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The optical pathway in a human eye is connected to the visual cortex (part of the brain which mediates the sense of light) via a nervous system. The nervous system comprises photo-receptors of two types – 1) Rods; 2) Cones (named on basis of the shapes of these nerve receptors). These photo-receptors, in turn, contribute to three types of visions with image forming functions and are important for normal daily function and life quality.

Scotopic (Rod) Vision-When the field luminances lie between 10⁻⁶cd/sqM to 10⁻²cd/sqM. This is a vision in the darkness. The world is grey and there is no sensation of colour.

Mesopic Vision- Most important vision from a Lighting Designer’s point of view. This occurs when the field luminance is raised upwards from 10⁻²cd/sqM to 10cd/sqM. As the luminance moves upward the luminosity of red increases more strongly than that of the blues due to changing contributions of the Rods and Cone receptors. This is known as the Purkinje phenomenon and is considered as an important aspect for designing effective road lighting, as it takes into account the luminance concept as well as the neurological aspects of road lighting.

Photopic (Cone) Vision- This is a 

daylight vision,i.e. when the luminance 

is above 10cd/sqM. Photopic vision is coloured.

The third type of photoreceptor was discovered (first in mice in 2002) and then in humans and primates in 2005. These receptors were found in mammalian retina and are called Intrinsically Photosensitive Retinal Ganglion Cells -3) ipRGCs. Melanopsin, a type of photopigment and belonging to a family of opsins (light-sensitive retinal protein)was found to form a mesh-work in ipRGCs  These photo-receptors were found to be sensitive to the absorption of short-wavelength blue in the visible region, peaking at around 480nM (deep blue). Unlike the classical photo-receptors Rods & Cones which contribute to the formation of images, the ipRGCs are non-image forming. These cells contribute to sub-conscious non-image functions. They serve as primary conduits through which photic information is relayed from the retina to non-image forming visual centres of the brain, ie direct communication to the circadian system which impacts mental and physical he­­­­­­­­­alth.­­­­ Extensive studies show that blue light after passing through our cornea and lens excites the melanopsin in ipRGCs with absorption sensitivity peaking at 482nm (deep blue region). Photo reversal of bleaching (found to be most effective with blue light) takes place if the duration of exposure to this wavelength is either excessive or subject to shorter exposures over prolonged periods. It is a photo-chemical reaction which augments the capability of rhodopsin molecules (photopigments contained in the rods) to absorb photons in large numbers. This may lead to a cascading effect allowing the molecules to reach the critical number of photons required to induce damage in the retinal cells. In view of the crystalline nature of lens in infants and children, the penetration of blue light is high. Thus this type of retinal damage is more pronounced in infants and children.

Digital devices such as smartphones, iPads, e-readers (kindles), and LED lights did not exist a decade ago. Over the last five years or so there has been a galloping increase in use of these devices. The LED which contains about 35% of harmful blue light has nowadays become a dominant technology for backlighting in all digital devices. But what is more disturbing is the gradual replacement of traditional light sources by more energy-efficient LED source.

After the discovery of a blue LED at the end of the twentieth century, extensive research started in order to commercialise the use of LEDs so that it can be used for general lighting. The first step was to get white light which was made possible by the use of yellow phosphors. Despite great developments in phosphor technology, it has been observed that over a course of time bleaching of phosphors (particularly in high power LEDs) causes a shift in colour temperature. The blue light is not absorbed as effectively and, in turn, the blue light increases over time. Of late the city of Calcutta (Kolkata) has witnessed an invasion of LED lights in the public domain. Change to a new concept is good, but not at the cost of standard norms of good lighting practice. Here almost all streetlight poles and guard rails of flyovers are getting adorned with blue and white strips of LED. The blue is deep blue i.e. in the shorter wavelength. As a driver, I have experienced a distracting and painful nature of drive through a maze of white and blue (mostly long stretches of blue) on Calcutta’s longest flyover from Race Course to Parama island at Eastern Metropolitan Bye Pass. It is not only distracting it gobbles the traditional streetlights on the flyover on which a motorist need to focus on safe driving. To add to the woes most of the strips are deep blue which adversely affects the ocular health of drivers frequenting such stretches.

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Biswajit Sengupta

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  • Top Member

    I believe that's where I read it. Very good information and thank you for sharing it here in the SE community. 

  • Top Commentor

    Yes, Terri, this is my original content. I published it in my blog long back. You will find my city Calcutta mentioned in the article. Maybe you have come across it in my blog.

  • Top Member

    Is this your original content? I believe I've read this before. 

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