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Optical Systems/Methods of Controlling Light

A luminaire, often called a light fixture, is a complete lighting unit that produces and distributes light to fulfill the design goals for the lighted space.

The primary methods of controlling light from a bare light source via a light fixture are reflection, transmission and refraction. Other methods include polarization, interference and absorption. In this article, we will discuss the first three.

Reflection, the most common form of controlling light, occurs when light rays impact and are then reflected from a surface. The types of reflection include:

  • Specular
  • Diffuse
  • Spread
  • Selective

Specular reflection is when light is reflected from a highly polished surface such as smooth polished metal, producing a consistent angle.

Diffuse reflection is when light is reflected from a rough surface, producing a variety of angles depending on how the light impacts each tiny part of the rough surface. Diffuse reflection is typically used to minimize glare, hot spots and shadows.

Spread reflection is when light is reflected into a cone of light rays from surfaces such as corrugated or etched metal, plastic or glass.

Selective reflection is when a colored surface is used so that only certain color wavelengths are reflected, as opposed to absorbed or transmitted.

The shape of the reflecting surface will determine how the beam is reflected. The most popular shapes for such surfaces include circular, parabolic, ellipsoidal and combination.

Transmission occurs when light rays are passed through a material. The types of transmission include:

  • Direct
  • Diffuse
  • Spread
  • Selective

Direct transmission is when light rays go through the material with no change to their direction or color. Example: Clear plate glass.

Diffuse transmission is when light rays are widely spread, useful when we want to obscure the light source and produce a uniform appearance of light on the transmitting surface. Example: Inside-frosted glass.

Spread transmission is when the maximum intensity of light rays passed through with little change in direction, producing a glow on the transmitting surface and a sense of sparkle.

Selective transmission is when selected color wavelengths are allowed to pass through the material. Example: Colored glass.

Refraction, used in prismatic lenses in fluorescent fixtures, floodlighting and streetlighting, occurs when light rays pass through one material and into another at a different intensity.