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Color Accuracyaccuracy WelcomeWelcome toto thethe FifthFifth Almost-AnnualAlmost-Annual BetterBetter LightLight OwnersOwners’’ ConferenceConference ……iitt’’ss ggoonnnnaa bbee HHOOTT!!!! COLORCOLOR ACCURACYACCURACY mmoorree tthhaann yyoouu eevveerr wwaanntteedd ttoo kknnooww - including - hhooww ttoo imimpprroovvee yyoouurr ccaammeerraa pprrooffileile ““CCoololorr AAccccuurraaccyy””…… ¬ HHooww aaccccuurraatteelyly ddooeess aa ddeevvicicee rreennddeerr ccoololorr?? ¬ ……ccoommppaarreedd ttoo wwhhaatt?? ¬ ……uunnddeerr wwhhaatt ccoonnddititioionnss?? ¬ ……uussiningg wwhhaatt tteerrmmininoolologgyy?? SSppeeccttrraall rreessppoonnssee mmeetthhoodd compares the actual response of a device with its theoretical response, based on spectral data for the reference chart being used, and for the device 1 1.00 0.9 dark brown 0.90 dark orange blue 0.8 white 0.80 light brown violet blue 0.7 green 0.70 gray 1 pale blue 0.6 0.60 medium red red ccd red gray 2 0.5 0.50 ccd green olive green dark purple ccd blue yellow 0.40 0.4 medium gray light purple yellow green 0.30 0.3 magenta dark gray 0.20 0.2 light cyan light orange cyan 0.10 0.1 black 0.00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 5 9 1 2 3 8 0 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 3 4 4 4 4 4 5 5 5 5 5 6 6 6 6 6 7 7 4 4 4 4 4 3 3 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 reference chart spectral data device spectral response data CCaallcucullaatetedd ddeevviicece reressppoonnssee toto eeaacchh rerefefererenncece chchaartrt cocolloorr ppaatchtch:: 1.00 0.90 0.80 0.70 red response 0.60 green response blue response 0.50 yellow patch ccd red 0.40 ccd green ccd blue 0.30 0.20 0.10 0.00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 3 4 4 4 4 4 5 5 5 5 5 6 6 6 6 6 7 7 area under each response curve represents the relative signal level for that color channel TThhisis mmeetthhoodd ininvvoolvlveess mmaannyy ccaalclcuulalattioionnss…… 1.00 0.90 0.80 0.70 calc R 0.60 meas R calc G 0.50 meas G 0.40 calc B 0.30 meas B 0.20 0.10 the 0.00 e 1 2 e n d a measuredd …e k n measured it y y y y c u e e w t e e e le n … n n e n le n h a a l r lo n a g u r p e g w w u e p a ra ra r r la b re l e y correspondsn l r e n o o l e r y w g g g g b g e g c correspondsa b d u r ra r r b r u c d k y a r t e p l g b b le g p lt e r o le m k e t o k lt a e lt a m k io d y l d p liv m d d v w o it h response lit very tle closely of s a u Better r to rp its ris but calculatedLight e correspondhow scanning well response does back to a the s standard device response?response WWhhaatt isis aa ssttaannddaarrdd rreessppoonnssee?? ¬ Characterizing the “native” response of a device is a good first step, but a device should also be capable of rendering colors according to an accepted standard, to provide a “standard response” ¬ This usually involves three-dimensional calculations to convert the device response to a standard response ¬ The above is equivalent to converting from the color space of the device to a standard color space CCoololorr ssppaaccee –– tthhee ffininaall ffrroonnttieierr…… Adobe RGB 1998 color space displayed by ColorThink DDeessccrribibiningg ccoololorr wwitithh aa 33DD ssppaaccee brightest point = white brightest point = white neutrals are increasing brightness, at center decreasing saturation max diameter = max saturation hue increasing brightness, increasing saturation darkest point = black DDeessccrribibiningg ccoololorr wwitithh aa 22DD ssppaaccee yellow green red Brightness (3rd saturation dimension) is hue perpendicular to gray this color plane cyan magenta blue DescribingDescribing colorcolor withwith aa standardstandard spacespace yellow b green red Lightness (3rd dimension) is a perpendicular to this color plane cyan magenta blue NNoott aallll ccoololorr ssppaacceess aarree eeqquuaall Adobe RGB ProPhoto RGB IdenticalIdentical RGBRGB datadata cancan representrepresent differentdifferent colorscolors inin differentdifferent colorcolor spacesspaces R=0, G=255, B=0 in ProPhoto RGB and in Adobe RGB AA standardizedstandardized methodmethod forfor evaluatingevaluating andand expressingexpressing colorcolor accuracyaccuracy ¬ a physical reference chart to photograph with the device being tested under standard conditions ¬ a reference chart color space with the ideal data values for the chart under standard conditions ¬ a way to relate or convert the device color space to the reference chart color space ¬ a way to measure and show errors in the device’s rendition of the reference chart AA standardizedstandardized methodmethod forfor evaluatingevaluating andand expressingexpressing colorcolor accuracyaccuracy standard device image data in color space illumination being tested device space conversion reference chart reference data in image data in standard space standard space ANALYSIS and REPORTING ThisThis samesame methodmethod cancan bebe usedused toto evaluateevaluate thethe accuracyaccuracy ofof aa profileprofile