Mesopic Vision - Road Lighting: Fundamentals, Technology and Application - page 71

Fig. 6.1 Spectral eye
sensitivity curves (also called
luminous efficiency curves)
for photopic vision V(

100

%

V'( λ ) V( λ )

80

) (CIE
1926 ) and scotopic vision
V ( ʻ ) (CIE 1951 )

ʻ

60

40

20

0

400 450 500 550 600 650 700

ʻ

(nm)

adaptation levels, lower than some 0.005 cd/m 2 (popularly referred to as being “pitch
dark”) only the rods are active and we speak of scotopic vision. 1
The rods have a higher sensitivity to light than do the cones. With scotopic or rod
vision, colour vision is impossible. The spectral sensitivity with scotopic vision is
characterised by the V ( ) curve. It reaches its maximum sensitivity at a wavelength

of around 505 nm, corresponding to the colours blue-green: relative to the V(

ʻ

) curve
there is a clear shift towards the blue end of the spectrum (Fig. 6.1 ). At adaptation
levels between approximately 5 and 0.005 cd/m 2 both the cones and the rods are
active. We then speak of mesopic vision. In the mesopic vision range, the activity
of the rods becomes more important from high to low adaptation levels. As a result,
the spectral sensitivity gradually shifts into the direction of small wavelengths—that
is to say in the direction of blue. 2 Fig. 6.2 shows this shift. The sensitivity scale
here is not relative as is commonly the case (see Fig. 6.1 ), but in terms of absolute
spectral luminous efficacy values, so that the effect of the larger sensitivity of rods,
as mentioned above, also becomes apparent.
Because of the shift towards the green-blue end of the spectrum, it can be claimed
that under road-lighting conditions, where we are mostly in the mesopic vision range,
light sources containing more green-blue light (cool-white light) than yellow-red light
(warm-white light) are more efficient for vision. Claims are sometimes exaggerated
and sometimes made when they are not valid at all (van Bommel 2009 ). As will be
explained in the next section, such claims are valid for peripheral vision but not for
on-line vision.

ʻ

1 On an asphalt road surface full moon results in an average of 0.02-0.03 cd/m 2 , which is clearly
too “bright” for scotopic vision. Dimly-lit residential streets have an average luminance of some
0.2 cd/m 2 to 0.5 cd/ m 2 , which is far too bright for scotopic vision.
2 Because of this shift, our perception of colour is changed somewhat towards blue. This colour-
change effect is called the “Purkinje effect”.