APPLICATION NOTE Revision 3.0 MTD/PS-0534 August 13, 2008 KODAK IMAGE SENDORS COLOR CORRECTION FOR IMAGE SENSORS

Transcription

1 APPLICATION NOTE Revision 3.0 MTD/PS-0534 August 13, 2008 KODAK IMAGE SENDORS COLOR CORRECTION FOR IMAGE SENSORS

2 TABLE OF FIGURES Figure 1: Spectrl Response of CMOS Imge Sensor...3 Figure 2: Byer CFA Ptterns...4 Figure 3: Spectrl Response of CMOS Sensor with RGB CFA...4 Figure 4: Color Correction Mtrix for RGB-> R G B...5 Figure 5: Color Conversion Mtrix for R G B -> YcrC...5 Estmn Kodk Compny, Revision 3.0 MTD/PS-0534 p2

3 CCD nd CMOS solid-stte imge sensors contin n rry of photosensitive elements clled pixels. The pixels collect light from scene nd convert it into electricl signl. The signl is then digitized nd processed. Digitl cmers use CCD or CMOS imge sensors to cpture imges. Silicon-sed imge sensors re sensitive to light with wvelengths from up to 1050 nm, s shown in Figure QE (%) Wvelength (nm) Figure 1: Spectrl Response of CMOS Imge Sensor Imges cptured y solid ste imge sensor re, therefore, monochrome in nture. To generte color imge, three primry colors (Red, Green, Blue or Cyn, Mgent, Yellow) re required for ech pixel. Before the invention of color imge sensors, the color imge ws creted y superimposing three identicl imges with three different primry colors. These imges were cptured y plcing color filter in front of the sensor, llowing certin ndwidth of the visile light to pss through. In erly 1970s, Kodk scientist, Dr. Bryce Byer, relized tht n imge sensor with color filter rry (CFA) pttern like tht shown in Figure 2 would llow the reconstruction of ll the colors of scene from single imge cpture. This specific CFA pttern is clled Byer pttern nd is used in most of the world s color digitl cmer systems. Estmn Kodk Compny, Revision 3.0 MTD/PS-0534 p3

4 G1 R G1 R Y1 M Y1 M B G2 B G2 C Y2 C Y2 G1 R G1 R Y1 M Y1 M B G2 B G2 C Y2 C Y2 Figure 2: Byer CFA Ptterns After cpture, the signl of ech pixel is red out sequentilly; digitized nd stored in memory. The originl dt for ech pixel only contins informtion out one color, depending on which filter is positioned over tht pixel. However, informtion for ll three primry colors is needed to construct color imge. The missing informtion is extrcted sed on the informtion gthered y the neighoring pixels. This is clled color interpoltion. There re vriety of interpoltion methods, such s Nerest Neighor Interpoltion, Biliner Interpoltion, nd Bi-cuic Interpoltion, etc. These simple methods re descried in the literture. The qulity of the finl imge depends in lrge prt on which lgorithm is used. For this reson mny cmer mnufcturers consider their est color interpoltion lgorithms to e trde secrets nd do not pulish them BLUE GREEN RED Wvelength (nm) Figure 3: Spectrl Response of CMOS Sensor with RGB CFA The colors vlues otined through the color interpoltion process re clled ntive colors. Due to the spectrl chrcteristics of the optics (lens, filters) used in the system, the lighting conditions (such s dylight, fluorescent, or tungsten) of the scene, nd the chrcteristics of the color filters of the sensor, the ntive RGB or MYC dt my not provide fithful color rendition. Estmn Kodk Compny, Revision 3.0 MTD/PS-0534 p4

5 For this reson n dditionl step clled color correction is required. In most cses, the ntive spectrl RGB or MYC is first converted into stndrd R G B (srgb, for exmple) color spce y 3x3 color correction mtrix, shown in Figure 4. srgb is the stndrd color formt for most digitl imging input nd output devices M C R' B' R R' G = G' B B' Figure 4: Color Correction Mtrix for RGB-> R G B Generting the correct vlues for the 3x3 color mtrix requires gret del of knowledge in imge science. The spectrl response of the CCD is different from the spectrl response of n output device. In turn, the spectrl response of the output device is different from the spectrl response of the humn eye. Therefore, perfect color reproduction is not possile, ecuse we do not hve the spectrl informtion for the scene. Imge Sensor Solutions will provide nlyticl mtrix coefficients ( ij, ij ) to its customers given the chrcteristic of the optics nd certin lighting condition for specific sensor. This mtrix minimizes the color error for the rnge of hues of which the eye is most criticl. The ntive primries cn then e converted to the color spce to mtch the output device for optiml color reproduction. Mny imging systems function more efficiently y using luminnce/color-difference color spce, such s YUV or YCrC. This llows the system to process the rightness nd color signls seprtely. Figure 5 shows the color conversion mtrix from the stndrd srgb color spce nd the YCrC color spce for JPEG compression R' Y G' = C B' C r Figure 5: Color Conversion Mtrix for R G B -> YcrC Tody, the conversion mtrix opertion is generlly performed in digitl signl processor. After the processing, the dt cn e converted ck to color spce suitle for ny output device. Estmn Kodk Compny, Revision 3.0 MTD/PS-0534 p5

6 This pge intentionlly left lnk. Estmn Kodk Compny, Revision 3.0 MTD/PS-0534 p6

7 This pge intentionlly left lnk. Estmn Kodk Compny, Revision 3.0 MTD/PS-0534 p7

8 Estmn Kodk Compny, Kodk nd Pixelux re trdemrks.