The photopic luminous efficiency function, written V(lambda), which weights radiant power at each wavelength to give photometric quantities. It peaks at 555 nm and falls toward zero at about 380 and 780 nm, so a watt of deep red or violet light counts for very few lumens. Its night-vision counterpart, V-prime, peaks at 507 nm. Every lumen, lux and candela is defined through this curve.
The meter's V-lambda correction is poor in the deep blue, so it's reading those royal blue LEDs about 15 percent high.
Also heard as
- V(lambda)
- luminous efficiency function
- photopic curve
- eye response curve
Related terms
Photopic
Describes daylight vision, when cones dominate, providing color and sharp detail at luminances above a few cd/m squared.
Scotopic
Describes night vision at very low light levels, when only rod cells respond and color disappears.
Luminous flux
The total amount of visible light a source emits in all directions, weighted for the eye's sensitivity and measured in lumens.
Radiometry
Measurement of optical radiation in physical energy units such as watts, without weighting for human vision.