, which has a different set of color errors. This has led to a profusion of ad hoc methods to 'calibrate' various cameras and scanners, as are evident in software packages such as Apple's Color Sync, Agfa's FotoTune. There is a well-established field of colorimetry, described in standard texts such as MacAdam, Color Measurement, or Hunter and Harold, Measurement of Visual Appearance.
  • CTRs cathode-ray tubes
  • Some of these devices are placed near or in contact with a CRT display, and its color is read by computer while various color signals are put to it. In this way, the color distortions and other properties of the display are learned and that information is used by color management software to correct for deficiencies in the display.
  • CRI Boston, MA
  • 'VariSPEC a tunable filter termed the 'VariSPEC which enables one to acquire an image at any specified wavelength.
  • This filter By using this filter to take many images that span the visible spectrum, multiplying the pixel intensity values of each image by the numerical value of the X colorimetric weighting function, and summing the reading of all images at each pixel, one can obtain the exact colorimetric value for the X response at each point in the image. The weighting and summing may then be repeated to obtain the Y and Z colorimetric values, at which point one has a high- resolution image of the scene with colorimetric color rendering .
  • Gemex (Mequon, I) has made and marketed a gem-grading system which uses this approach to quantify the color of valuable gems, and to produce colorimetric- quality images.
  • many exposures are required, typically 20 or more, from which the spectral data is extracted.
  • liquid crystal filters in a time-sequential approach to color imaging.
  • the liquid crystal filters produce transmission curves that match the X,Y, and Z response functions divided by the spectral response of the electronic image detector.
  • the overall system captures the X,Y, and Z components of every point in the scene in a digital photographic image.
  • a variation of this further incorporates means to record the color properties of the ambient light.
  • Another embodiment of special use in medical imaging uses a spectrally-variable illuminator to illuminate the scene, which is then photographed at three illuminator settings.
  • the illuminator settings are chosen to be the spectrum of a desired illuminant, times the X, Y, and Z response, divided by the spectral response of the detector.
  • the overall system captures the X, Y, and Z components of the scene in a digital photographic image taken under controlled illumination conditions such as daylight or CIE C, CIE A, or the like.
  • the preferred embodiment further incorporates means for converting the raw images into XYZ space, taking account of overall signal levels and the relative exposure times employed.
  • a further means for transforming from XYZ colorimetric space to an L*a*b space which has the property that equal distances correspond to equal perceptual differences, so digital storage of the pixel appearance is efficient and compact for storage and transmission purposes.
  • This invention enables one to obtain images with greatly reduced color errors, or simply put, to take digital photographs with markedly better color quality. This will be an immediate benefit to digital photography users.
  • Some embodiments incorporate means for transforming the image from XYZ colorimetric space to RGB space in a quantitative fashion that permits transformation back to XYZ or other colorimetric spaces without loss or degradation of color information.
  • a key long-term benefit of the Invention is that, since it records images in a quantitative fashion, it is possible at all subsequent steps to refer to the primary colorimetric data and know what colors were present. So, while a given monitor, printer, or display may have inherent color limitations and errors which prevent the perfect rendering of the image, one may utilize this Invention to preserve a true record of the colors as they ought to be present in the image. Presently, since a camera' s errors are comparable to those of any other component, it is normal practice that the color balance is altered by eye at each step: acquisition, layout, proofing, color separation, and printing. The resulting accumulation of errors, and the absence of any quantitative objective basis for end-to- end color checking, is eliminated by the present invention.
  • Figure 1 shows the spectral response of a prior-art Kodak DCS200 digital color camera.
  • the response of the red, green, and blue channels are indicated by R, G, and B.
  • Figure 2a shows the spectral response of the X, Y, and Z colorimetric functions as specified by the 1931 CIE for a 2° field-of-view.
  • Figure 2b shows the spectral response for several imaging detectors including a KX-085 (Sony, Tokyo), a back-illuminated Hamamatsu CCD (Bridgewater, NJ) , and an amorphous silicon detector from Silicon Vision (Siegen, Germany) .
  • Figure 3 shows the spectral response of the weighting functions X' , X' ' , Y, and Z as described in this patent application.
  • Figure 4 shows the construction of a liquid crystal switchable filter element as described in U.S. Patent 5,892,612 entitled “Tunable Optical Filter with White State”.
