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Diffuse reflection
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Specular Reflection
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Mixed specular and diffuse reflection
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Transmission of a ray of light through a translucent material
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Refraction of a ray of light through a prismatic panel.

Controlling Light

When we light an object, be it a space such as a room or a sports field, or part of a luminaire such as a louvre or diffuser, we do not see the light that falls onto a surface or object. What we actually see is the effect of light upon the object. Different materials affect light in different ways, for example paper reflects light differently to polished metal and the lit effect is different again for glass. To understand how a surface or object will look we need a basic understanding of reflection, transmission and refraction, the principal ways materials react to light.

3.1 Reflection
As mentioned above paper reflects light differently to polished metal. This is because paper exhibits what we term matt or diffuse reflection whilst polished metal exhibits what we term specular reflection. With diffuse reflection the light reflected from a surface is scattered equally in all directions.

With specular reflection the light reflects from a surface as if from a mirror, producing a sharp-mirrored image. For any ray of light striking a specular surface the angle of incidence of the light is equal to the angle at which the ray of light is reflected.

Some surfaces exhibit a mixture of diffuse and specular reflection, showing a fuzzy mirrored image. For this the peak reflection still obeys the rule of angle of incidence equals angle of reflection but light is also diffusely scattered around this peak.

3.2 Transmission
Certain materials have the ability to transmit and diffuse light. When light falls on a translucent (light transmitting) material some light will be reflected in a specular manner, and some light will pass through the material. For a clear material, such as clear glass, the light will pass through with a minimum of scattering. However for materials such as opal plastic the light is scattered or diffused, therefore spreading the brightness of the light ray over a larger area.

3.3 Refraction
When light passes from one transparent medium to another of different density (e.g. air to glass) it bends. This is known as refraction and this principle is used to control light, for example using prisms. In luminaires prisms are used to direct light away from areas that could cause glare or waste light and into areas that produce more useful light, thereby making the luminaire more efficient at illuminating a task or object.