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Illuminance: the different types

Illuminance E as a measure of luminous flux incident per surface unit A.

Illuminance (E) describes the quantity of light incident on a surface. It is the ratio of the incident luminous flux (Φ) to the size of the surface (A). The unit of measure of illuminance is foot candle (Fc). This is defined as lumens per square foot (1Fc = 1lm/sq ft). Illuminance recedes with the square of the distance from the light source (law of photometric distance).

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Compact Light Knowledge: Photometry

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Illuminance: an overview

What is the difference between illuminance and luminance?

Figure showing the difference between illuminance E and luminance L.

lluminance and luminance are two important photometric parameters in lighting design. Both take into account the brightness sensitivity curve of the human eye. Illuminance is a measure of the light incident on a surface, whereas luminance describes the light reflected from it. If a white and a black sheet of paper are illuminated with the same intensity, the same illuminance will be measured on both sheets. However, the white sheet reflects significantly more light than the black sheet and thus has a higher luminance.

What is point illuminance?

In contrast to the nominal illuminance (E_m), which gives an indication of the average illuminance of a surface, point illuminance (E_p) describes the precise illuminance at a point.

Using the law of photometric distance, it is possible to calculate the illuminance at individual points in space. Indirect lighting components are not taken into account in the calculation, but can be included via an additional calculation. Calculating point illuminances plays a role mainly in lighting design for narrowly defined areas illuminated by individual luminaires, e.g. exhibits in an exhibition.