Light is an electromagnetic radiation which travels at very high speed (300.000 km/s) in the form of waves.

The distance between the individual waves (wavelength) determines the colour of light. Wavelengths (colours) that are visible to the human eye range from 300 nm (1 nm is a millionth parth of a millimeter, so 0.000000001 m) which is violet light, through blue, green, yellow and orange to red, which has a wave length of 800 nm. There are also wavelenghts outside this range, but they can nott be seen by the human eye (UV light, infra-red light, roentgen radiation, etc.).

We see colours because the surface of an object reflects a certain wave length, which can be seen by the eye. In other words, a red object absorbs all wavelengths except red (800 nm), which reflects and is observed by the eye. On the other hand, white light is a combination of all wavelengths, so a white surface reflects all wavelengths, while black absorbs all wavelengths.

The colour of light is expressed as temperture (degrees Kelvin). It is determined by heating up a metal plate until it starts emitting light. The light that is emitted at the lowest temperature is red, and has a temperature of about 3000oK. When the metal is heated up more, it starts emitting the other colours, until it finally emits blue light, which has a temperature of approximately 7000oK. Day light has a temperature of approximately 6500oK.

The intensity of light is expressed as lux, which is measured as lumen per m2, where lumen is a measurement of the intensity of the radiation. Standard lux meters simply measure the intensity of the radiation in the range between 300 and 800 nm (visual light).

A lux meter registers the intensity of blue light in the same way as the intensity of red light (by the amount of radiation per m2 of surface).
The human eye as well as the chicken eye observe blue and green light (light with high temperature, often called cool white light) as more intense than red light (low temperature light, often called warm light) of the same intensity. That is why lux meter sometimes give a different reading than what is observed by the human eye especially if the color spectrum of the light is different.