Age Effects - Road Lighting: Fundamentals, Technology and Application - page 83

Table 7.1 Relative maximum
pupil size. (Nakamura et al.
2009 ; Weale 1992 )

Age Pupil size (%)

25 100

50 65

65 55

Fig. 7.1 Theeyeofa
24-year-old ( left ) and of a
66-year-old person showing
the great difference in pupil
size. (Photographs taken
under the same lighting
conditions)

Fig. 7.2 Illustration of the
yellowing of the crystalline
eye lens with age

25 yr 50 yr 65 yr 75 yr

on nearby objects more difficult (presbyopia). It can be compensated for by the use
of reading glasses or lenses. Bifocal or multifocal spectacles can also be employed.
Normally, presbyopia does not result in real problems for motorists in carrying out
their visual tasks. However, the gradual yellowing of the eye lens illustrated in Fig. 7.2
reduces its transmittance and so reduces the amount of light reaching the retina. The
figure shows that the yellowing process has already started in the 25-year old lens.
Most of the yellow, green and red light can pass through a yellow lens, but
blue light (which in the spectrum is farthest away from yellow) is largely absorbed.
Figure 7.3 , which gives the transmittance of the lens according to wavelength, shows
that the yellowing of the lens indeed reduces the transmittance, especially in the blue
part of the spectrum. This effect of the yellowing of the eye lens is called the “blue
light loss effect”.
Light sources with a relatively large part of their output in the blue region of
the spectrum (cool-white light sources) are less effective for older people. This is
because only a small part of the blue light reaches their retina. Table 7.2 gives
the transmittance, calculated from the curves of Fig. 7.3 , for a typical warm-white
phosphor LED of 2700 K and of a cooler-white phosphor LED of 4000 K for 50 and
65 year old persons relative to that for a 25 year old.