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I read on a paper from kodak the differences between the two, but I'm still not sure if I understand it correctly. I'm pretty certain I understand the spectral sensitivity of film, as it is pretty synonymous to the trichromacy of our eyes or the ways digital sensors interpret light. However, I am a bit confused to what Spectral Dye Density is and how it relates to the spectral sensitivity curve.

Does the dye density refer to the density of the dye present in the film, if so why would they vary and does this variation lead to the spectral sensitivity of the film stock to emulate human vision? Or is the density some sort of figure conveying a relationship between exposure and the density obtained?

The explanations from Kodak:

  • "Spectral sensitivity measures the sensitivity of the film as a function of the wavelength of the exposing light"
  • "The spectral dye density measures the density of each imaging dye (cyan, magenta, and yellow) in the processed film as a function of wavelength"
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3 Answers 3

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Photo films, be they black & white or color, are sensitive only to violet and blue, in their natural state. By accident, in an attempt to mitigate halation’s, Professor Vogal, Berlin Technical, dyed an emulsion yellow. This solved the halation problem, as a bonus, sensitivity was extended into the green region. His graduate students continued; other dyes extended sensitivity into the red. This discovery yielded orthochromatic and panchromatic emulsions. Spectral sensitivity measures color sensitivity.

Modern color films are basically black & white films. Dyes are caused to replace the metallic silver that comprise the developed-up images. Cyan dye (blue green) in the red sensitive emulsion, magenta (red-blue) in the green emulsion and yellow dye in the blue emulsion. The color and intensity of the dyes in the film are measured by Spectral Dye Density.

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  • \$\begingroup\$ The fathers of this science are Ferdinando Hurter & V.C. Driffield, published in 1890. No calculators, computers, they used the slide rule which is based on logarithmic notation. The basic they used is the f-stop, a 2x change in blackening (density). In log notation base 10 this is written as 10 elevated to the 0.30 power. Often the decimal point is dropped thus 1 f-stop = 30 units of density. 1 stop =30 --- 2/3 f-stop = 20 units ---- 1/2 f-stop = 15 units --- 1/3 f-stop = 10 units – 1/6 f-stop = 5 units. \$\endgroup\$
    – Alan Marcus
    Commented Oct 24, 2023 at 15:22
  • \$\begingroup\$ thank you for the clear answer and the inclusion of the history of it. I am however a bit confused on how adding yellow dye would extend the silver halides sensitivity into the the green and red regions. I am assuming these dyes are separate from the dyes produced when the developer reacts with the color couplers to produce the appropriate dyes. Seeing as orthochromatic film was just black and white and much more sensitive to blue and uv without the impregnation of any dyes. Or am i misunderstanding something? \$\endgroup\$
    – vannira
    Commented Oct 25, 2023 at 21:59
  • \$\begingroup\$ You also mention how the the dyed emulsion mitigated halation, but isn't halation still present in films stocks, especially in the very bright regions. Did it just severely reduce halation or are there different types of halation. I thought halation was the reflection of the light that managed to penetrate the emulsion layers and are then reflected off of the base of the film, hence why it appears usually around the highlights. \$\endgroup\$
    – vannira
    Commented Oct 25, 2023 at 22:01
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    \$\begingroup\$ Dyes alter the frequencies the crystals absorb. Halation's are caused by highlights that penetrate the emulsion and reflect off the pressure plate, back into the film from the rear. The yellow dye is a blue blocker. It severely limited internal blue light transmission. \$\endgroup\$
    – Alan Marcus
    Commented Oct 26, 2023 at 3:00
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    \$\begingroup\$ Modern films have a backing that is flat-black or green or the color the film is least sensitive to. The idea is to absorb and thus kill light energy that traverses the emulsion thus mitigating its reflection back into the film. \$\endgroup\$
    – Alan Marcus
    Commented Oct 26, 2023 at 5:10
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Dye density is a measurement of the films response... the density in response to exposure as you alluded to.

This answer covers all of the relevant charts: How to read a film color response chart

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The spectral dye density curves denote the normalised spectral absorbance per dye layer of the film. Basically they measure the ratio of light transmitting through the film per wavelength with a spectrophotometer, this is known as spectral transmittance, where spectral density is then computed via

-log10(transmittance(λ))

If it’s colour negative film, often they’ll plot analytical densities of the 3 dye layers and d-min (base+fog and masking) separately, these are measured in isolation, which as you can imagine is far easier for the manufacturers to do. The analytical densities need to be summed per wavelength along with d-min to obtain the integral spectral dye density, of which is the density across all relevant layers.

The relationship between spectral dye densities and density shown in a characteristic curve is made via industry standard density metric responsivities, which for colour negative film is Status M (ISO 5-3:2009).

You would first take the integral spectral dye density and calculate spectral transmittance per wavelength via

10^-density(λ)

Next ensuring you have Status M responsivities per channel covering the same wavelength range as the spectral transmittance, start by taking the summation across all wavelength values for each responsivity channel, this will produce the normalised coefficients for the red, green, and blue channel densities. Then the final Status M densities for each channel is calculated via

-log10(SUMPRODUCT(spectral_transmittance(λ_range), responsivities(λ_range)(channel)) * normalised_coefficient(channel))

Given the analytical spectral dye densities are normalised, they will produce arbitrary Status M densities if you do this. In reality, dye couplers follow the Beer Lambert Law, where the concentration would need to be specified. The concentration produced of course will depend on the scene spectral irradiance over time and the spectral sensitivity of the stock.

Often the spectral sensitivity curves are plotted in log sensitivity reciprocal of exposure in erg/cm2 required to produce a specified density.

Put more simply, 10^-log_sensitivity(λ) represents the irradiance in erg/cm2 required per wavelength to achieve the specified density for that dye layer.

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  • \$\begingroup\$ if spectral dye density denotes the spectral absorbance per wavelength per day layer of the film. How does that that differ to the spectral sensitivity of the film. I read the spectral sensitivity of the film shows the color gamut as it represents the range of colors that can be captured. So from those two definitions, they seem to be measuring the same thing, the spectral absorbance, although one has a y axis of log sensitivity (which Im assuming is tightly related to density) and the other with density on the y axis. So I am a bit confused. \$\endgroup\$
    – vannira
    Commented Oct 30, 2023 at 9:43
  • \$\begingroup\$ I thought spectral dye density refers to the degree of developed (density) colored dye on the film, due to the reaction of the colour couplers and the oxidised developer. I may have misread something. I would also like to ask if you perhaps have a link to all the stuff you mentioned in your answer above as it all seems very interesting, but for me like a lot to learn just from a singular reply. Thank you for your previous replies \$\endgroup\$
    – vannira
    Commented Oct 30, 2023 at 9:48
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    \$\begingroup\$ Spectral sensitivity is measuring how much irradiance is required to produce a particular Status M density, this is a response since it relates scene light to resolved densities after development. But the spectral dye density curves do not tell you anything about the film's response to light, it simply describes the spectral absorption properties of the dyes used for the film post development. Kodak publishes spectral dye density curves because they are typically useful for optimising scanners and printers. The spectral dye density curves do not refer to the degree of developed dye. \$\endgroup\$
    – srb633
    Commented Oct 30, 2023 at 10:04

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