07 Sep Shedding Light: How do your eyes affect the quality of light? – monthly column featured in Sparks Electrical News (Crown Publications)
I appreciate that I am older in years, which is perhaps the reason why time seems to flash by so quickly. What a thought that it is almost time to get the Christmas shopping done!
As I ended off my column last month, I said that this month I would focus on the subject known as Equivalent Visual Efficiency (EVE) which has its foundation in the spectral quality of light. Of course, LED lighting has the highest spectral quality characteristics of all light sources.
So, what does this mean? How does it benefit a lighting design for a project?
Let’s think about a typical office environment for a moment. The occupants are probably varied, from different backgrounds, different lifestyles and different ages, which also means differing levels of ocular health. If I asked you to give me the typical human field of vision, would you be able to tell me? Probably not, and that is nothing derogatory. The typical human field of vision is 120 degrees. See the diagram.
However, because we have binocular vision, although each eye has a slightly bigger range, the interesting fact is that our brain combines the images received in each eye, giving us a 120-degree field of vision. As you would have noticed in the diagram, we have lateral as well as vertical fields of vision.
What is the significance of this? When we are exposed to light, it has a greater impact on us than the light measured with a typical photometer which most of us know as a lux meter. It measures for an equivalent visual field of only two degrees at V(l). V(l) represents photopic or daylight vision.
Now we get to the really interesting part. When LED light sources are used, they produce short wavelength light which is, of course, blue light. Therefore, it promotes a V(l) or scotopic response from our vision or, put into greater perspective, a mesopic response is prompted. The end result is that our eye and therefore human vision has been found from the available research proven to function best when the size of the pupil is between 2 and 2,8 mm.
Cast your mind back to an earlier section in this article; a typical office population has occupants of different ages and therefore different ocular health as well.
Before I continue, it is very important to be aware that the illuminance levels in the standards are typically based on the young human eye from age 20 to 30 years. As the human eye ages, various ocular ailments can become evident. When that happens, higher illuminance levels are required to have the same level of visual acuity (the ability to see clearly and all detail). Therefore, we need a method to calculate the correct illuminance value for the space based on the assumed average age mix of the occupants.
The method that we apply is referred to as the Equivalent Visual Efficiency (EVE) factor. This takes the spectral quality and characteristics of the light source and especially LED light sources, which are spectrally rich.
Next month, I will continue this discussion and explain how we actually do the calculation and how we can also include this in Relux desktop when doing a lighting design.
BHA SCHOOL OF LIGHTING – 7 September 2021
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Copyright © 2021 BHA School of Lighting, All rights reserved.
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