Why Less Light is Sometimes More

Occasionally we see a product application that is just, well, perfect. Take, for example, the unfortunate installation of 52 CRI warm white 40W circline fluorescent fixtures in the common area halls of a large multi-family apartment complex in New York City. Residents there hated the quality of the light emitted by the light fixtures, but there was not enough money available in the budget to replace them.  We pointed out that we could correct the ghastly color rendering, and also cut the energy consumption by more than 50% with the installation of two GU24 9W 4,100 Kelvin CFLs in place of the existing circline bulb and ballast. The customer was skeptical, because the initial lumens for existing vs. new would drop from 2,800 to a measly 1,200. As we suspected. the mockup proved that their concerns were unfounded, and the hallways are now perfectly illuminated. 

Despite the fact that a light meter will measure a drop in footcandles in the halls, the space actually appears to be brighter than it was. Much of this is due to the superior color rendering of the new light bulbs, but the rest of the effect is due to the way the human eye perceives light, especially in low-light interior applications such as this.

Here is a photo:

A marked improvement.

The reason this worked so well, despite what the lumen data would presuppose, is explained best in Dr. Sam Berman's excellent study, The Reengineering of Lighting Photometry. In a nutshell, the higher the color temperature of a light source, the brighter it will appear to the eye. To arrive at the "perceived lumens", (scotopic lumens as registered by the rods in the eyes), the factor we apply is called the S/P ratio, and the specific values are identified in Dr. Berman's study. 

An additional benefit to the apartment complex, besides excellent energy savings and better-looking hallways, is vastly lower maintenance costs because the ballast is now integral to the low-cost GU24 light bulb.