from those of younger persons in two ways:
(1) the lens of the eye thickens, decreasing
the eye’s ability to change its shape to prop-
erly focus at varying distances (figure 1.6),
and (2) pupil size becomes reduced, decreas-
ing the amount of light reaching the retina.
Older persons require higher illuminance
values for the same tasks (a sixty-year-old
requires three times the illuminance of a
twenty-year-old for equal retinal illumi-
nance). At the same time, glare sensitivity
within the field of view is significantly
increased.
IESNA illuminance recommendations
assume an age of forty to fifty-five. As a rule-
of-thumb, for persons under age forty,
illuminance values may be reduced by up to
one-third; for persons over age fifty-five,
illuminance values may be increased by up
to two-thirds.
Illuminance Calculations
Although people see not footcandles but
luminance contrast, the question of how
much light is necessary for tasks must still
be answered. Calculations are performed
during the design process to obtain informa-
tion about lamp and luminaire performance,
to evaluate design alternatives, or to refine a
particular design.
Following are descriptions of simple cal-
culation methods, with examples for deter-
mining illuminance at a specific point and
the average illuminance on a specific plane.
Illuminance at a Point
To find the value of incident illuminance at a
specific point produced from a compact
source, theinverse-square methodis used.
This method closely approximates the illumi-
nation where the distance from the source is
at least five times the maximum dimension
of the source.
Source aimed at a target
Illuminance is proportional to the luminous
intensity of the source in the given direction,
and inversely proportional to the square of
the distance from the source.
To calculate illuminance (fc) from a
source aimed at a surface perpendicular to
the source:fc=I
D^2
whereIis theintensity of the source (in can-
delas) in the direction of the point andDis
thedistance from the source to the point
(figure 10.4).Example
A 60PAR/HIR/SP10 lamp is aimed at a point
on a surface 12 ft away. From a luminous
intensity distribution chart (figure 10.3, left),
find that the 60PAR/HIR/SP10 lamp pro-
duces 20,000 cd at 0°:fc =20,000
122
= 139
INTERIOR LIGHTING FOR DESIGNERS
Figure 10.4Illuminance on a surface perpendicular to
the source.