Mesopic Vision - Road Lighting: Fundamentals, Technology and Application - page 73

Fig. 6.3 Density of cones and
rods on the retina of the eye

line of vision

retina retina

fovea

blind spot

rods

cones

60 40 20 0 20 40 60

off-line angle (degrees)

interconnected so that the brain signal loses information as from where the signal
originates and consequently the peripheral image formed is not sharp but blurred. 4

6.3 S/P Ratio of Light Sources

In order to quantify the effect of the spectrum of a light source on peripheral vision in
the mesopic luminance range, we have to characterise the proportions of the spectrum
of the light source in the V( ) and V ( ) areas respectively. The so-called S/P ratio has

shown to be a suitable measure for this. It is the ratio between the scotopic-weighted
spectrum, according to V (

ʻ ʻ

ʻ ), and the photopic-weighted spectrum, according to

V( ). The S/P ratio can easily be calculated from the spectrum of the light source.

As an example, let us take a theoretical monochromatic light source with wavelength
of 500 nm. From Fig. 6.1 we see that the relative scotopic V (

ʻ

ʻ ) value for this

4 The eye has 126 million rods and cones but only 1 million nerve fibres. In the far periphery of the
retina some hundreds of rods are connected to a single nerve fibre (Tovée 1996 ).