What is light?

A distinction is made between radiometric and photometric quantities. Radiometric values are purely technical figures that do not take the effect on people into account. The quantities are derived from the unit of watts.
 
When calculating photometric quantities, the spectral brightness sensitivity curve (V (λ) curve) of the human eye is always included in the equation. As a result, the photometric values are specifically tuned to suit human perception. Photometric quantities are derived from the unit of lumen.
 
The brightness sensitivity curve is made up of the following dependencies: the radiation visible to the human eye lies between wavelengths of 380nm (blue) light and 780nm (red). The eye is most sensitive in the yellow-green spectrum at 555nm. The sensitivity of our sensory organ, the eye, is less for longer and shorter wavelengths. This means that to maintain the same impression of brightness, a greater radiant power is needed.

The relationship between the radiant power at 555nm (1nm = 10-9m) and the radiant powers for the different wavelengths of the visible spectrum is called the degree of sensitivity to spectral brightness V (l) and its graph is called the V (l) curve.