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• To find the value of incident illuminance at a specific point produced from a compact source, the inverse-square method is used. This method closely approximates the illumination when the distance of the illuminated object or surface from the light source is at least five times the maximum dimension of the light 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:
Illuminance = I/ D 2

where I= intensity of the source in candelas in the direction of the point
D= distance from the source to the point.

Source aimed at an angle to a horizontal surface

The illuminance at the target on a horizontal surface is the intensity of the beam at nadir reduced by the cosine of the angle from the source to the target.
Fc= I/ D2 x  cos θ°

Source aimed at an angle to a vertical surface
The illuminance at the target on a horizontal surface is the intensity of the beam reduced by the sine of the angle from the source to the target.
Fc= I/ D2 x  sin θ°