This weighting factor, or luminous efficacy (V λ), allows for conversion of Radiant Flux to Luminous Flux at any wavelength. The eye's nighttime sensitivity, called scotopic vision, shifts toward the blue end of the visible, peaking at 507 nm and falling to 10 -4 at 340 and 670 nm. This constitutes the range of daylight sensitivity, or photopic vision. The sensitivity of the eye peaks at 555 nm and falls off to approximately 10 -4 at 380 and 750 nm. It is a weighted average because the human eye does not respond equally to all visible wavelengths. Thus, luminous flux is a weighted average of the Radiant Flux in the visible spectrum. More specifically, it is energy radiated over wavelengths sensitive to the human eye, from about 330 nm to 780 nm. Luminous Flux (Φ v) is energy per unit time (dQ/dt) that is radiated from a source over visible wavelengths. Units for other quantities in photometry contain the lumen, such as the lux (lumens/meter 2) Comparison of luminous performance of light sources The luminous efficacy is 1 at that frequency.Ī typical 100 watt incandescent bulb has a luminous flux of about 1700 lumens. The luminous flux is the part of the power which is perceived as light by the human eye, and the figure 683 lumens/watt is based upon the sensitivity of the eye at 555 nm, the peak efficiency of the photopic (daylight) vision curve. Luminous flux in lumens = Radiant power (watts) x 683 lumens/watt x luminous efficacy In terms of radiant power (also called radiant flux) it can be expressed as: The abbreviation is lm and the symbol is Φ v. It can be defined as the luminous flux emitted into unit solid angle (1 sr) by an isotropic point source having a luminous intensity of 1 candela. It is an SI derived unit based on the candela. The lumen is the standard unit for the luminous flux of a light source. The standard definition is as follows: Radiometry and photometry This power must be factored by the sensitivity of the human eye to determine luminous flux in lumens. The radiant power is the total radiated power in watts, also called radiant flux.
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