White light has an even representation of all colours in the spectrum i.e. all wavelengths represents an equal amount of energy.
This spectrum is true for the sunlight but artificial light sources such as electrical lamps typically have a differently composed spectrum.
Standard light bulbs with incandescent (i.e. glowing) elements have a spectrum with a bias to the red end of the spectrum, while fluorescent lamps tend to have a very uneven spectrum with peaks in the green and blue areas of the spectrum. This therefore influences the perceived colours of any screen – or other object – we see.
The light we see is measured in a number of different ways and the following is the definition of the units relevant to displays: