Due to the great adaptability of the human sight, photometric parameters such as illuminance, luminance or reflectance are difficult to estimate with the naked eye. Light must therefore be measured in units corresponding to the brightness sensitivity of the human eye.

This is commonly referred to as Photometry. Photometric measurements are restricted to the visible spectrum from 380 to 780 nm. It is thus considered that the sensitivity of the human eye depends on the optical radiation of the wavelength. The values of photometric quantities are, therefore, the impressions of brightness as we truly perceive them.

In contrast to photometry, radiometry simply indicates the total radiated power of different wavelengths, without reference to human perception.
Symbols for photometric and radiometric quantifies are distinguished with the subscript "v" for visual and "e" for energy, respectively.

 
Luminous flux (lumens)
The luminous flux is the electromagnetic radiation flux measured with the V (lambda) curve (spectral light sensitivity of the human eye) and represents the luminous output emitted by a light source or reflected by a body. The luminous flux describes the total light output of an emitting source independent of the direction of emission, and represents the unit of power in the photometric measurement system.

Unit of Measure: lm (lumen)
The corresponding radiometric quantity is the radiation power in (W)

 
Luminous intensity (candelas)
The luminous flux (lm) emitted in a given direction at a solid angle, based on this solid angle (sr). It represents the directionality of the emitted light flux. The unit is lm / sr and is usually specified as candelas (cd).
Unit of measure: cd (candelas)

1 cd = 1 lm / sr (lumen / steradian)
The corresponding radiometric quantity is the radiant intensity in (W/sr)

 
Luminance
The luminance describes the luminous flux per area and solid angle. The human eye perceives luminance differences as differences in brightness. The luminance represents the measure of the "visually perceived brightness".

Unit of Measure: cd/m² (candela / square meter)
The corresponding radiometric quantity is radiance in (W/sr m2)

 
Illuminance or light intensity (lux)
It measures the luminous flux reaching a surface. If a luminous flux of 1 lm is uniformly projected on a 1 m² area, the corresponding illuminance would be 1 lx.

Unit of Measure: lx (lux) 1 lx = lm/m2
The corresponding radiometric quantity is the irradiance in (W/ m²)