I'd known of the different speed ratings in darkroom dataguides for years, and never thought too deeply of why until recently...
Originally Posted by mr.datsun
I picked up a (new to me, but I'd had one before) Weston Master II recently, and noticed the needle really jumps when aimed at a regular light bulb. At the same time I was reading LP Clerc. Section 402. Daylight and Artificial Light Speeds. Explained several factors: "spectral distribution of the artificial light, colour sensitivity of the film and spectral response of the exposure meter." "Photo-sensitive -- and especially photo-conductive -- cells tend to respond more to the longer wavelength bands than an average panchromatic emulsion." "This tungsten speed rating is thus an equivalent rating rather than an actual emulsion speed"
(p.s. CdS cells are the photo-conductive cells).