  • Figure 5 shows a colorimetric imaging system in accordance with the present invention, comprising a liquid crystal filter, a detector, and control electronics.
  • Figure 6 shows the transmission of a three- element liquid crystal filter in the clear state and in three filtering states designed to match the colorimetric curves for X', Y, and Z.
  • Figure 7 shows the transmission of a three- element liquid crystal filter in three filtering states designed to have spectra that match the X',Y, and Z curves divided by the response curve of a back- illuminated Hamamatsu CCD. The filter curves are shown as solid lines, and the desired response is shown as dashed lines.
  • Figure 8 shows the transmission of a three- element liquid crystal filter in three filtering states designed to have spectra that match the X' , Y, and Z curves divided by the response curve of a Sony KX-085 detector .
  • Figure 9 shows the transmission of a three- element liquid crystal filter in three filtering states designed to have spectra that match the X' , Y, and Z curves divided by the response curve of an amorphous silicon detector produced by Silicon Vision.
  • Figure 10 shows a block diagram of the steps involved in obtaining an XYZ colorimetric image using the present invention.
  • Figure 11 shows a block diagram of a colorimetric imaging system in accordance with the present invention, further incorporating means for sensing the colorimetric value of the ambient light.
  • Figure 12 shows the block diagram of a spectral illuminator which is described in the copending application "Spectral Imaging System", Serial No.
  • Figure 13 shows a colorimetric imaging system in accordance with the present invention, utilizing a spectral illuminator, detector, and control electronics.
  • Figure 14a shows a diagram of the conversion of an XYZ colorimetric image into conventional RGB space while preserving quantitative color information.
  • Figure 15 shows a block diagram of a digital camera system for obtaining images in L*a*b colorimetric space .
  • the embodiments shown make use of a detector with a certain spectral response, and an illumination system or a filtration system which accommodates this spectral response and further imposes either the X, the Y, or the Z colorimetric weighting function. That is, the filter or illumination system has a spectral response given by:
  • Ki are scalar constants
  • the system takes three exposures which comprise the X, Y, and Z content of the scene presented to the camera, with overall signal levels multiplied by arbitrary scale factors. It is necessary to calibrate the scale factors k , and to take into account the relative exposure time used for each image, in order to ascribe a quantitative value of X, Y, and Z to each point in the image. If desired, it is further possible to utilize information about the lens setting and overall exposure times to determine the absolute intensity level, but this is often not a goal and may be omitted in most cases.
  • the spectral response of the X, Y, and Z weighting function is relatively smooth and free of high-frequency spectral features, which simplifies the construction of a filter or illuminator which realizes the desired responses as listed in Equations la-lc.
  • many CCD and CMOS detectors incorporate thin- film structures such as poly-silicon gates and the like, which exhibit interference phenomena and high-frequency spectral features. This can be seen in looking at the Kodak KAF-1400 detector, for example, shown in Figure 2b.
  • the interline configuration has a slow roll-off in the infrared, relative to non-interline devices. This is often beneficial, since the goal in a colorimetric system is to reproduce the visual response functions, in which case the camera must be rendered insensitive to the infrared band. That can be a demanding requirement when working with a detector that has little blue response, but significant infrared response, such as the KAF-1400. Infrared blocking is thus simplified when working with an interline detector like the interline KX-085.
  • KAF-1400E Another example with intermediate qualities is the Kodak KAF-1400E.
  • This detector is similar to the KAF-1400 CCD, except the polysilicon layers are replaced with indium-tin oxide (ITO), which is much more transparent in the blue spectral range, and which has a lower refractive index (RI) that leads to significantly reduced interference effects compared to polysilicon- based devices.
  • ITO indium-tin oxide
  • RI refractive index
  • CMOS complementary metal-oxide-semiconductor
  • CMOS complementary metal-oxide-semiconductor
  • the CAESER has an integral amorphous silicon photodiode array deposited on top of the read-out circuitry, and the photodiode array has a bias voltage which may be modulated to alter the spectral response between exposures, if desired.
  • One preferred embodiment consists of a liquid crystal tunable filter (LCTF) , a digital camera, and software.
  • the LCTF can be constructed utilizing the design described in U.S. Patent 5,892,612, "Tunable Optical Filter with White State".
  • a diagram of a filter stage of this type is shown in Figure 4.
