Color Models
Result of some rendering modes (such as interpolated) is an RGB image in the color space of the color process used to take the photogrpah. In order to display it correctly, it is necessary to convert it to modern color space (such as sRGB, ProphotoRGB or XYZ). Color models define how colors are converted. We intensively seek for any published data on colors and spectra measurements of original or aged dyes used by individual color processes supported.
Color models are either bases on XYZ (or xy or xyY) values of the primaries or they use spectra measurement. If spectra is available, then it is possible to accurately simulate different types of backlight. Early color photographs were usually viewed with daylight or projected using Tungsten light.
Color models specify different intensities of red, green and blue filters. For additive color screen process to work well, the color screen needed to be reasonably close to neutral gray. Because the published data does not always match the proportions of color patches in the filter, it is possible to correct the white balance using dye_balance parameter. It can be one of:
-
none: No color balancing is performed. This is useful especially when displaying aged dyes where color cast is expected. -
brigness: Only adjust overall brigness to compensate for light captured by the screen. -
bradford: Use Bradford adapatation which is a model simulating human's eye capacity to adapt to different colors of light. This assumes that the color screen white balance is realistic and observer is seeing it in a projection or diascope. -
neutral: Adjust intensity of individual filters to obtain neutral color. This assumes that there is mismatch between patch sizes and intensities of dyes, which is a common case. -
whitepoint: Scale XYZ values to correct whitepoint. This is imprecise equivalent of Bradford correction.
For Dufaycolor there is also mode which attempts to restore original colors of the screen based on simulated reaction of the film using given set of dyes in the color screen filter.
Spectra can be exported using colorscreen utility. For example
colorscreen lab save-dyes wratten-red.dat wratten-blue.dat wratten-green.dat wratten_25_58_47_kodak_1945
will save wratten filters into wratten-red.dat, wratten-blue.dat, and wratten-green.dat files.
Perform no color transformations. Useful for debugging
Shown red, green or blue separation
"We may, therefore, say that there are three fundamental or primary colors in the spectrum, by the admixture of which we can form all intermediate colors and white. The positions of these fundamental colors are shown by the heavy vertical lines R. G. B,, in Fig. 3, and the curves show how the intermediate colors are formed. It will be seen from the diagram that the fundamental red is at about 6700, and this excites but the one sensation; but the green at 5180 not only excites the green-sensitive nerve fibril but also the red and the while the blue blue,at 4600 excites the green and red also, though but slightly."
This is interpreted as xy values
- red: 0.7319933,0.2680067 (670nm)
- green: 0.059325533,0.829425776 (518nm)
- blue: 0.143960396, 0.02970297 (460nm)
RGB values which looked good for early set of scans of paget plates available. This setting will be replaced by better one once paged color dyes are understood
This is interpreted as xy values
- red: 0.674, 0.325
- green: 0.059325533,0.829425776
- blue: 0.143960396, 0.02970297
Colors used in Goertz projector (designed by Miethe) as reconstructed in Wagner, J. (2006) Die additive Dreifarbenfotografie nach Adolf Miethe. Master thesis. Technische Universität München.
This is interpreted as xy values
- red: 0.674, 0.325
- green: 0.182, 0.747
- blue: 0.151, 0.041
Colors of aged filters used in Goertz projector (designed by Miethe) as measured in Wagner, J. (2006) Die additive Dreifarbenfotografie nach Adolf Miethe. Master thesis. Technische Universität München.
This is interpreted as xy values
- red: 0.620, 0.315
- green: 0.620, 0.315
- blue: 0.182, 0.135
Those are colors of Wratten filters 25, 58 and 47 which were used as standard filters for three-color photography. Based on MacAdam, D. L. Colorimetric specifications of Wratten light filters. Journal of the Optical Society of America 35, 10 (1945), 670–675.
These are XYZ values
- red: 0.3038, 0.1425, 0.0001
- green: 0.0891, 0.2451, 0.0219
- blue: 0.0862, 0.0240, 0.4730
Colors based on spectrum of Wratten filters as published by Kodak "Wratten light color filters, 1945 edition.". Those are tables XYZ values above are based on. Advantage of having full spectrum is that colors can be viewed with different backlights.
Digitized data based on published table
Spectra based on charts in the Dufaycolor manual. Those are clearly not realistic, but represents the intended sensitivity of the film.
Published chart
Digitized data
xyY specification of Dufaycolor reseau filter in Cornwell-Clyne, Adrian (1951) Colour Cinematography. Third. Champman & Hall. Available at:https://archive.org/details/dli.ministry.11295.
Table is (in xyY):
- red: 0.633, 0.365, 0.177, Dominating wavelength 601.7
- green: 0.233, 0.647, 0.43, Dominating wavelength 549.6
- blue: 0.140, 0.089, 0.037, Dominating wavelength 466.0
We believe the table is faulty since the dominating values does not correspond to the xyY values specified. Also it makes the overall reseau filter to look green.
Our attempt to correct the table (in xyY):
- red: 0.633, 0.365, 0.177
- green: 0.233, 0.647, 0.43
- blue: 0.164, 0.089, 0.087
Colors based on dominating wavelength data in the table discussed above
Published chart
Digitized data
"Camera profile" based on the previous spectral measurement and estimate of the spectral resonse of Dufaycolor emulsion. This compensates for some weak points of the process (such as desatured yellows) and can make correction of color cast based by different spectra of the light when photograph was taken.
Published chart (better scan would be greatly appreciated)
Digitized data
Spectrum published in Harrison, Horner: Principles of Dufaycolor printing, The photographic journal, 1939
"Camera profile" based on spectrum above.
Published chart (better scan would be greatly appreciated)
Digitized data
Published chart (better scan would be greatly appreciated)
Digitized data
Measurements of spectra aged Dufaycolor, Spicer-Dufay and Cinecolor dyes by Giorgio Trumpy, funded by ERC Advanced Grant FilmColors . See https://filmcolors.org/timeline-entry/1257/
Colors based on measurements in Casella L, Tsukada M. Effects of low-oxygen environments in the light fading of six dyes present in the autochrome color screen. Journal of the American Institute for Conservation. 2012 Sep 1;51(2):159-74.