A distinction is made between spectrometry values and photometric values.
 
Spectrometry values are purely technical values that do not relate to the effect of light on human beings. The values are derived from the watt.
 
Photometric values always take into consideration the spectral luminous efficiency curve (V(λ) curve) of the human eye. Consequently, photometric values are linked specifically to human sensitivity. Photometric values are derived from the lumen.
The luminous efficiency curve results from the following relationships: Radiation that is visible to the human eye lies between wavelengths of 380 nm for blue light and 780 nm for red light. The eye is most sensitive in the green area of the spectrum around 555 nm. At longer and shorter wavelengths the eye is less sensitive, which means that a higher radiated power is needed for these wavelengths to achieve an impression of identical brightness. The ratio between the radiated power at 555 nm (1 nm = 10^-9 m) and the radiated powers for the various wavelengths in the visible spectrum is called the spectral luminous efficiency V (λ). A graph of these values is called the V (λ)-curve.