What’s the difference between a film’s spectral sensitivity and spectral dye density?
Asked 10/23/2023
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I’m trying to understand two terms used in film datasheets: spectral sensitivity and spectral dye density.
My understanding is that spectral sensitivity describes how strongly the film responds to different wavelengths of light during exposure. What confuses me is spectral dye density: is it about how much dye is present after processing, or is it a measure of the film’s density/transmission at each wavelength?
How are these two curves related on color film? For example, how do the sensitivity of the emulsion layers and the cyan/magenta/yellow dye densities connect in the final image?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
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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.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
2y ago
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Spectral sensitivity and spectral dye density describe two different stages of color film.
- Spectral sensitivity = how each emulsion layer responds to exposing light by wavelength.
- Spectral dye density = how the processed dyes in the developed film absorb/transmit light by wavelength.
So sensitivity is about exposure; dye density is about the result after processing.
In color film, the layers are essentially silver-halide emulsions sensitized to different parts of the spectrum. After development, the silver image is replaced by dyes: typically cyan in the red-sensitive layer, magenta in the green-sensitive layer, and yellow in the blue-sensitive layer. Those dye density curves show the spectral absorbance of those dyes, usually measured with a spectrophotometer as density = -log10(transmittance).
The curves are related because the wavelengths that expose a given layer determine where image dye is formed, but they are not the same curve. One shows input sensitivity; the other shows output filtering/absorption in the processed film.
Also, “density” here usually means optical density (how much light the processed film blocks), not simply the physical amount of dye, even though more dye generally means higher density.
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