  • the filter has three such stages, each of which has two color states.
  • One state is designed to match the response of a given colorimetric function (such as X', Y, or Z), and the other state is a clear state which is highly transmissive at all visible wavelengths. In practice, the transmission will not be unity in the clear state, especially for the Y state at wavelengths near the blue or red end of the spectrum.
  • Each stage comprises an entrance polarizer, a first network of retarders, a liquid crystal variable retarder, and a second, symmetrically related network of retarders.
  • the entrance polarizer of a given stage forms the analyzer polarizer for the previous stage, and there is one additional polarizer at the end of the assembly that comprises an exit polarizer for the third stage.
  • a passive color filter incorporated, which comprises a piece of BG-39 filter glass.
  • the liquid crystal cells exhibit a half-wave retardance in the relaxed state, and approximately zero retardance in the driven state. All retarders in a given network have the same value of retardance, and produce a nominally white state when the liquid crystal cell interposed between the two networks exhibits a half-wave retardance.
  • One such arrangement is achieved by choosing the angles of the second retarder network as:
  • ⁇ 2 (i) is the orientation angle of the slow axis of the i-th retarder in the second network
  • ⁇ (K+l-i) is the orientation angle of the slow axis of the K+l-i th element in the first network
  • each of the two networks has N elements.
  • the angles ⁇ (i) are chosen to achieve a desired spectral filtration state when the liquid crystal cell exhibits approximately zero retardance. The theory and design of such networks is described in detail in the aforementioned U.S. Patent 5,892,612.
  • the filter design can be tabulated in terms of the angle and retardation value of the retarder films employed, and the thickness and nominal half-wave retardance of the liquid crystal cell, for each stage.
  • All retardation films are NRZ type, made by Nitto Denko and sold by Nitto Denko America. It is also possible to construct the multiple layer elements using photopolymer methods as described by Chiulli et . al.
  • All liquid crystal cells are pi-cells, using construction methods that are known in the art and further recited in Patent 5,892,612. Use of thin substrates is favored, to minimize overall optical path length.
  • All polarizers are NPF-1225DU type, made by Nitto Denko, and have their transmission axis oriented at an angle of 0°.
  • a passive color-glass filter of BG-39 is incorporated as well, and its thickness is tabulated as well.
  • the filter design can be performed in several ways.
  • One approach is to take the desired filter functions F x ( ⁇ ), F y ( ⁇ ), and F z ( ⁇ ) from equations [la] [lc], and further divide them by the spectral transmission of all other elements in the filters, such as the BG-39 color shaping glass, the polarizers and any other components. These are then fitted as a Fourier series expansion in terms of cos (m ⁇ ) for various m. Typically between two and four terms are sufficient to produce a good fit for colorimetric imaging purposes. The Fourier expansion of a given function yields a set of coefficients a ⁇ that may be used as the input to the synthesis procedure recounted in the aforementioned patent "Tunable optical filter with white state".
  • the retarder is chosen to have integral order at peak wavelength of the filter function, which is not generally the same as the peak of the colorimetric matching functions X, Y, and Z, due to the spectral response of the detector and filter components.
  • Figure 5 shows a colorimetric imaging system in accordance with the present invention, consisting of a liquid crystal switchable filter, a detector, and control electronics.
  • Table 1 shows the design for a filter that realizes the filter response curves of Figure 6. These are a good quantitative match to the colorimetric functions X', Y, and Z, and the filter may be used with a spectrally-neutral detector to directly image a scene in colorimetric terms.
  • a spectrally-neutral detector is the ferroelectric array camera developed by Texas Instruments, to which a thin surface of gold-black has been deposited as described in Foukal. This detector is spectrally neutral to within 2 percent across the visible and near-infrared. This system, where the filter directly produces the CIE colorimetric curves and the detector is spectrally neutral, is easily modeled and its performance directly confirmed.
  • a third filter design is tabulated in Table 3 which matches the response of equations la-lc for the Sony KX-085 interline detector. Cameras that incorporates this detector include the Apogee KX-085, the Cooke SensiCam (Tonawanda, NY) , and the Hamamatsu Orca (Bridgewater, NJ) . The response of this third filter is shown in Figure 8, superimposed on the desired curves for this detector. Again, near-ideal colorimetric response is attained.
