The ratio known as noise factor, originally used as a measure of performance at microwave frequencies, seems likely to be useful also at higher frequencies; namely in the infrared and optical regions. It seems to be a convenient measure for most terrestrial applications at all frequencies, and for both terrestrial and space applications at the higher frequencies.While, perhaps, most useful for summarising the measured performance of a practical device, examples are given of estimated bounds. Thus, the minimum noise factor of a molecular amplifier rises from unity at low frequencies to 2 at high frequencies; and the minimum noise factor of a photocathode heterodyne mixer may be unity if an unattenuated fluctuation-free local oscillator is used, but it rises to 2 if the local oscillator is attenuated.