Cornell University Ergonomics Web
Other aging effects in addition to decreased transmission are:
Pupil diameter varies between 2-8 mm.
Nominally changes in pupil diameter should change the luminous flux over a range of 16:1 BUT because of directional sensitivity the effective range is 10:1. HOWEVER, the operating range of the eye is 1010 :1, so the pupil variation is only a minor aspect of adaptation of the visual system to prevailing conditions.
Retinal structure
Cones are concentrated in the fovea and receptive fields are small in this region.
Peak sensitivity for cones = 555 nm, but combination of R, B, and G cones. Peak sensitivity for rods = 505 nm.
The human visual system isn't equally sensitive to all wavelengths. Human spectral sensitivity is measured in terms of the equivalence of visual effect e.g. a person viewing two equally presented fields of the same wavelength and radiance sees them equal in all respects. (Radiance = luminance, or light output, in a given direction in an imaginary cone.)
Whenever wavelengths are the same but radiances differ then the field with higher radiance will be seen as brighter. When both wavelength and radiance differ then the best visual effect that can be achieved is brightness equivalence by changing the radiance of one of the two fields until they appear equally bright.
By establishing the equivalences of many pairs of wavelengths (and some maths interpolation) it is possible to express the sensitivity of the visual system at each wavelength relative to its sensitivity at an arbitrarily chosen standard wavelength (usually that for which sensitivity is the maximum). The resulting curve is the relative spectral sensitivity curve for the human visual system. Unfortunately, there is no such thing as a single relative spectral sensitivity (RSS) curve for the human visual system for all people! There is, however, an internationally agreed curve (or rather, curves). Since the human visual system comprises 2 types of photoreceptor (cones and rods), which operate under different lighting conditions, there are usually 2 RSS curves: one to describe photopic conditions (when light is plentiful) and one to describe scotopic conditions (when light is scarce). As one moves from photopic to scotopic conditions or vice versa, there is also a third curve, called the mesopic condition, which sometimes is drawn though this is not accurately know. Mesopic vision applies quite commonly e.g. under road lights at night. In daytime (photopic) nearly all photopigments in the rods are "bleached": vision is predominantly a function of cones. We see color and acuity is high (because of foveal vision). At night (scotopic) vision is determined by rods - we don't see colour and acuity is low. In effect we have 2 functionally separate retinas.
The change from scotopic to photopic is called the Purkinje shift but because these systems cannot operate simultaneously, there is an adaptation delay in moving from light to dark (slow) or vice versa (fast).
When light is absorbed, pigment breaks down, energy is released, and the nerve impulse is interpreted as light. In the dark, regeneration of pigments.
Adaptation - can look into a flashlight in daytime but not at night without being dazzled. This is because retina adapts to changes in prevailing lighting conditions.
Dark adaptation - moving from light into dark. Slow. May be temporarily blind but gradually recover some vision. 25 minutes - 80% adaptation. 60 minutes - 100% adaptation.
Light adaptation - moving from dark to light. Fast. 2 phases.
When there is bright light in any part of the visual field, the overall sensitivity of the retina is decreased. Therefore it is not a good idea to put VDU screen against bright background like a window.
Partial adaptation - If visual field contains bright or dark area also local or partial adaptation ( and afterimage e.g. stare at bright light then look away into light, gives dark spot).
Also adaptation of one eye has some effect on other eye e.g. in bright sunlight, closing one eye leads to decreased sensitivity of retina of other eye. In the dark, closing one eye decreases dazzle from car headlights and closed eye retains greater level of dark adaptation.
BUT lighting design is not just for visibility; it is also for appearance of space and psychological comfort. Generally:
Blur - If eye cannot focus image on retina, there is blur. Blur can also be consequence of: 1. Uncorrected refractive errors - eyes can't focus image.
Aberration - even when eye is perfectly corrected for refractive errors, there will be individual differences in:
These aberrations become increasingly important as light level decreases.