Illuminance is the quantity of light, or luminous flux, falling on a unit area of a surface. It is designated by the symbol E. The unit is the lux (lx). One lux equals one lumen per square meter (lm/m²). The illuminance is independent of the direction from which the luminous flux reaches the surface. Some practical examples of common lux levels are given below:

Summer, at noon, under a cloudless sky 100,000 lux
Street lighting 5 – 30 lux
Full moon, on a clear night 0.25 lux

The illuminance is measured by the instrument called illuminance meter or lux meter. It uses a photocell that must be corrected by a special filter in order to match the V(λ) sensitivity curve of the human eye.

Constant Light Output (CLO) is a system to compensate for the depreciation of luminous flux and to avoid excess lighting at the beginning of the installation’s service life. In fact, the luminous depreciation that takes place over time must be taken into account to ensure a predefined lighting level during the luminaire’s useful life. Without remote management, this simply means increasing the initial power upon installation in order to make up for luminous depreciation. By precisely controlling luminous flux, one can control the energy necessary so as to reach the required level – no more, and no less – throughout the luminaire’s life.

Luminance is the concept for the luminous intensity emitted per unit of area of a surface in a specific direction. It measures the light as perceived by the human eye. The visibility of all surfaces and objects being in our field of vision is due to their luminance, while illuminance levels are in fact not perceived. The unit is the candela per square metre (cd/m2).

Surfaces with different reflecting properties will have the same illuminance, but different luminance.
The main criterion requested by the standard for road lighting applications is the luminance of the road surface. The good knowledge of the light reflection characteristics of the road pavements is consequently of main importance for achieving an accurate lighting design

Luminous flux is the total quantity of light energy emitted per second by a light source. It is designated by the symbol F and is expressed in lumens. Luminous flux measurement of artificial light sources is carried out within a perfectly diffusing white sphere, called an Ulbricht sphere. This equipment measures the total amount of lumens emitted by the tested light source by comparison with a calibrated lamp.

Luminous intensity is the concept for the concentration of light in a specific direction, radiated per second. It is designated by the symbol I. The unit is the candela (cd). The luminous intensity can be defined as: The luminous flux in a certain direction, radiated per unit of solid angle.

Intensity is not a function of distance
The photometrical performances of a luminaire are derived from the luminaire’s measured luminous intensity distribution. These measurements of luminous intensity are carried out in specialised laboratories, using goniophotometers.

A selective dynamic lumen output allows the luminous intensity to be adjusted according to the traffic density. Dimming profiles can be programmed to provide just a little light at night when it is quiet and more light during rush hour. This is done in accordance with international lighting standards.

Spectral distribution represents the distribution of the energy radiated at different wavelengths in the visible part of the spectrum. It can be defined for every type of light source. For example, as illustrated below, the spectral distribution of a high-pressure sodium vapor lamp has the main part of energy radiated in yellow-orange colors while the spectral distribution of a metal halide lamp has its energy radiated in all parts of the spectrum resulting in white light emission.