The Science Behind Good Lighting

Rodney Heller LC, CLEP, IES

When specifying lighting, you want to make the right choice. This is a huge investment for your company. I am not referring to the actual cost of the job because that is minimal compared to after installation benefits. Comfort and productivity are two of the top benefits of getting your lighting right. You get these two right and your lighting job will be paid for in months regardless of the cost of the job or the energy savings!

To begin making the right decision, we need to know how we see. Let’s start with a little bit about the biology of the eye.

The Eye

The eye has a number of components such as the lens, cornea, iris, optic nerve, etc. These are all critical components of the eye, but the most important parts for interpreting vision are our cones and rods.

The cones are responsible for our photopic vision. This vision is low sensitivity because it works best with light above 3 foot-candles. It is our day and color vision. There are 6-7 million cones in the eye and they are located directly behind the lens in the fovea.  The vast majority of external stimulus is processed through the cones

The rods are responsible for our scotopic vision. This vision is highly sensitive because it works best with light below 1 foot-candle. It is our night vision and has poor color and detail discrimination. There are 130 million rods in the eye and they are located all around the inside of the eye except for the fovea. Compared to the cones, little external stimulus is processed through the rods.

Interestingly enough though, the rods do control the pupil size. This is important to know because in low light levels your pupil size increases. This lets in a large amount of peripheral light to help the eye see. This is a natural response to a natural phenomenon.

Normally this is a good response, unless you are working indoors at a computer, reading fine print or doing other detail oriented work. By letting more peripheral light in your eye, you are actually “polluting” the good light you need to complete fine tasks. In other words your visual acuity drops.

Now the question is, how do you increase your visual acuity?

The most common method of increasing visual acuity is to increase the light levels. The problem with this method is as energy prices rise this method becomes cost prohibitive. This method can also make performing a task more uncomfortable, i.e. glare, contrast, etc.
 
A relatively new method is using a higher quality light source. By choosing the right light source, you can improve visual acuity, increase worker comfort, and cut energy costs.

Let’s start out with the most important qualifier for good lighting, visual acuity. I stated previously by lowering light levels, pupil size will increase and visual acuity will decrease. Consequently, working indoors at lower light levels will always decrease your visual acuity, right? Wrong.

The rods in the eye along with a newly found photoreceptor, retinal ganglion cells, respond to light with more blue content in its spectrum. This blue light not only affects the pupil size but also your circadian regulation. The more blue light in the light source, the smaller the pupil size. This will yield higher visual acuity. The blue light also increases the perception of brightness even at the same foot-candles.

Since most lighting is measured on the photopic response and now we have to consider the scotopic/mesopic response to measure acuity and relative brightness, researchers have developed a scotopic/photopic ratio. The higher the ratio, the higher the relative brightness and acuity. Following is a table of common lamps.

This table is also intended to be used for maximizing energy efficiency. By increasing the S/P ratio, you can reduce the ballast factor or the number of lamps you use without affecting relative brightness or visual acuity. The lower half of the table refers to the relative brightness increase in using an 850 lamp compared to the lamps listed under the S/P ratio column.