Age Effects - Road Lighting: Fundamentals, Technology and Application - page 84

Fig. 7.3 Transmission of the
human eye,

100

˄ eye ,fora50and
65-year old relative to that for
a 25-year-old. (Based on Van
De Kraats and Van Norren
2007 ; CIE 2012 )

˄ eye

50/25 yr

75

(%)

50

65/25 yr

25

0

400 500 600 700

ʻ

(nm)

The combined effect of reduced pupil size and reduced transmittance of the eye
is given in Table 7.3 . The light on the retina for 50-65 year-old persons is reduced
to 43-28 %. As a comparison: normal sun glasses fall in the transmittance range of
30-45 %.
The effects are illustrated in Fig. 7.4 where the same view is shown as “seen”
through the eyes of a 25 and a 65-year old person.
Eventually, the mechanisms causing lens yellowing usually lead to cataract (as,
for example, the lens of the 75-year old in Fig. 7.2 ). With relatively simple surgery,
the crystalline lens can then be replaced by an artificial one, a so-called intraocular
lens (IOL). There is a great debate going on as to whether that artificial lens should
be chosen such that it transmits all visible light equally or that it should absorb some
part of the blue light by the use of a so-called blue-block lens (Henderson and Grimes
2010 ). Blue-block lenses have spectral transmittances similar to the crystalline lens
of a person of around 50-years, meaning that full transmittance does not return. The
dispute relates on the one side to the, as yet, unproved hypothesis that retinal damage
due to life-long exposure to blue light increases the risk of age-related macular
degeneration (AMD), leading to loss of vision in the centre of the visual field. The
yellowing of the crystalline lens is by the supporters of this theory seen as a natural
protection of the retina against prolonged energy-rich blue light. On the other side,
some scientists not supporting this hypothesis point to the benefits of a clear lens for
visual and especially for non-visual biological effects of light via the ipRGC light-
sensitive cells in the retina These cells, discovered only recently (Berson et al. 2002 )
have a high sensitivity for short-wavelength light viz. blue light. They are important
as regards circadian and neuro-behavioral regulation (Lucas et al. 2013 ).

Table 7.2 Transmission of warm-white and cool-white LED light through eye lenses of 50 and
65-year-old persons relative to that of a 25-year old lens
Transmission relative to 25 year old (%)

Age Warm-white LED 2700 K S/P ratio: 1.28 Cool-white LED 4000 K S/P ratio: 1.47

50 66 55

65 62 51