A high intensity carbon arc lamp is used with an ellipsoidal mirror as a source of radiation giving a maximum intensity of 55 w/cm2 The area over which the intensity of radiation is greater than 90 per cent of the maximum is 3 cm2. A venetian blind type of shutter may be operated either by a solenoid to produce a timed pulse of radiation of oonstant intensity; or by a cam driven mechanism to produce a pulse having the srune time intensity relationship as that from an atomic bomb. A device has been developed which will detect and record the instant of ignition of an irradiated specimen.