Mesopic Vision - Road Lighting: Fundamentals, Technology and Application - page 72

Fig. 6.2 Spectral luminous
efficacy curves for scotopic,
V (

lm/W

V'( λ )

1500

) and
mesopic vision Vmes(

ʻ ), photopic, V( ʻ

adaptation
luminance )

(broken lines). Maximum of
V(

ʻ

1000

ʻ ) is 683 lm/W, of V ( ʻ )is

1700 lm/W

500

V( λ )

0

400 450 500 550 600 650 700

ʻ

(nm)

6.2 On-Line and Off-Line Vision

Cones are concentrated on the part of the retina called the fovea. It is the area of the
retina on which a sharp image is formed of the scene contained in a small 2 ⓦ cone
centred around the line of vision. The fovea contains no rods. Outside the fovea the
number of cones diminishes very rapidly to a low and more-or-less constant number
(Fig. 6.3 ).
So, for “central” or “on-line” vision (also-called foveal vision), only cones are
used. The V( ) curve provides the prediction of on-line performance, and for this

aspect of vision the normal photopic light units should be used, also for mesopic
lighting levels (CIE 2010 ). 3
The field of view of foveal vision is small, only 2 ⓦ . Nevertheless, we experience
a sharp image, in colour, of a much larger area. This is because our eyes, subcon-
sciously, shift their gaze continuously towards different fixation points in the scene.
Part of these fixation-point changes, called saccades, are stepwise and take a very
short time: between 0.04 and 0.2 s. The duration of a fixation subsequently takes
around 0.25 s. During each saccadic movement, image forming is suppressed so that,
contrary to the case of a fast moving camera, no blurred or smeared image is obtained.
Through these eye movements many different sharp foveal images are composed by
the brain into one sharp picture of a larger area than the 2 ⓦ field (Fig. 6.4 ).
The concentration of rods increases on the outer (peripheral) area of the retina,
while the concentration of the cones decreases. The maximum concentration of rods
is at about 15 ⓦ from the direction of view (Fig. 6.3 ). Rods are thus important for “off-
line” or “peripheral” vision. Cones at the fovea all have individual nerve connections
into the brain so that sharp images are formed (Fig. 6.5 ). Most rods, however, are

ʻ

3 Visual performance tests under foveal mesopic viewing conditions give conflicting results. Some
show a positive effect for cool-white light (Fotios et al. 2005 ; Fotios and Cheal 2007 ; Szabó et al.
2010 ), while some others show a positive effect for warm-white light (Bisketzis et al. 2009 ; Fotios
and Cheal 2011 ). Others show no specific spectral effect (Kebomaki et al. 2003 ; Alferdinck and
Hogervorst 2013 ). It may be expected that there exists some interaction between rods and the three
different types of cones that influences foveal mesopic vision (Ripamonti et al. 2009 ).