Believe it or not, quite a bit of science goes into selecting the correct lighting for any given space. Whether performing a building assessment or building commissioning, understanding the way the lighting works is critical to a productive environment. In order to select the correct luminaire — the combination of lamp, fixture type, and ballast/driver — one must understand how the human eye sees and perceives light.
As you may remember from your high school anatomy class, the eye uses cones and rods to sense varying wavelengths of light. There are three conditions under which the human eye perceives light and color — photopic, scotopic, and mesopic. Under well-lit conditions (i.e. daylight), photopic vision allows for color perception via our cone cells. Scotopic vision allows for us to see in low-lit conditions (i.e. nighttime). During these low-lit conditions, the cone cells in the human eye become dormant and rods in our eyes begin to perform. The human eye performs best under mesopic condition, which is the combination of scotopic and photopic fields of vision and uses both our cones and rods. Lighting professionals use this knowledge to determine the proper lighting for a given task plane.
There are two measurements to describe a lamp’s color perception — the color rending index (CRI) and the correlated color temperature (CCT). Essentially, CRI measures photopic levels and CCT measures scotopic. The best lamps will have high CRI and CCT values as they allow for greater clarity while consuming the least amount of energy. A great example can be found when we compare high pressure sodium (HPS) lamps to newer technologies such as metal halide (MH) light, LED, or induction lamps. HPS lamps produce an orange to yellow light caused by a low CRI and CCT, making it hard to distinguish true color. HPS was commonly used in the past because it produced a high number of footcandles (FC), allowing individuals to work in dark environments;however, color perception was always a problem. Metal halides allow for a reduced wattage of a lamp while providing proper light to the task plane compared to HPS, which would require a greater wattage for the same task plane. To put it another way, consider a facility that has six wallpacks operating one 250W HPS lamp. Maintenance staff can replace all of those wallpacks with ceramic metal halide lamps or with LED purpose built luminaires. Either option will allow the company to lower power consumption while providing more visually effective light.
Here is another example of using the science behind the way our eyes perceive light to save energy. Take a corridor with 8 recessed troffers each housing 4 F34T12 lamps. These luminaires are producing 40FC on the floor and the T12 lamps have a CRI of about 75 and a CCT of 3,500ºK. With the knowledge we have about the way the human eye operates, we know that if we raise the CRI and CCT, then we can safely lower the FC measurements. We can safely replace the 4 T12 lamps with 2 T8 lamps that have a CCT of 4,500ºK or higher and a CRI of 85 or higher. The wattage is effectively reduced by approximately 41% and some occupants will perceive the corridor as being brighter than it was before. In the lighting business, we call that a win-win.
In reality, there are more factors behind deciding to take this path, such as how much light is actually getting out of the fixture (i.e. luminaire efficiency) as well as codes and standards for use areas, but this essential science is the foundation.
Ultimately, the way the human eye perceives lights is one of quite a few factors that go behind our recommendations for lighting retrofits and replacements, but it’s a pretty interesting subject to explore. Feel free to download our white paper about LED technology and keep checking in for more information about the latest in lighting.
Lonnie Coleman is a lighting subject matter expert for Sain Engineering Associates, Inc.