5.2.5 Spectral sensitivities of an image capturing device

The amplitude of each of the three color-separated signals received at the PMT 26, 27, 28 outputs behind the broadband (spectral) RGB filters 22 - 24 is determined by the spectral sensitivity η(λ) of the corresponding PMT and the spectral content of the light entering it. As can be seen from the scheme in figure 5.8, the latter depends in turn not only on the spectral reflection ρ(λ) of the sampled original area but also on the spectral power distribution of illumination p(λ), spectral transmission τ_t(λ) and reflection τ_r(λ) of dichroic mirrors, transmissions α(λ), β(λ), γ(λ) of color filters in three optical channels. As the expressions 5.4 show, the instantaneous values of these signals are proportional to the area under the curve obtained by multiplying all these spectral curves:

5.4

The integration bounds correspond here to the spectral sensitivity range of the photo detector, and the dimensionless coefficients a, b, c ensure the equality of the color-separated signals for the neutral, achromatic fields of the original. In practice, this balance is achieved by adjusting the light intensity in optical channels with gray filters of variable optical density and adjusting the gain of the output signals.

With the exception of the original reflection ρ (λ), the products of the remaining factors of above expressions represent the spectral sensitivity characteristics of the color separating channels.

Subsequent nonlinear transformations of color-separated signals are accompanied by losses due to the effecting of the reproduction system noise. In order to eliminate or minimize such transformations, the sensitivity characteristics of the three channels must correlate to those of the originals and their intended display media.

Quite often argue that scanners work in a certain "RGB color model". However, in order to the color-separated signals represent the color coordinates of any colorimetric system, the spectral sensitivities of scanner channels must correspond to the so-called Color Matching Functions (CMF) for such system primary colors. Any set of real primaries is corresponded by CMFs having sufficiently extended areas of negative values. So, the “negative” sensitivities are required to directly produce, for example, the RGB signals of the CIE RGB system of monochromatic (700; 546.1 and 435.8 nm) primaries. They can be obtained just indirectly, for example, by calculating them from CIE XYZ ones related to positive CMFs of virtual, unreal primaries [5.5].

Most scanners, as will be noted further, do not work colorimetric but as densitometers, carrying out the color separation in three wide spectral bands (red, green and blue) of the visible spectrum.

Selection of spectral sensitivities for color channels can take into account:

- spectral characteristics of the dyes for the used type of originals (photographic prints, transparencies or negatives; prints of conventional, toner based, thermal transfer or other kind of printing);

- characteristics of process colors in the intended image copying (on the monitor, by photographic recording, conventional or digital printing, etc.);

- light spectrum used for the copy viewing.

Then the values of the signals are more closely related to the colors of synthesis (amounts of inks, phosphors or LEDs intensities, etc.) and do not need significant additional nonlinear transformations, fraught with loss of information. Such characteristics are provided by the proper selection of color filters with calculation their transmission spectra according to equation 5.4.

For many years, until the color management creating for open reproduction environment, there were successfully used densitometric scanners. Their sensitivity characteristics are close to the absorption spectra of pigments of photographic originals and printing inks. So, acceptable was a direct, via Color Look-up Table (CLUT) conversion of red, green and blue channels signals in the process inks amounts without going to the colorimetric metrics. As will be shown in chapter 10, such kind of color separated signals processing stays effective until now for the closed, direct (one input – one output) systems.