device/light device being profile color space light source tested conversion reference chart image data in device/light space reference data in image data in standard space standard space ANALYSIS and REPORTING Ideally,Ideally, thethe device/lightdevice/light profileprofile isis generatedgenerated byby aa differentdifferent referencereference chartchart device being image data in light source tested device/light space reference chart device/light profile reference data in standard space profile generator PPootetenntitiaall ssoouurcercess ooff eerrorrorr reference chart exposure may profile may conversion may may be improperly be incorrect be inaccurate be inaccurate illuminated device/light device being profile color space light source tested conversion reference chart image data in device/light space reference data in image data in standard space standard space ANALYSIS physical chart and analysis may may not match REPORTING be inaccurate reference data IItteemmss nneeeeddeedd ffoorr tthheessee tteessttss:: ¬ ccoonnssisistteenntt liglighhtt ssoouurrccee ¬ rreeffeerreennccee cchhaarrtt ffoorr mmaakkiningg pprrooffileile ¬ rreeffeerreennccee cchhaarrtt ffoorr eevvaaluluaattiningg aaccccuurraaccyy ¬ pprrooffilinilingg ssooffttwwaarree ¬ ccoololorr ssppaaccee ccoonnvveerrssioionn ssooffttwwaarree ¬ aannaalylyssisis aanndd rreeppoorrttiningg ssooffttwwaarree ¬ ccuurrioiossitityy aanndd ppeerrssisistteennccee AAnnaalylyssisis aanndd RReeppoorrttiningg ssooffttwwaarree inexpensive color accuracy evaluation software is now available from www.imatest.comwww.imatest.com IImmaatteesstt CCoololorrCChheecckk ¬ compares the color accuracy of an image of a Macbeth ColorChecker captured by the device being tested, expressed in your choice of standard color spaces, to your choice of reference data ¬ many different versions of reference data for the ColorChecker exist – for different color spaces and/or illuminants ¬ ColorCheck requires the device image to be converted to the selected standard color space (does not recognize or use ICC profiles) ¬ expresses results in several easy-to-understand ways, including graphic plots and visual comparison charts CCoololorrCChheecckk wwoorrkkffloloww (optional) device/light ICC profile device being Photoshop: tested convert to standard color space, using any supplied image of input profile ColorChecker from device selected ColorChecker image from device in reference data imatest standard color space ColorCheck CCoololorrCChheecckk vvaarriaiabbleless (optional) device/light ICC profile change profile device being Photoshop: tested convert to change exposure, standard color color balance, space, using contrast any supplied image of input profile ColorChecker from device choose different color space choose different reference data selected ColorChecker image from device in reference data imatest standard color space ColorCheck IICCCC pprorofifillee vvaaririaabblleess change exposure, color balance, contrast device being image data in light source tested device/light space reference chart device/light profile reference data in standard space profile generator choose different (change reference reference data illuminant) SSoo mmaannyy vvaarriaiabbleless,, ssoo litlitttlele ttimimee…… ¬ reference chart should be uniformly illuminated ¬ position chart in the center of the image area ¬ reference chart image should be neutral-balanced and properly exposed for the gamma of the reference data set ¬ adjust the Tone curve (if used) to fine-tune gray scale for proper gamma, if necessary ¬ ColorCheck reference data color space and target standard space should be the same MMoorree ssimimppliflificicaattioionnss ¬ use your Photoshop working space as the target standard color space (presuming ColorCheck supports this space as a reference data color space) ¬ choose a reference data set using the reference data color space (= working space) ¬ only need one perfectly neutral-balanced and exposed ColorChecker image adjusted for the target (working space) gamma SSoo wwhhaatt AARREE wwee cchhaanngginingg?? ¬ our intention is to produce a device profile and workflow that consistently delivers the most accurate color response compared to a standard response ¬ we can fine-tune the profile by optimizing the profile reference chart exposure, and by changing profile generation characteristics ¬ we can fine-tune the workflow itself GGeettttiningg ssttaarrtteedd ¬ capture a properly-exposed and neutralized image of a ColorChecker adjusted for the gamma of your Photoshop working space (e.g., 2.2 for Adobe RGB 1998) RGB values from RGB values expected Robin Myers: by ColorCheck: white = 243 white = 242 gray1 = 201 gray1 = 201 gray2 = 161 gray2 = 161 gray3 = 122 gray3 = 122 gray4 = 85 gray4 = 84 black = 53 black = 54 TTeessttiningg aann uunnpprrooffileiledd imimaaggee since ColorChecker image has been adjusted for proper neutral balance, exposure, and gamma, it can be presumed to already be 1.
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