基于TCS230颜色传感器的色彩识别器的设计 外文翻译.docx

1、基于TCS230颜色传感器的色彩识别器的设计 外文翻译Sensing color with the TAOS TCS230The TAOS TCS230 is a small, highly integrated color sensing device packaged in a clear plastic 8-pin SOIC. It reports, as analog frequency, the amount of shortwave (blue), mediumwave (green), longwave (red), and wideband (white) optical po

2、wer incident onto the device. It can be used in a variety of color sensing applications. Details of the device can be found in its datasheet. This white paper details the concepts and calculations involved in color sensing using the TCS230. We will use the ColorChecker chart as an optical stimulus t

3、o work through a numerical example of color sensing. The chart, depicted in Figure 1, is manufactured and distributed by GretagMacbeth. The chart measures approximately 13 inches by 9 inches (330 mm by 230 mm); it contains 24 colored patches arranged in a 6 by 4 array. Figures 2 through 5 overleaf s

4、how the spectral reflectance of the patches in each of the four rows of the chart that is, the fraction of incident light that is reflected (with respect to an ideal diffuse reflector), as a function of wavelength from 350 nm to 750 nm.Figure 1 The ColorChecker contains 18 colored patches and a 6-st

5、ep gray series.Figure 2 ColorChecker spectra, top row.Figure 3 ColorChecker spectra, second row.Figure 4 ColorChecker spectra, third row.Figure 5 ColorChecker spectra, bottom row (neutral series)Figure 6 Cone sensitivities of cone photoreceptors are shown. Theshortwave-sensitivephotoreceptors are mu

6、ch less sensitive than the other two types. The responses of the mediumwave and longwave photoreceptors have a great deal of overlap. Vision is not sensitive to the precise wavelength of the stimulus: Whatatters is optical power integrated under each response curve.Introduction to color vision Photo

7、receptor cells called cones in the retina are responsible for human color vision. There are three types of cone cells, sensitive to longwave, mediumwave, and shortwave radiation within the electro-magnetic spectrum between about 400 nm and 700 nm. Because the cone sensitivities are very roughly in t

8、he parts of the spectrum that appear red, green, and blue, color scientists denote the cell types as , and , the Greek letters for r, g, and b. (To denote the sensors R, G, and B would wrongly suggest a closer correspondence.) Estimates of the spectral response of the cone types are graphed in Figur

9、e 6 above.Light in the physical world can be characterized by spectral power distributions (SPDs). Colored objects can be characterized by spectral reflectance curves, such as those of the ColorChecker. However, vision is insensitive to the exact wavelength of a stimulus: According to the modern the

10、ory of color science, all that matters is the integral of optical power underneath each response curve. That there are exactly three types of cone cells leads to the property of trichromaticity: Three components are necessary and sufficient to characterize color. Some people might use the phrase “co

11、lor as sensed by the eye,” but I con-sider that qualifier to be redundant at best, and misleading at worst: Color is defined by vision, so there is no need to use the qualifying phrase “as sensed by the eye,” or to use the adjective visible when referring to color.Overview of CIE ColorimetryThe spec

12、tral responses of the cone cells that I graphed in Figure 6 were unavailable to researchers in the 1920s. Researchers at the time used psychophysical experiments, such as the famous color matching experiment, to tease out the data. The CIE is the international body responsible for color standards.In

13、1931, that organization adopted the color matching functions denoted x (), y (), and z (), graphed in Figure 7.Figure 7 CIE 1931, 2 color-matching functions. A camera with 3 sensors must have these spectral response curves, or linear combinations of them, in order to capture all colors. However, pra

14、ctical considerations make this difficult. These analysis functions are not comparable to spectral power distributions!Weighting a physical SPD under each of these three curves (that is, forming the wavelength-by-wavelength product), and summing the results, forms a triple of three numbers, denoted

15、X, Y, and Z. In continuous mathematics, three integrals need to be computed; in discrete math, a matrix product is sufficient. The X, Y, and Z tristim-ulus values characterize color. They are linear-light quantities, propor-tional to optical power, that incorporate the wavelength sensitivity of huma

16、n vision. The Y value is luminance, which is ordinarily expressed in units of candela per meter squared (cdm-2). If you are measuring reflectance, the reflected tristimulus values depend upon the spectral characteristics of the illuminant, and their amplitudes scale with the power of the illuminatio

