Surface Illuminance

The illuminance of a surface by a point source depends upon the pointance (intensity), the distance to the surface, and the angle of the surface. The illuminance is measured in lux, but the older unit footcandle is still encountered.

A source with luminous intensity Iv = candela

is placed at a perpendicular distance of y = m = ft from a surface.

At a horizontal distance of x1 = m = ft

the radial distance is r1 = m = ft and the angle θ1 = °.

At this point, the source provides an illuminance of Ev = lux = footcandles.

At a horizontal distance of x2 = m = ft

the radial distance is r2 = m = ft. If the surface at this point is tilted to an angle α = °, then the angle θ2 = °.

Under these conditions, the source provides an illuminance of Ev = lux = footcandles.

Perpendicular areance
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Photometry concepts

Vision concepts
 
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