difficult to eliminate because it is equal
to the Exitance of a Lambertian
surface.
Luminance Unit relationships
L = M /
These relationships hold only for a Lambertian Surface.
1 cd/ft
2
=
lms/ft
2
1 cd/m
2
=
lms/m
2

14
Inverse Square Law
E = I / D
2
= cd / ft
2
= lm / ft
2
= lm/sr/ft
2
Note – This is how luminous
intensity is measured.
Inverse Square Cosine Law
E = I cos
i
/D
2
i
Accounts for the effective surface area
which catches the lumens arriving
from a particular direction (projected area).
sr is unitless
cd=lm/sr

15
8 ft.
I = 2000 cd
E = I cos
i
/D
2
E = 2000 cd / 8
2
E = 31.3 fc = 336 lux
8 ft.
I = 2000 cd
E = I cos
i
/D
2
E = 2000 cos(20) / 8
2
E = 31.3 cos(20) = 29.4 fc
n
20°