Light is the single most important element when it comes to taking photographs. Whatever type of camera you use – film or digital – a wet or dry darkroom, you are involved in portraits, landscapes, under water, aerial or any of the myriad photographic areas, you cannot escape light and its use. If you desire to understand photography then you need to learn as much as you can about light.

The following article provides the terminology used in the study of illumination and how it relates back to your camera and associated equipment.

Intensity describes the amount of light. This determines the shutter speed and aperture needed to make a photograph. Intensity is measured by a light meter.

Direction to the way the light falls on the subject, relative to the camera. Changing the direction at which light meets the subject significantly changes perception of relationships between subjects.

Specular implies contrasty lighting because specular light is from a single source, often called a point source. Specular light makes very sharp-edge shadows. The sun, a single spot or flash without diffusion are examples of point sources.

Diffuse is the opposite of specular; it appears to come from many directions. Diffusion softens the edges of shadows and lowers contrast. Diffuse lighting in nature is created by atmospheric moisture, dust, haze, pollution that refracts and reflects until it seems to have an undefined direction. Studio lights are diffused with screens that vary the direction of the light rays.

Source is ambient or supplemental: these are often called natural or artificial illumination. These terms suggest the physical origin of the light. Artificial light is often momentary (flash) that exists only for the camera.

Artificial light is added to a scene to modify ambient light to create the desired quality of light. Artificial lighting may be steady state and predictable (hot lights [quartz/tungsten] candle, fire) or momentary and transient (electronic flash). The precise effects of momentary light on the subject can usually only be guessed, or by digital samples or a Polaroid proof can be made. Professional studio flash units have modelling lights which aid the photographer in anticipating overall results.

The sun is the principle continuous light source and may be modified by diffusion or refection and may be supplemented by artificial illumination.

Colour refers to both the subjective and descriptive terms we use to describe light as cool, meaning toward blue, or warm, toward red. When colour is described scientifically, the opposite is true, colder light is redder and hotter light is bluer. The scientific description of light is radiation from a black body at a certain temperature measured in degrees Kelvin. All photographic light sources are measured by their colour temperature.

Visible illumination ranges from deep red to blue-violet (6,800^oK). The hotter a light source is, the bluer the light appears to the eye. Colour temperature is important in B/W photography because films are not equally sensitive to all colours and our subjective responses to the colour of light.

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