Introduction

Abstract

Visible light represents a very small part of the electromagnetic spectrum, and corresponds roughly with radiations within the wavelength range 380–780 mµ (1 mµ = 10 Å = 10^-7cm). Daylight covers this range and extends into the ultraviolet region (less than 380 mµ), but its composition varies with the season, time of day and climatic conditions. The light from artificial illuminants is nominally white, but varies widely according to the type of lamp. There is a deficiency of blue in the light of tungsten lamps and of red in that of simple discharge lamps. Correction is possible by using filters to absorb the excess of red or blue, but this results in great loss of efficiency. In mercury-vapour lamps the red deficiency can be largely overcome by introducing fluorescent powders within the discharge tube which absorb very short waves and emit light of greater wavelength, so that a good approximation to daylight results. The human visual system is able to assess colours with some allowance for the deficiencies of artificial light, so that a white object is recognised as such even when seen in yellowish light.