Basic lighting technology

Luminous flow
Indicates the amount of light emitted by a source in a time unit and depends on the radiating energy emitted by the light sources and by the sensitivity curve of the eye.
It is measured in lumens (lm) using special laboratory tools (Ulbricht sphere).

Average illumination
The ratio of the luminous flow reaching a surface to its size. It is measured by means of a luxmeter and is expressed in lux (lx = lm/m2).
The levels of illumination, often considered as the only important parameter of a system, are only a part of the achieved effects. Each individual may prefer different levels of illumination, and the same individual may prefer more or less lux in his or her environment depending on passing moods and physical conditions. While 100 lux is enough to illuminate a room in which ordinary activities take place (diffused illumination), more complicated activities, such as applying makeup or cooking, require 200 to 500 lux (focused illumination); 500 to 1,000 lux (spot illumination) are required to draw attention to a painting, an object, or a detail.
The human eye is able to distinguish forms with as little as 5 lux, to perceive forms and colours with at least 30 lux, to read and work with 150 lux or more. Light outdoors on a sunny day can be equivalent to 200,000 lux or more.
The UNI 10380 Standards can be applied to make an accurate and safe choice. If only projectors are used, the technical data provided in the catalogues can be checked. A simple performance chart is often supplied for every light source with built-in reflector and for actual projectors, specifying: the light cone, the diameter of the light spots, and the levels of illumination obtained at different distances between the source and the illuminated space.

Lighting efficiency
The ratio of the emitted luminous flow to the power input of the source. Measured in lumen/Watt (lm/W).
Allows to compare different types of artificial light sources in terms of running costs.
Luminous intensity
CIndicates the light emission in relation to its direction. For a spot source, intensity is calculated as the ratio of the luminous flow issued in a tiny cone to the size of the solid angle of the cone, expressed in steradiants.
It is measured in Watts (w).
If the luminous intensities emitted by an appliance in space are represented with vectors issuing from the centre (actual or conventional) of the source, the ends of these vectors are located on a surface enclosing a volume known as "photometric solid".
In lighting technology terms, a lamp or a lighting appliance is defined by the distribution of the emitted intensities (i.e. luminous flow from the photometric solid) in space and by the geometry of the surface.
Catalogues usually show both photometric curves resulting from the intersection between the photometric solid and two normal surfaces - longitudinal and transversal with respect to the lamp.

Percent reflectance
The percent ratio of the luminous flow reflected by a surface to the total luminous flow falling onto it. Depends on the type, colour, and texture of the reflecting surface.
Average luminance in a given direction (W/m2)
Luminance L indicates the amount of light that comes from a surface in a given direction vs. the size of the concerned surface. It is measured in Watts per square metre (W/m2) and is the ratio of the luminous intensity emitted by a surface in a given direction to the apparent size of such surface. In practice, it indicates the feeling of luminosity received from a light source.
Luminance is associated with the state of adjustment of the eye, as well as with the concept of intensity of the visual sensation and any discomfort caused by the glare.

Visual task
This term conventionally indicates the objects, the details, and their immediate background that need to be observed to carry out a given activity.
Luminance contrast (C)
Luminance contrast is defined by the following ratio:
where "Ld" is the luminance of the detail (for instance the letters of a document for reading) and "L" that of its background (the sheet and its immediate surroundings). "L" is generally also the value to which the eye has adjusted.
The presence within the field of vision of an area of very high luminance compared to the average luminance of the field (adjustment luminance) reduces the visual capacity of an individual. This phenomenon is known as glare and occurs, for example, whenever we encounter a car with high beams at night or whenever light sources fall into our area of normal observation, perhaps due to reflection off glossy surfaces.
Two types of glare exist:
- disability glare, which causes immediate impairment of the visual functions,
- discomfort glare, which occurs as a sense of visual discomfort that does not always cause serious visual discomfort, but in time results into visual fatigue, stress, difficulty of concentration, reduced attentiveness, increased propensity to make errors, and reduced performance.