The purpose of this part of IEC 61300 is to describe a test to determine the suitability of a fibre optic device to withstand the environmental condition of high humidity and change of temperature which may occur in actual use, storage and/or transport. The test is primarily intended to determine the suitability of fibre optic components under conditions of high humidity - combined with cyclic temperature changes and, in general, producing condensation on the surface of the device under test (DUT). Absorption of moisture may result in swelling that would destroy functional utility, cause loss of physical strength, and cause changes in other important mechanical properties. Degradation of optical properties may also occur. Although not necessarily intended as a simulated tropical test, this test can, nevertheless, be useful in determining moisture absorption of insulating or covering materials.