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Geometry of Light

Light comes in 3 general forms; point sources, linear sources and area sources:
Point source which produces the sharpest shadows and the most intense illuminations. It's illumination decreases inversely with the square of the distance, doubling the distance from light source reduces the light level by 1/4. Meaning if a surface were to be located at a distance of 1 metre away from a point light source with an illuminance of 100 reading of lux (the amount of light hitting a surface), then moved to a distance of 2 metre its illuminance would be reduced to 75 lux.
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A linear source is a point source extended along an axis. Shadows produced by line sources are perpendicular to their axis. Unlike point sources, when the distance is doubled from the linear light source the illuminance levels are reduced by half. Using a the same example but changing the light source from point to line, if a surface were to be located at a distance of 1 metre away from a line light source with an illuminance reading of 100 lux (the amount of light hitting a surface), then moved to a distance of 2 metres its illuminance would be reduced to 50 lux. 
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As for an area source, it is a light source extended along 2 perpendicular axes. An example of this would be sunlight whether direct or overcast. Though the sun may seem like a point source itself, the natural like we receive comes from the general illumination it produces when diffused through clouds or reflected off building and cars. As it is an overall illumination, it does not decrease with distance. 
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