As you dial in higher and higher values for any of three dichroic glass filters, it will be necessary to apply a correction (filter factor that increases exposure). I can provide approximate values but there are many valuables therefor you will need to run your own tests. Enlarging exposure meters are available however you can use most any exposure meter, placed on the baseboard; it will show light level reductions as you dial in more filtration.
Dial in .10 = 1/3 stop
Dial in .30 = 2/3 stop
Dial in .50Y = 1 stop
Why dichroic? All filters fade in time however enlargers are particularly bad because of the bright light and high heat. We started color printing with dyed-in-the-mass gelatin (CC filters Color Correcting) and acetate (CP filters Color Printing). Because these fade quickly in the lamphouse, we turned to dichroic glass. A dichroic filter is made by depositing micro thin coats of metals or oxides on heat resistant glass. These work by the interference principles like the rainbow you see on soap bubbles. Anyway, the metal or oxide coats are inert. Dichroic filters reflect away heat and the colors of light they do not pass thus they run much colder than their dyed-in-the-mass cousin. Thus a yellow dichroic looks blue by reflected light and yellow by transmitted light etc. (Greek for twice). A magenta dichroic looks magenta by transmitted light and green by reflected light. A cyan dichroic looks cyan by transmitted light red by reflected light.
When we print color negative film, with the enlarger lens wide-open, the paper is hit with an abundance of all three light primary colors (red – green – blue). We stop down the enlarger lens to reduce these light level. As you continue to stop down, the red light energy becomes correct first. This is because of the ISO assignments (actually paper speeds). With the red exposing energy correct, we address the green and blue light energy, both remain in excess.
Now we impose a green blocking filter. This is the magenta filter. We dial the magenta value until the green light energy is correct. Next we dial in a blue blocking filter. This is the yellow filter. In this way we adjust the exposure of the paper/negative combination to cause the paper to receive correct red – green – blue exposures. Again -- Red achieved by stopping down the lens, Green achieved by adjusting magenta filter and blue achieved by adjusting yellow filter.
This scheme avoids the use of the cyan filter. We avoid because the cyan filter is difficult to make and all cyan filters pass unwanted frequencies. The yellow filter is almost a perfect blue blocker. The magenta is an OK green but far from perfect. The cyan filter is so bad, it has a high filter factor due to the fact that we must dial in more and more to do the job. This induces cross-talk. Better to use the iris diaphragm to control the red energy level. Thus the paper layer speeds are set so this scheme can be used.
