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Imatest Documentation Noise display: Pixel noise or SNR selects the noise display for the lower-left plot of the first figure (noise/SNR for grayscale row 4) and the upper-right plot of the second figure. (noise/SNR for BGRYMC row 3). The effects of gradual illumination nonuniformities have been removed from the results. All displays are derived from Noise in pixels (the third selection below). The five display types are identical to the types in the lower noise detail plot in Stepchart. In the notation below Ni is RMS noise and Si is signal (pixel) level for patch i. Noise (%) normalized to White - Black (Zone 19 - 24) Noise (%) normalized to 255 pixels (max of 100) Noise in pixels (maximum of 255) Noise in pixels ( Ni ). Values 0-255. 100% * (Noise in pixels) divided by (White patch (19) pixel level − Black patch (24) pixel level) = 100% N/(SWHITE−SBLACK ). Useful because it references the noise to the scene: noise performance is not affected by camera contrast. 100% * (Noise in pixels) / 255 = Ni/2.55. Values 0-100. This noise measurement will be worse for higher contrast cameras. (It is affected by the gamma encoding.) Pixel S/N (Signal in patch/RMS noise) dimensionless (Signal (pixel level)) / Noise = Si/Ni for each patch. Dimensionless. Pixel SNR (dB) (20*log10(S/N)) 20 Log10(Signal/Noise) for each patch = 20 log10( Si/Ni ). Units of dB (decibels). Doubling S/N increased dB measurement by 6.02. For this selection, SNR_BW is also displayed. SNR_BW is an average SNR based on White-Black patches (Zone 19 - Zone 24; density difference = 1.45). SNR_BW = 20 log10((SWHITE-SBLACK )/Nmid ), where Nmid is the noise in patch 22 (the 4th patch in row 4; middle gray; nominal density = 0.7). Color space: You can select among the following. Danny Pascale's A Review of RGB Color Spaces is recommended for readers interested in an in-depth explanation of color spaces. sRGB Adobe RGB (1998) Wide Gamut RGB ProPhoto RGB The default space of Windows and the Internet. Limited color gamut based on typical CRT phosphors. Gamma = 2.2 (approximately), White point = 6500K (D65). Medium gamut, with stronger greens than sRGB. Often recommended for high quality printed output. Gamma = 2.2, White point = 6500K (D65). Extremely wide gamut with primaries on the spectral locus at 450, 525, and 700 microns. One of the color spaces supported by the Canon DPP RAW converter. 48-bit color files are recommended with wide gamut spaces: banding can be a problem with 24-bit color. Gamma = 2.2, White point = 5000K (D50). Extremely wide gamut. Gamma = 1.8, White point = 5000K (D50). Described in RIMM/ROMM RGB Color Encodings by Spaulding, Woolfe and Giorgianni. Apple RGB Small gamut. Used by Apple. Gamma = 1.8, White point = 6500K (D65). ColorMatch RGB Small gamut. Used by Apple. Gamma = 1.8, White point = 5000K (D50). Color error display: You can select between Standard ∆C and ∆E*ab (CIE 1976) measurements, CIE 1994 measurements (somewhat more accurate), and CMC mesurements (widely used in the textile industry) color difference formulas. CIEDE2000 measurements ( ∆C00 and ∆E00 ). The emerging standard. See Gaurav Sharma's CIEDE2000 Color-Difference Formula page. The CIE-94, CMC, and CIEDE2000 mreasurments reflect the eye's reduced sensitivity to hue and saturation changes for highly saturated colors. Mean CIE-94 numbers are typically around half of the standard measurements; CMC measurements are between the two. CIEDE2000 measurements will be become more familiar as time passes. Patch for noise spectrum (Imatest Master only): You can select any of the patches in the bottom two rows for calculating the noise spectrum. (Row 3 contains B, G, R, Y, M, and C primaries; row 4 contains grayscale values.) The spectra for the Y (luminance), R, G, and B channels is displayed. The third patch in row 4 (middle gray; density = 0.44) is the default. a*b* color error: The upper-right of the a*b* color error plot displays mean color errors (∆C... and ∆E...) and either the standard deviation of the color error (σ) or the maximum color error (max), selected in this box. Incident lux (for ISO sensitivity calculations) When a positive value of incident light level (not blank or zero) in lux is entered in this box, ISO sensitivity is calculated and displayed in the Stepchart noise detail figure. More details are on the ISO Sensitivity and Exposure Index page. Output The example was photographed with the Canon EOS-10D at ISO 400, and RAW converted with Canon FVU using automatic color balance. 294 of 451
Imatest Documentation The first figure: Gray scale tonal response and noise shows graphic results derived from the grayscale patches in the bottom row, including tonal response and noise. Selected EXIF data is displayed, if available. The upper left plot is the density response of the colorchecker (gray squares). It includes the first and second order fits (dashed blue and green lines). The horizontal axis is Log Exposure (minus the target density), printed on the back of the ColorChecker. Stepchart provides a more detailed density response curve. The exposure error is shown in pale red if it is less than 0.25 (the maximum recommended error) and bold red otherwise. The upper right plot shows the noise in the third colorchecker row, which contains the most strongly colored patches: Blue, Green, Red, Yellow, Magenta, and Cyan. In certain cameras noise may vary with the color. Problems may be apparent that aren't visible in the gray patches. Note that the x-axis scale (log exposure) is reversed from the plots on the left. The lower left plot shows the RMS noise or SNR (signal-to-noise ratio) for each patch: R, G, B, and Y (luminance). Several display options are described above. The selected display in the above figure is Noise (%) normalized to White - Black (Zone 19 - 24), i.e., RMS noise is expressed in percentage of the difference between the white and black patches.The average R, G, B, and Y noise for the gray zones (2-5) is also reported. The lower right region displays EXIF data, if available. The bottom left plot has several display options, listed above. Pixel SNR (dB) is shown on the left. SNR (dB) = 20 log10(Si/Ni), where Si is the signal (mean pixel level) of patch i and Ni is the noise (standard deviation of the pixel level, with slow variations removed) of patch i. SNR_BW is an average SNR based on White - Black patches (patch 19 - patch 24; density difference = 1.45) SNR_BW = 20 log10((SWHITE-SBLACK )/Nmid ), where Nmid is the noise in patch 22 (the 4th patch in row 4; middle gray; closest to nominal chart density = 0.7). 295 of 451
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