  • Table 4 shows the design of a filter for use with the Silicon Vision detector, when operated at fixed bias - i.e. with no modulation of detector spectral response via bias voltage. The response of this filter is shown superimposed upon the desired response curves, and again the match is excellent.
  • a passive filter such as BG-39 color glass
  • it makes it easier to synthesize a filter that matches the X' response, due to the particular shapes of the X' response and of the passive filter response.
  • Other filter materials may also be used instead of BG-39 glass to achieve purposes such as improved spectral quality, lower cost, thinner overall filter construction, and the like. Dyed or coated plastic films may be preferred for reasons such as reduced cost and weight. Such decisions will be made according to well-known arts of optical design and engineering .
  • the best image quality is usually obtained when the liquid crystal filter is located in a portion of the optical path where the rays are telecentric (or nearly so) , as that arrangement insures there is no variation of incidence angle through the filter, versus position in the image. This achieves the most consistent color across the image, free of color shading. However, this is not always possible, as many photographic systems use lenses that are not telecentric. In that case, it is best to minimize the field-of-view requirement on the filter, by placing it in that portion of the optical path where there is least angular spread. Another consideration is that of aperture, which must be kept low for reasons of economy.
  • the filter is located away from image planes, as a defect or blemish at or near an image plane will result in an artifact in the image.
  • This effect is well- known in the art of optical system design, as are techniques for evaluating what distance is suitable for a given f/ number and defect size. All the above factors will be considered in the optical design, as is well known in the art .
  • the system is operated as follows.
  • the liquid crystal filter is tuned to a first state, such as the state corresponding to the X' response, and an image is obtained from the digital camera.
  • the liquid crystal filter is tuned to a second state, say the state corresponding to the Y response, and a second image is obtained.
  • the filter is tuned to the final state, corresponding to the Z response, and a third image is obtained from the digital camera.
  • the exposure times are recorded as t x , t y , and t z .
  • These exposures may be taken in rapid sequence to minimize blur and color break-up, which can result from motion of the object being photographed, although for still life compositions there is no such requirement.
  • the system made by Silicon Vision GmbH is suitable for high-speed acquisition, as it has storage means on-chip that record the signal level for X' , Y, and Z exposures, which may be subsequently read out.
  • CAESAR detector has a means to adjust its spectral response for each exposure, this is not essential for the present invention, which could be achieved using a variant of the CAESAR which maintains the same spectral response for all exposures.
  • Other detectors which provide for multiple, rapid exposures would be suitable as well.
  • Each raw image is corrected for dark-current as is known in the art.
  • the dark-corrected images may be further corrected for spatial variation in brightness at different pixels (a so-called 'flat-field' correction) .
  • This correction typically uses a 'white' image of a uniformly bright target. Such an image may be obtained through opal glass (Edmund Scientific, Barrington, NJ) if a uniformly bright target is not available.
  • the flat- field correction may be omitted completely if the pixel- to-pixel variation, also sometimes called fixed-pattern noise, is acceptably small and the liquid crystal filter is situated in a telecentric beam or the range of angles is small enough that its transmission is sufficiently alike for all points in the image.
  • the flat-field correction is usually performed separately for each image (X', Y, and Z), using a 'white' image taken while the filter is in the corresponding filter state, since typically the off-axis intensity does not vary in like fashion for the different filter states.
  • the resulting images are used to derive the colorimetric image.
  • the steps involve scaling by the exposure time, and by the factors k , k , and k .
  • the latter may be determined by a one-time calibration where a white halon target is illuminated by a known llluminant such as CIE A or CIE C, and imaged by the system.
  • the k factors may then be determined by means such as e.g. regression against the measured readings m the corrected component images.
  • the unit-to-unit variation may be small enough that such calibration is only performed on a sample device, or at intervals during production, and used for all cameras m a lot.
  • the X'' function is reconstructed from the X' and Z functions, if such an arrangement is used.
  • the three scaled images are combined to form a colorimetric image.
  • a representative beam is obtained by use of a diffusing element which may be planar or hemispherical, and this is directed through the filter element to a photodetector .
  • the photodetector may be a small region of the same detector used to record the image, or it may be another detector such as a single-pixel photodiode which has a comparable spectral response to that of the image detector. From the readings of the ambient-light color detector one derives the X, Y, and Z (or Z' ' ) properties of the llluminant. Other arrangements which perform this goal would be equally acceptable for this purpose.
