an apparatus for testing the optical output of light - emitting devices in the presence of ambient light , with means for compensating for said ambient light , is described . the d . c . and periodic components of the ambient light are measured and stored during a time when the apparatus is not being subjected to radiation from the device under test . when radiation from the dut is received , the resulting signal is processed with the stored ambient light intensity data , and an output proportional to the intensity of the radiation from the dut and compensated for the ambient light is provided .