It’s only this part of the colour spectrum that’s affected, depending on the light colour of your enlarger. It should be virtually impossible to achieve proper density.
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I’m not entirely sure which part of the colour spectrum you mean by ‘this part’. But it doesn’t really matter. Your reasoning is wrong. The spectrum of standard enlarger lamps is certainly sufficient to fully activate the spectral sensitivity of the colour paper.
It has to be, when you realise that colour enlargement is a subtractive process; that is, when you apply your yellow-magenta filtering – as is customary in colour enlargement – certain spectral components (in this case blue-green) are
removed from the original ‘white’ lamp light, or rather, these are attenuated. The light then passes through a colour negative, which also acts as a subtractive filter. If the light filtered in this subtractive manner is still sufficient at the end to produce an image with the full colour spectrum and black, then the unfiltered light from the lamp is certainly sufficient to activate all three emulations and produce black!
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It also makes no difference whether you use a classic colour-mixing head with an NV halogen lamp or a condenser head with an opal lamp. As a classic filament lamp, the opal lamp has a continuous spectrum, i.e. unlike the halogen lamp, it has no spectral gaps. That is why the opal lamp can also cover the full spectral sensitivity of the photographic paper. How else would people have been able to produce colour enlargements using condenser units and colour filters back in the day, when colour mixing heads were not yet so widespread?
