Adaptations to Light and Dimness

Abstract

The earth receives, almost exclusively, radiation from the sun; the spectrum of which encompasses ultraviolet to infrared radiation (Fig. 5.1). Within this, only a narrow region of the continuum (visible light) is detectable by the eyes of living organisms. The visible light, defined in terms of human sensitivity, ranges from about 400 nm (violet) to 700 nm (red) (Fig. 5.1). To the human, the sunlight’s spectral composition appears white. This natural source of illumination can vary in intensities by a factor of 10 from a clear, bright, sunny day to a dark, moonless night (Table 3). Thus to function efficiently the eye must ‘adapt’ to the incident light; it must be able to capture the few photons available in darkness and to protect itself in bright light when bleaching of the visual pigments can become a problem. Remarkably, the human visual system can operate over most of this range (Table 5.1). The ability to adapt to different light intensities is required not only during the gradual change from day to night and vice versa, but also during the much faster changes associated with movements between areas of different light intensities. When we leave a brightly lit area and enter a darkened area we experience temporary ‘blindness’ until our vision improves.