Energy Manager

Lighting efficiency series: What you see isn’t necessarily what you get

Light is a funny thing; we are surrounded by it, yet so often what we see is just a minimal fraction of what’s really there.  The human eye itself only responds to light with wavelengths between 380 nm and 750 nm, whereas some laboratory tests have shown a response from 2 nm to 2500 nm. 

September 14, 2009  By Greg Jones

The fact that there is all this light that we humans are unable to perceive raises some skepticism in the lighting community; if we cannot detect it, is it there? Even if we can’t detect it and it is there, should we be paying for it?

Numerous studies have been done to show how the rods and cones in a human eye react to light in different scenarios. It was long thought that cones are responsible for daytime vision and rods are responsible for night time vision. Actually, rods and cones react based on certain frequencies of light; scotopic light controls rod sensitivity, while photopic light controls cone sensitivity.

Scotopic light appears to have a greater level of perception, and therefore brightness, while photopic light is detected on a wider spectrum. Scotopic light is more easily detected by the human eye, while photopic light is best measured with standard light meters. Knowing how these different types of light react with the human eye is important in the lighting business because it enables us to know what types of lighting are best implemented in certain environments. 

Scotopic and photopic light exists in a balance, referred to as the S/P Value. The S/P ratios of commercially available lighting systems varies, and a lighting designer can use this information to provide lighting that is perceived to be of high brightness while minimizing energy use.


A good example would be to consider an industrial space that is using high pressure sodium (HPS) high bay fixtures. These fixtures were presumably selected because they are highly energy efficient and satisfy the foot candle requirement at the floor level. The light level would be verified at time of construction with a standard light meter. However, the poor colour of the light and the low S/P ratio provide a work environment that is very uncomfortable. Even with a high foot candle reading very few people would perceive that the space is bright. 

Using a lighting product with a higher S/P ratio such as a T8 fluorescent product with 5000oK lamps we can get a much better result. The T8 technology is not quite as efficient as the HPS but we can generally reduce the power input by 30-50 per cent and people will perceive that the light is just as bright as it was before. In reality, a light meter reading photopic light will show that light levels have been reduced by 30-50 per cent as well. 

We commonly apply this principle in our designs. There is no point in paying for light that the people can’t see. We should always keep in mind that we don’t always get what we see.

Greg Jones ( is President of Nexstar Lighting.

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