Often task lighting refers to office lighting, where the task light is used to increase illuminance

Illuminance

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...

 on the reading area. However, the illuminance level is not the only factor governing visibility. Contrast

Contrast (vision)

Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 is also important, and a poorly positioned light source may cause contrast reduction, resulting in loss of visibility. Therefore, it can be argued that the most important purpose of task lighting in the office is not increasing illuminance, but improving contrast.

Different strategies for office task lighting exist. The three main approaches are:

• Localised average lighting, where a lamp supplies both ambient light and task light
• Freely adjustable task light
• Asymmetric task light, where the lamp is placed at the side of the work area

There are also other approaches to office task lighting, for example under-shelf luminaires.

Other instances of task lighting are in machinery, where a specific work area needs illumination, and in workshops, where a task light may illuminate the actual working area. Special instances of task lighting are examination and operation lights for medicine

Medicine

Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 and surgery

Surgery

Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...

, as well as the dentist

Dentist

A dentist, also known as a 'dental surgeon', is a doctor that specializes in the diagnosis, prevention, and treatment of diseases and conditions of the oral cavity. The dentist's supporting team aides in providing oral health services...

's lamp. The actual task may range from very small up to about as far as you may reach with your hands or available tools. Lighting of larger areas is beyond the scope of task lighting.

Localised average lighting

Localized lighting consists of a luminaire that provides ambient light as well as task light. Often it is an uplighter with a light source that is directed downward. It is intended to be mounted immediately over the workplace, and it can be either hung from the ceiling, mounted on the desk or a dividing wall, or it can be a free-standing floor lamp.

Freely adjustable task light

The main feature of the freely adjustable task light is evident; one may adjust it freely at any whim or to suit one's needs. The lamp presents few limits to how one may position or orient the light. A freely adjustable lamp may include means for glare control, as a honeycomb or parabolic louvre that restricts the light output angle.

Asymmetric task light

The asymmetric task light is intended to be placed at the side of the actual task. The luminaire directs the light obliquely over the desk, with the highest illuminance typically about 1' to 1½' to the side of the lamp head. It mostly has an arm system that holds the lamp head horizontally irrespectively of the arm movement - a parallel arm. Asymmetric lamps often cause more reflected glare than other lamps. In workplaces where people use different table heights, an asymmetric lamp may cause direct glare due to its absence of means for glare control (ref:1).

Contrast reduction

Contrast reduction in the office workplace refers to reading objects having decreased contrast compared to an estimated ideal contrast. If a lamp is placed so that printed letters reflect some of the light, their contrast against the paper background will decrease. This happens when a light source is reflected as in a mirror from the print into the eyes of the observer. A poorly placed lamp may render text illegible, regardless of illuminance level.

Contrast reduction by veiling glare: In the left picture, the lamp is positioned where it does not cause glare, in the right picture it is placed where it produces maximum reflected glare.