A high‐pressure optical cell, modeled after the Drickamer cell I, has been developed for absorption and luminescence in the 0–40 kbar and in the 77°–383°K ranges. The windows consist of a NaCl‐sapphire combination in order to retain transparency down to 77°K. The window transmission varies as a function of temperature and/or pressure between tolerable limits. The largest contribution to nonreproducible errors in intensity measurements arises from the sample—salt interfaces. The pressure scale has been approximately determined at low temperatures by means of sudden changes in light transmission during the KCl phase transformation around 20 kbar. The liquid—solid phase transition in heptane is also studied.