  • a second class of preferred embodiment is based on selective illumination using spectrally variable illuminators. These devices are discussed m a co-pending application, "Spectral Imaging System", U.S. Serial No. 60/147,636, the contents of which are hereby incorporated into this application. A diagram is shown of one such illuminator, in Figure 12. It provides for rapid computer control of the illumination spectrum.
  • Figure 13 shows a colorimetric imaging system using such an illuminator, together with a detector and control electronics.
  • the object being photographed is recorded under three sets of known lighting conditions.
  • the spectral balance of the illuminants are proportional to the X, Y, and Z functions (or X' , Y, and Z functions), divided by the spectral responsivity of the detector, so that the net system response (illuminator times detector) is a known colorimetric function in each exposure. It is possible to further weight the illumination so as to incorporate the spectral balance of a standard illuminant such as CIE A, daylight, or some other desired value. In that case, the illuminator response setting for acquiring the X colorimetric image is
  • a standard illuminant such as CIE A, daylight, or some other desired value.
  • the embodiment utilizing a spectral illuminator one obtains a quantitative colorimetric image of an object under known lighting conditions. Because the lighting spectrum is controlled, the image may be thought of as recording the properties of the objects in the scene, whose reflected colors may be deduced from the image (if one neglects inter- reflections between objects). In contrast, the embodiments based on switchable filters record the color and brightness of a scene in quantitative colorimetric terms, precisely as a human would perceive it, were they to stand at the camera's position. But since the lighting is not controlled, it is a record of the visual appearance of the scene, with its unknown lighting, and one can make few conclusions about what color the objects would exhibit under controlled lighting such as daylight, CIE A, and so on.
  • the spectral illuminator may have fine spectral resolution, beyond that necessary to achieve the goal of matching the desired functions F x ( ) , F y ( ) , and F z ( ) . This may enable simplification of construction and reduction of the parts count involved.
  • each column in the P matrix is the chromaticity vector of the corresponding RGB primary.
  • the L*a*b is a nonlinear conversion, defined as :
  • X w , Y w , and Z w are the X,Y, and Z values of a white object when viewed with the scene illumination.
  • the white values can reasonably be imputed from a measurement of the X,Y, and Z values of the ambient illumination, using a sensor that detects this, as described above.
  • the values for X w , Y w , and Z w are known from the properties of the illuminator.
  • FIG. 15 A block-diagram of the image-acquisition process and conversion to L*a*b space is shown in Figure 15.
  • the conversion from XYZ to L*a*b may be achieved using circuitry that is incorporated on the detector chip itself, if CMOS circuitry is employed. Alternatively, it may be performed in software, either by direct calculation or by look-up tables, as will be recognized by those skilled in the art of imaging systems and computation. In either case, it requires knowledge of the color.
  • the invention may be used to produce images of conventional photographic scenes, and indeed this is a primary application for it. However, it may also be used to produce images of printed or displayed images, for the purpose of providing a quantitative measure of the colors as reproduced by printing, or as displayed on a computer screen and the like. In the case where a luminous screen is to be measured, the invention cannot be an embodiment based on controlled illumination, but must be an embodiment based on filtration.
  • a measurement of the XYZ values (or other colorimetric index) of a printed or displayed image will be limited by the gamut and brightness range of the output medium. That is, there are certain pure colors which lie outside the printable (or displayable) range; these are sometimes spoken of as being outside the gamut of the device. But colors within the gamut may be displayed and their values corrected to within the measurement accuracy of the invention, by comparing the actual displayed color and brightness against the value produced when the image was acquired.
  • the image may be presented in RGB space (or some other space)
  • the XYZ values corresponding to the RGB values can be calculated without error by means of the matrix methods above and others which are known in the science of colorimetry.

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • General Physics & Mathematics (AREA)
  • Color Television Image Signal Generators (AREA)

Abstract

La présente invention concerne un système d'imagerie et un procédé qui permettent d'obtenir la valeur colorimétrique de plusieurs points d'une scène lorsque celle-ci est éclairée par une source d'éclairement. Le système comprend un détecteur avec formation d'images; un dispositif optique de formation d'images qui reçoit la lumière de la scène et la dirige vers le détecteur; un filtre à accord variable sensible à des signaux électriques qui filtre le spectre optique qui le traverse; et un circuit de commande pour acquérir et stocker plusieurs images provenant du détecteur tandis que le filtre à accord variable exprime plusieurs fonctions de réaction du filtre préétablies en réponse à des signaux électriques appliqués, et pour déterminer les valeurs colorimétriques à partir des images stockées, chaque fonction de réaction du filtre permettant une transmission considérable à plusieurs longueurs d'onde.
