Remember that as you dim an incandescent lamp the peak wavelength gets longer (ie the light gets redder) so you are probably reducing the actinic efficiency of the lamp and thus you are likely to get less actinic power for a given amount of heat - the 'actinic watts' per electrical watt* generally goes down, particularly with ortho film. It's usually better to run the lamps as hot as they are designed for, which is why the good old P1 photofloods have a colour temperature of 3400 K (tungsten Type A) and a short, but relatively efficient (for that type of incandescent) life.
*lumens per watt isn't appropriate for films that don't match the eye's spectral response - which is just about every B&W film - but it is a guide in the absence of anything else.
Best,
Helen
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