Perceived Brightness

Index

Vision concepts

Reference
Williamson & Cummins
p177-178
 
HyperPhysics***** Light and Vision R Nave
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Efficacy Tables

Luminous Efficacy

The curves represent the spectral luminous efficacy for human vision. The lumen is defined such that the peak of the photopic vision curve has a luminous efficacy of 683 lumens/watt. This value for the photopic peak makes the efficacy the same as the scotopic value at 555 nm.

The scotopic vision is primarily rod vision, and the photopic vision includes the cones.

The response curve of the eye along with the spectral power distribution of a luminous object determine the perceived color of the object.

Examples
Why is a candle flame yellow?
Index

Vision concepts

Reference
Williamson & Cummins
p173
 
HyperPhysics***** Light and Vision R Nave
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Photopic Vision

The Photopic efficacy curve was extrapolated from testing done on 'Standard Observers'. This was done by taking a person with normal vision, and having them compare the brightness of monochromatic light at 555 nm, where the eye is most sensitive, with the brightness of another monochromatic source of differing wavelength. To achieve a balance, the brightness of the 555 nm source was reduced until the observer felt that the two sources were equal in brightness. The fraction by which the 555 nm source is reduced measures the observer's sensitivity to the second wavelength. This exercise is repeated through many wavelengths and many observers. The average of the results gives us the relative sensitivity of the eye at various wavelengths. In 1924, the International Commission on Illumination adopted the "relative sensitivity curve for the C.I.E. Standard Observer".

Each wavelength has a relative value for the Standard Observer's sensitivity, the luminous efficacy at that wavelength, Vλ. The value of Vλ is designated as unity at 555 nm and decreases to zero at the ends of the visible spectrum. This is associated with the daylight vision of the human eye, also known as photopic vision. In lowlight conditions, the efficacy curve shifts toward the blue end of the spectrum due to the sensitivity of the eye. Chemical changes in the eye at night shift our vision to the scotopic range. This differentiation between light and dark vision is caused by the activity of the rods and cones in the retina, and their sensitivity to light.

At 555 nm, this efficacy translates to a luminous flux of 683 lumens/W, and thus a fraction of that value at wavelengths to either side of the visible spectrum. This value is derived from the definition of the candela directly.

Index

Vision concepts

Reference
Williamson & Cummins
p173
 
HyperPhysics***** Light and Vision T Beaulieu, R Nave
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Scotopic Vision

The Scotopic Efficacy curve was established in the same manner as the photopic curve. It's sensitivity is shifted however to peak at 507 nm, and decreases in proportionally the same manner as the photopic curve. This results in the scotopic curve reaching a relative value of zero sooner in the visible spectrum than the daylight curve. As a consequence of this fact, our nighttime vision does not see red!

The scotopic efficacy curve is assigned a value of unity at 507 nm, and is represented by the symbol Vλ. To determine spectral Luminous Efficacy, the scotopic efficacy value, Vλ , must be multiplied by 1700 lumens per Watt. This value was adjusted from 1754 to allow both curves to obtain the same value of 683 lumens/W at 555 nm. So, a source we see with our dark adapted vision at 507 nm produces 1700 lumens for every Watt radiated, and any other wavelength produces a fraction of that value based on the efficacy curve.

Rods do not see red!
Index

Vision concepts

Reference
Williamson & Cummins
p173
 
HyperPhysics***** Light and Vision T Beaulieu, R Nave
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