Lighting Metrics: Brightness

Luminance (Photometric Brightness)

The light that we can actually “see,” brightness can be measured as the light leaving a lamp, or the light reflecting from an object's surface. It ais measured in footlamberts or candelas/sq.ft. (English) or candelas/square meter (metric).

Brightness, or photometric brightness or luminance, can be measured in footlamberts or candelas/square inch or square meter as either the light emitted by a lamp or reflected from a surface. It is therefore a function of the output of the light source and the reflectance of surfaces that are lighted.

Brightness can be used for a variety of purposes, from producing a sense of drama to creating sparkle and glitter elements in a space. For the common purpose of getting work done, the brighter a task is, the easier it is to see and the lower the amount of light that is required. Too much brightness that is uncontrolled, however, can produce glare that either impair or prevent a desired task from being performed. Glare can be described as direct or reflected glare, which can then result in discomfort or disability.

Direct glare comes straight from the light source. Reflected glare shows up on the task itself, such as a computer screen.

Discomfort glare does not prevent seeing but makes it uncomfortable. Disability glare prevents vision. A popular example is holding a glossy magazine page at a certain angle; a veiling reflection results, impairing our reading of the page. A very common example of both discomfort and disability glare is computer screens, vertical surfaces that reflect images of lighted fixtures or other light. 

Brightness Ratios

Brightness ratios in a space can affect how it is perceived. While high ratios of bright to dark in the space can produce contrast or a sense of drama, it can also be visually fatiguing during transient adaptation, which describes the eye adapting to changes in brightness. This can reduce productivity and can even be hazardous.

Uniform light and brightness levels across a space is often desirable but may also be boring; in applications demanding a more visually interesting environment, sparkle elements, color and/or other methods can be employed to create visual interest without causing fatigue.

The right approach is determined by the application; the IESNA has recommended brightness ratios for a wide range of environments in its Lighting Handbook.