As you probably realize, the human eye consists of cones and rods. It’s now known that rods and cones share responsibilities, but simply speaking, the cones are most engaged during daylight or lighted conditions, and the rods are most used at night or in dim environments. Scientists differentiate these two types of seeing by referring to daylight vision as photopic and night vision as scotopic.
So, what’s this all have to do with light bulbs? Well, until recently those in the lighting industry only measured photopic light. No one really took into account that our complex eyes perceive light differently than manmade measuring devices, and a bulb of a specific lumen can appear differently to our eyes depending on whether we view it in the light or dark. In other words, not all lumens are created equal. It depends on who’s doing the viewing — man or machine. This lack of awareness meant measuring tools only gauged light in one dimension, and photopic measurements became the standard for labeling the brightness of bulbs.
However, thanks to advances in technology, that’s all changed now. Today, many lighting experts are advocating two ways to label the lumens of a particular light: by its photopic lumens (for high light levels) and its scotopic lumens (for low light levels).
This Means Energy Savings for You
Because our eyes have the ability to perceive more light than what is technically emitted as lumens, we can switch to a bulb with high scotopic lumens and actually see more light than what’s put out by a higher wattage bulb. This can result in significant energy savings, since you can use lower watt bulbs and still maintain the same or better quality light.
How Scotopic Lumens are Measured
To determine the scotopic lumens of any particular light source, its Scotopic Ratio (S/P Ratio) is multiplied by its number of photopic lumens. The S/P Ratios vary depending on the light source, but the lighting industry has already standardized these numbers.
For instance, to compare the scotopic lumens of a150W HighMax CFL light versus a 250W high pressure sodium light, you’d perform the following calculations:
150W HighMax = 9,200 photopic lumens X 1.96 S/P ratio = 18,032 scotopic lumens
250W HPS = 24,500 photopic lumens X 0.62 S/P ratio = 15,190 scotopic Lumens
As you can see from the above example, if a bulb has a high enough S/P Ratio, it can easily deliver brighter perceived light even when compared to bulbs with higher wattages.
Should You Switch to Lights with High Scotopic Lumens?
Currently there is some debate about whether scotopic measurements are useful for bulbs used in typical, indoor environments. However, it’s clear there’s some overlap in the “duties” of the eyes’ cones and rods, and the rods actually do play a part in lighted conditions. This suggests, even for high light environments, we can’t solely rely on photopic measurements.
The one area where most lighting experts agree high scotopic lumens come in handy is in outdoor, nighttime applications, such as street lamps, parking garage lights, and parking lots. These types of lights are most relied on in dim conditions, so it makes sense to measure them according to night vision standards (scotopic). In fact, the Illuminating Engineering Society of North America is currently reevaluating their street light standards to include scotopic measurements.
If you use any type of outdoor light and are concerned with energy savings, you may want to consider changing to bulbs with a high S/P Ratio. Of course, you’ll also want to keep in mind other energy saving traits, like long life, durability, and watt requirements.
There’s still a great deal of research going on in this arena, and it will be interesting to see how pivotal scotopic lumens become in the future. For now, we suggest staying aware of this new form of measurement and keeping it in mind as a possible strategy in your energy savings plan.