Mesopic vision

Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions.[1] Mesopic levels range approximately from 0.01 to 3.0 cd/m2 in luminance. Most nighttime outdoor and street lighting conditions are in the mesopic range.[2]

Human eyes respond to certain light levels differently. This is because under high light levels typical during daytime (photopic vision), the eye uses cones to process light. Under very low light levels, corresponding to moonless nights without artificial lighting (scotopic vision), the eye uses rods to process light. At many nighttime levels, a combination of both cones and rods supports vision. Photopic vision facilitates excellent color perception, whereas colors are barely perceptible under scotopic vision. Mesopic vision falls between these two extremes. In most nighttime environments, enough ambient light prevents true scotopic vision.

In the words of Duco Schreuder:

There is not one single luminescence value where photopic vision and scotopic vision meet. [Rather,] there is a wide zone of transition between them. Because it is between photopic and scotopic vision, it is usually called the zone of mesopic vision. The reason that the zone of mesopic vision exists is because the activities of neither cones nor rods is simply switched 'on' or 'off'. There are reasons to believe that the cones and the rods both operate in all luminescence conditions.[3]

As a result of guadually switching from cones to rods in processing light, a number of visual effects occur:[4]

  • The rods have a different wavelength sensitivity, causing blue objects to appear brighter and red objects to appear darker. This is called the "Purkinje shift".[4]: 7 
  • Color appears desaturated and hues change, drifting towards a dull purple.[4]: 8 
  • Spatial acuity decreases linearly with log-luminance. A varying "noise" slowly becomes more prominent.[4]: 8 

Cinematographers intentionally emulate mesopic effects to make scenes look darker than a display can actually achieve.[4]: 1 

  1. ^ Stockman, A.; Sharpe, L. T. (2006). "Into the twilight zone: the complexities of mesopic vision and luminous efficiency". Ophthalmic Physiol. Opt. 26 (3): 225–39. doi:10.1111/j.1475-1313.2006.00325.x. PMID 16684149. S2CID 6184209.
  2. ^ CIE Publication No. 41. Light as a true visual quantity: principles of measurement. 1978.
  3. ^ Schreuder, D. (2008). Outdoor Lighting: Physics, Vision and Perception. Berlin & New York: Springer. p. 237. ISBN 9781402086021.
  4. ^ a b c d e Jacobs, David E.; Gallo, Orazio; A. Cooper, Emily; Pulli, Kari; Levoy, Marc (8 May 2015). "Simulating the Visual Experience of Very Bright and Very Dark Scenes" (PDF). ACM Transactions on Graphics. 34 (3): 1–15. doi:10.1145/2714573.