Mesopic vision

Mesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations.^[1] Mesopic light levels range from luminances of approximately 0.001 to 3 cd m⁻². Most night-time outdoor and traffic lighting scenarios are in the mesopic range.^[2]

Humans see differently at different light levels. This is because under high light levels typical during the day (photopic vision), the eye uses cones to process light. Under very low light levels, corresponding to moonless nights without electric lighting (scotopic vision), the eye uses rods to process light. At many night-time levels, a combination of both cones and rods supports vision. Photopic vision facilitates excellent color discrimination ability, whereas colors are indiscriminable under scotopic vision. Mesopic vision falls between these two extremes. In most night-time environments, there is enough ambient light at night to prevent true scotopic vision.

The effect of switching from cones to rods in processing light is called the "Purkinje shift". During photopic vision, people are most sensitive to light that is greenish-yellow. In scotopic vision, people are more sensitive to light which would appear greenish-blue.

The traditional method of measuring light assumes photopic vision and is often a poor predictor of how a person sees at night. Typically research in this area has focused on improving street and outdoor lighting as well as aviation lighting.

Prior to 1951, there was no standard for scotopic photometry (light measurement); all measurements were based on the photopic spectral sensitivity function V(λ) which was defined in 1924.^[3] In 1951, the International Commission on Illumination (CIE) established the scotopic luminous efficiency function, V'(λ). However, there was still no system of mesopic photometry. This lack of a proper measurement system can lead to difficulties in relating light measurements under mesopic luminances ^[4] to visibility. Due to this deficiency, the CIE established a special technical committee (TC 1-58) for collecting the results of mesopic visual performance research.^[5]

Two very similar measurement systems were created to bridge the scotopic and photopic luminous efficiency functions,^[6][7][8] creating a unified system of photometry. This new measurement has been well-received because the reliance on V(λ) alone for characterizing night-time light illumination can result in the use of more electric energy than might otherwise be needed. The energy-savings potential of using a new way to measure mesopic lighting scenarios is significant; superior performance could in certain cases be achieved with as much as 30 to 50% reduction in the energy use comparing to the high pressure sodium lights.^[9]

Mesopic weighting function

The mesoscopic weighting function at wavelength $\lambda$ can be written as the weighted sum^[10]

$V_m(\lambda) = (1-x) V'(\lambda) + x V(\lambda)$,

where $V(\lambda)$ is the standard photopic weighting function (peaking at 613 lm/W at 555 nm) and $V'(\lambda)$ is the scotopic weighting function (peaking at approx. 1700 lm/W at 507 nm). The parameter $x$ is a function of the photopic luminance $L_p$. Various weighting functions are in use, for blue-heavy and red-heavy light sources, as proposed by two organizations, MOVE and Lighting Research Center (LRC).^[10] Some values for $x$ are shown in the table below.

$L_p$	Blue-heavy		Red-heavy
(cd/m^2)	MOVE	LRC	MOVE	LRC
0.01	0.13	0.04	0.00	0.01
0.1	0.42	0.28	0.34	0.11
1.0	0.70	1.00	0.68	1.00
10	0.98	1.00	0.98	1.00

References

1. Stockman A, Sharpe LT (2006). "Into the twilight zone: the complexities of mesopic vision and luminous efficiency". Ophthalmic Physiol Opt 26: 225–39. PMID 16684149.
2. CIE Publication No. 41. Light as a true visual quantity: principles of measurement. 1978.
3. "Mesopic Vision and Photometry" (PDF). Retrieved 2011-06-09.
4. CIE Publication No. 81. Mesopic photometry: history, special problems and practical solutions. 1989.
5. Yandan Lin, Dahua Chen, Wencheng Chen. "The significance of mesopic visual performance and its use in developing a mesopic photometry system", Building and Environment, Volume 41, Issue 2, February 2006, Pages 117–125.
6. Rea M, Bullough J, Freyssinier-Nova J, Bierman A. A proposed unified system of photometry. Lighting Research & Technology 2004; 36(2):85.
7. Goodman T, Forbes A, Walkey H, Eloholma M, Halonen L, Alferdinck J, Freiding A, Bodrogi P, Varady G, Szalmas A. Mesopic visual efficiency IV: a model with relevance to nighttime driving and other applications. Lighting Research & Technology 2007; 39(4):365.
8. "Driver Response to Peripheral Moving Targets under Mesopic Light Levels" (PDF). Retrieved 2011-06-09.
9. "Mesopic Street Lighting Demonstration and Evaluation Final Report for Groton Utilities Groton, Connecticut Prepared by Peter Morante, Lighting Research Center Rensselaer Polytechnic Institute" (PDF).
10. Photopic and Scotopic lumens - 4: When the photopic lumen fails us