CdS cells have quite a few problems that have made them obsolete for photographic purposes: temperature dependence, slow response and memory effect. They need a significant time to adjust to a different level of illumination to give a normal response. The response becoming non-linear with age has also been reported. For a long time cameras have been using silicon photodiodes instead. As an example of the memory effect, the manual for a Durst darkroom exposure meter, which I have just been given and which uses a CdS cell, says to illuminate the CdS cell immediately beneath the lens of the enlarger for 15-20 seconds before making a measurement. They do work well all the same, most of my cameras that actually have an exposure meter use CdS cells. And they seem to be easily available. /Jonas