PCT/US2000/028465 1999-10-13 2000-10-13 Systeme d'imagerie colorimetrique WO2001028224A2 (fr)

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US60/159,277 1999-10-13

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WO2001028224A9 WO2001028224A9 (fr) 2002-08-15

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WO2002057752A1 (fr) * 2001-01-21 2002-07-25 Color Aix Perts Gmbh Procede et dispositif de controle de la qualite de coloration et/ou de brillant de tissus et de materiaux analogues
US7336323B2 (en) 2005-09-27 2008-02-26 Chemimage Corporation Liquid crystal filter with tunable rejection band
US7417796B2 (en) 2006-09-29 2008-08-26 Chemimage Corporation Wavelength discrimination filter for infrared wavelengths
EP2222077A2 (fr) 2009-02-23 2010-08-25 Sony Corporation Dispositif d'imagerie à l'état solide et appareil électronique
WO2014207742A3 (fr) * 2013-06-24 2015-02-26 Technology Innovation Momentum Fund (Israel) Limited Partnership Système et procédé d'acquisition d'images en couleurs
US10827152B2 (en) 2015-07-15 2020-11-03 Technology Innovation Momentum Fund (Israel) Limited Partnership Tunable MEMS etalon

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US5151751A (en) * 1989-07-19 1992-09-29 Nippon Paint Co., Ltd. Method for evaluating color tone of coating film and device for this evaluation
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US4467438A (en) * 1982-01-18 1984-08-21 Dset Laboratories, Inc. Method and apparatus for determining spectral response and spectral response mismatch between photovoltaic devices
US5151751A (en) * 1989-07-19 1992-09-29 Nippon Paint Co., Ltd. Method for evaluating color tone of coating film and device for this evaluation
US5432609A (en) * 1993-01-06 1995-07-11 Minolta Co., Ltd. Two-dimensional colorimeter

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2002057752A1 (fr) * 2001-01-21 2002-07-25 Color Aix Perts Gmbh Procede et dispositif de controle de la qualite de coloration et/ou de brillant de tissus et de materiaux analogues
US7336323B2 (en) 2005-09-27 2008-02-26 Chemimage Corporation Liquid crystal filter with tunable rejection band
US7417796B2 (en) 2006-09-29 2008-08-26 Chemimage Corporation Wavelength discrimination filter for infrared wavelengths
US8493452B2 (en) 2009-02-23 2013-07-23 Sony Corporation Solid-state imaging device and electronic apparatus having a light blocking part
EP2222077A3 (fr) * 2009-02-23 2012-05-02 Sony Corporation Dispositif d'imagerie à l'état solide et appareil électronique
US8243146B2 (en) 2009-02-23 2012-08-14 Sony Corporation Solid-state imaging device and electronic apparatus
EP2222077A2 (fr) 2009-02-23 2010-08-25 Sony Corporation Dispositif d'imagerie à l'état solide et appareil électronique
USRE46729E1 (en) 2009-02-23 2018-02-20 Sony Corporation Solid-state imaging device and electronic apparatus having a light blocking part
USRE46769E1 (en) 2009-02-23 2018-03-27 Sony Corporation Solid-state imaging device and electronic apparatus having a light blocking part
WO2014207742A3 (fr) * 2013-06-24 2015-02-26 Technology Innovation Momentum Fund (Israel) Limited Partnership Système et procédé d'acquisition d'images en couleurs
US9652827B2 (en) 2013-06-24 2017-05-16 Technology Innovation Momentum Fund (Israel) Limited Partnership System and method for color image acquisition
US10229476B2 (en) 2013-06-24 2019-03-12 Technology Innovation Momentum Fund (Israel) Limited Partnership System and method for color image acquisition
US10827152B2 (en) 2015-07-15 2020-11-03 Technology Innovation Momentum Fund (Israel) Limited Partnership Tunable MEMS etalon

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WO2001028224A9 (fr) 2002-08-15

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