17、n. Relative luminance is the ratio of reflected luminance to the luminance of the illumination; it is also known as the luminance factor.Figure 8 SPDs of various illuminants are graphed here. Illuminant A, shown in orange, is representative of tungsten light sources; it is deficient in shortwave pow

18、er, and may cause errors in sensing blue colors. The blue line graphs the SPD of a Nichia white LED. There is a peak in the blue portion of the spectrum: Uncorrected, the sensor would report excessive blue values. The other four lines represent CIE standard illuminants C, D50, D55, and D65.In many a

19、pplications, tristimulus signals (including luminance) scale with the illumination, and are otherwise uninteresting in themselves. What is more interesting is the ratios among them, which characterize color disregarding luminance. The CIE has standardized the projective transformation of Equation 1,

20、 in the margin, to transform X, Y, Z values into a pair of x, y chromaticity coordinates that represent color disregarding luminance. These coordinates are suitable for plotting in two dimensions on a chromaticity diagram.Eq 1 Chromaticity coordinatesIllumination A nonemissive object must be illumin

21、ated in order to be visible. The SPD reflected from an illuminated object is the wavelength-by-wave-length product of the illuminants SPD and the spectral reflectance of the object. Before light reaches the eye, the interaction among light sources and materials takes place in the spectral domain, no

22、t in the domain of trichromaticity. To accurately model these interactions requires spectral computations. When applying the TCS230, attention must be paid to the spectral content of the illumination and to poten-tial interaction between the illumination and the samples to be sensed. Generally, the

23、less spiky the spectra, the better. Figure 8 graphs several illuminants.Your application may involve sensing color, in which case the preceding description applies. However, some applications of the TCS230 involve not so much estimating color as seen by the eye but rather sensing physical parameters

24、 associated with optical power in the visible range. In such applications, to approximate the visual response may not be the best approach: It may be more effective to take a more direct approach to estimating the parameters of the underlying physical process.The Color Checker Equipped with knowledg

25、e of how spectra are related to colors, the plotting of chromaticity coordinates, and the dependence of colors upon illumination, we can return to the ColorChecker. GretagMac-beth doesnt publish or guarantee the spectral composition of the patches of the ColorChecker. However, nominal CIE X, Y, Z va

26、lues are published. The patches in the bottom row of the ColorChecker contain neutral colors; the numeric notations in the legends of Figure 5 reflect one tenth of the lightness (L*) values of those patches.Thespectra graphed on pages 2 and 3 represent the physical wave-length-by-wavelength reflecta

27、nce of the patches. These spectral reflec-tances have been measured by color measurement instrument called a spectrophotometer. If you had access to a light source having perfectly even distribution of power across the visible spectrum, then the reflectance curves graphed here could simply be scaled

28、 to repre-sent the reflectance in your application. Practical light sources do not have perfectly even spectral distributions, so compensation is neces-sary: You must compute the wavelength-by-wavelength product of the illuminants SPD with the spectral reflectance of the chart.We will first calculat

29、e the CIE X, Y, Z values from the chart. (These values should agree with the figures provided by Gretag.) Then we will calculate the R, G, B values that will be detected by a TCS230.To calculate CIE X, Y, Z, we take the 313 matrix representing the color matching functions (CMFs) of the CIE Standard

30、Observer, and perform a matrix product with 31 spectral response values as corrected for illumination. This produces the X, Y, Z tristimulus values. When chromaticity coordinates x, y are computed from X, Y, Z through the projective transform in Equation 1, then plotted, the chromaticity diagram in

31、Figure 9 results. The horseshoe-shaped figure, closed at the bottom, contains all colors: Every non-negative spectral distribution produces an x, y pair that plots within this region. The lightly-shaded triangle shows the region containing all colors that can be produced by an additive RGB system us

32、ing sRGB (Rec. 709) primary colors. This region typifies video and desktop computing (sRGB). The points plotted in Figure 9 are the colors of the ColorChecker. White and gray values are clustered near the center of the chart.Figure 9 Coordinates of ColorChecker patches are graphed on the CIE x, y chromaticity diagram. The horseshoe encloses all colors; the triangle encloses the colors that can be represented in video (Rec. 709) and in desktop computing (sRGB).The TCS230 Figure 10 shows the responses of the four channels of the TCS230. The black curve shows the response of the