Luminous Flow

The luminous flow is the visual size which corresponds to the luminous power emitted by a source. One should not confuse luminous flow with the radiant energy flux emitted by this same source: indeed, according to its wavelength, an electromagnetic radiation produces a visual feeling of very variable intensity, even not of feeling of the whole if one is located in-outside field of the visible light.

The unit IF of luminous flow is the lumen, symbol lm (of the Latin word meaning light ). It corresponds to the flow emitted in a solid angle of 1 Stéradian by a source whose uniform intensity is worth 1 Candela.

To take account of the sensitivity different from the human eye with each wavelength of the radiation, one corrects the power of the electromagnetic Rayonnement by a calculation of integral holding account, for each wavelength of the visible field, of the sensitivity of the eye. For that, one uses the standardized functions of spectral sensitivity v (λ) and v' (λ) defined by the tables below. These functions become null for the infra-red or the ultra-violet which are located out of the visible field.

With a radiant energy flux of one Watt monochromatic radiation wavelength 555 Nm (green-yellowish, where the maximum is reached: v (555 Nm) = 1), corresponds a luminous flow of 683 lumens, while with the same power luminous flow will be less half (344 lumens) for a green (510 Nm) or an orange (610 Nm): v (510) = v (610) = 0,503.

Calculation for a polychromatic light is the following: $F = K_M \ int_ {380 \ Nm} ^ {780 \ Nm} F_ {E, \ lambda} \, v (\ lambda) \, D \ lambda$ with $K_M$ = 683 lm/W

where $F_ {E, \ lambda}$ is the spectral concentration of radiant energy flux (radiant energy flux for a very narrow band wavelength given $\ lambda$ ), which is expressed in Watts/meter.

It is what explains the difference in luminous output between the various modes of lighting, according to whether the power is diffused in the zone close to the maximum of sensitivity (case of a fluorescent Lampe, of a Electroluminescent diode) or on the contrary far from this zone and even out of the visible one (case of a Ampoule to traditional incandescence or halogenous).

relative Apparent brightness. photopic Vision (diurnal)

relative Apparent brightness. scotopic Vision (twilight)

Anglo-Saxon measuring units

The Measuring unit of luminous flow is the spherical candle power (spherical candle) (1 cd×4π.sr ≈ 12,566 lm)

See too

Random links: