Light is a physical phenomenon but is interpreted by psychological processes in our brain. It is, therefore, a bit more complex to measure than other physical processes. Some prerequisites have to be established in order to make objective measurements. One of these is the bandwidth of the light frequencies considered, and this is usually from 400 nm to 700 nm. All of the frequencies contribute to the light energy radiated by the source.

Let us, firrst of all, make clear the kind of light sources we have. The basic division is into two major groups:

• Primary sources (the sun, street lights, tungsten lights, monitor CRTs, LEDs)
• Secondary sources (all objects that do not generate light but reflect it)

We do not apply the same type of measurement when measuring the amount of light radiated by a tungsten globe, for example, and the light reflected by an object. It is not the same if we are analyzing light radiated from a source in all directions, or just in a narrow solid angle. These are some of the reasons we have so many different units of light measurement.

The science that examines all these different aspects is called photometry, and the units defined are called photometric units.

Many different units have been defined by various scientists, depending upon the point of view taken

Luminous intensity (I)

Luminous intensity (I) is the illuminating power of a primary light source, radiated in all directions. The unit that measures this kind of light is the candela [cd].
One candela is approximately the amount of light energy generated by an ordinary candle. Since 1948 there has been a more precise definition of a candela as the luminous intensity of a black body heated up to a temperature at which platinum converges from a liquid to a solid state.

luminous flux

luminous flux is the luminous intensity but in a certain solid angle.The unit for luminous flux is lumens [lm].

Because the sensation of brightness depends on the human eye sensitivity, the luminous flux depends on the wavelength as well. For example, 1 watt of light power with 555 nm color (green) produces approximately 680 lm, whereas all other wavelengths, with the same light power, produce proportionally fewer lumens (see the eye spectral sensitivity curve). It is therefore meaningless to express light power in watts, even if, theoretically, light energy like any other energy can be expressed in watts.

Illumination (E)

• Illumination (E) is the most commonly used term in CCTV, especially when referring to the camera’s minimum illumination characteristics. Illumination is very similar to the luminance except that we are now referring to objects that are secondary sources of light.
• Therefore, the illumination of a surface is the amount of luminous flux on a unit area
• When luminous flux of 1 lumen falls on an area of 1 m2 (square meter), it is measured in lumens per square meter or meter candelas, but it is better known as lux [lx].