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The average spectral sensitivity of the human eye's perception of brightness is called the luminosity function or luminous efficiency function. Lights of different wavelengths are perceived differently by your eyes - bright light and dull light. It is not accurately perceived, but studies point it out as being a good representation.
A recent study conducted by scientists at the Yokohama National University aimed to understand the brightness perception by the human eye. The study was able to gather a new understanding of the importance of light-sensing cells in the retina that process visual information  .
The cells in the study focus were the melanopsin cells which are photoreceptors or a type of photopigment that belongs to a larger family of light-sensitive retinal proteins, and photoreceptors are the cells in the retina that respond to light.
Entirely New Type Of Cells
With various researchers looking into the aspects of vision and its related factors, an entirely new type of cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs) have been found. Unlike the known cells like cones and rods, ipRGCs contain melanopsin - a photopigment sensitive to light  .
Until now it was believed that when light struck the retina, rods and cones (cells that precess light) were the only two kinds of cells that react. But the current study centralised on developing clarity on the role played by the melanopsin cells.
Role Of Melanopsin Cells In The Perception Of Visual Environment
By isolating the functions of the melanopsin cells, the researchers demonstrated the central role it plays in the perception of the visual environment. The study and its results help create a new understanding of the biology of eyes and how the visual information is processed  .
"We've found that melanopsin plays a crucial role in the human ability to see how well-lit the environment is. These findings are redefining the conventional system of light detection that so far has only taken into consideration two variables, namely brightness and the amount of incoming light. Our results suggest that brightness perception should rely on a third variable -- the intensity of a stimulus that targets melanopsin", asserted the researcher Masahiko Yamakawa  .
The role of retinal melanopsin cells and how they contribute to the perception of the brightness of the light was never accurately understood, until now.
Perception Of Brightness
The study explored and pointed out the role of cones and melanopsin and how it combines to perceive brightness. To understand the role played by melanopsin in the detection of light, the signals produced by melanopsin were isolated from cones and rods. The isolation aided in a clearer understanding of the melanopsin signal alone  .
Using a tracking software to measure the study participants' pupil diameters under each visual stimulus further helped in understanding the relationship between brightness perception and the actual visual stimulus intensity on the retina  .
On A Final Note...
The study reached the definite conclusion that melanopsin should not be considered a minor aspect in the human eye's perception of brightness but plays a central role in brightness perception.
-  Yamakawa, M., Tsujimura, S. I., & Okajima, K. (2019). A quantitative analysis of the contribution of melanopsin to brightness perception. Scientific reports, 9(1), 7568.
-  Brown, T. M., Tsujimura, S. I., Allen, A. E., Wynne, J., Bedford, R., Vickery, G., ... & Lucas, R. J. (2012). Melanopsin-based brightness discrimination in mice and humans. Current Biology, 22(12), 1134-1141.
-  Bullough, J. D. (2015). Spectral sensitivity modeling and nighttime scene brightness perception. Leukos, 11(1), 11-17.
-  Science Daily. (2019, August 18). Shedding light on how the human eye perceives brightness [Blog post]. Retrieved from https://www.sciencedaily.com/releases/2019/08/190818101642.htm
-  Chellappa, S. L., Steiner, R., Oelhafen, P., & Cajochen, C. (2017). Sex differences in light sensitivity impact on brightness perception, vigilant attention and sleep in humans. Scientific reports, 7(1), 14215.