The nature of electromagnetic radiation is explained in relation to photochemical activity. Ultraviolet radiation below 400 nm is noxious, visible radiation 400-760 nm is harmful, and infrared radiation through its heating effect causes desiccation. The use of ultraviolet filters is described. A lighting level of 50 lux and limited exposure is advised. Stable temperature and relative humidity are essential, since fluctuations cause swelling and shrinkage in objects.