LED light quality not determined by the specs on the box
I recently read an article in MIT Technology Review (How to Choose an LED Light Bulb) that provides ways to distinguish between the good and not-so-good LED light bulbs on the market. One point of reference was the quality of light emitted. Since light quality is extremely important to us at Leapfrog (it’s our main focus in our LED lamps), I was especially interested in how buyers are advised to assess the quality of light.
Three specification values were specifically cited:
- Number of lumens – The higher the lumens the brighter the light.
- Color temperature (or CCT) – The lower the CCT the more yellow the light. The higher the CCT the more white the light. 2700K – 3000K is the norm for most indoor lighting. Note: For a CCT light appearance scale, see our FAQ under What is CCT?
- Color quality, known as CRI – The measure of a light source’s ability to render color accurately compared to that of the sun. A perfect CRI would be 100, meaning the light source renders color the same as the sun; however, good color rendering is achieved with a CRI of at least 85.
Lumens, CCT, and CRI are undeniably valid metrics in measuring light quality. With recent LED performance improvements, a good ranking of these metrics is getting much easier to achieve than previously. As long the metrics are understood, a simple glance at the box or lamp datasheet will provide all three specifications. However, they paint an incomplete picture when isolated from other information.
What else plays a role in the quality of light? While there are many, many parts that affect light quality, here are three that we pay a whole lot of attention to:
- Optics: Lens design is extremely important in providing a glare-free, evenly distributed, soft-edged light. This is generally attempted through light diffusion or reflection techniques, though all manufacturers use different designs to try to reach this effect. Good results are hard to achieve. For a more in-depth analysis, see Light Quality of LED lamps. Ask your lighting representative about glare, light distribution, and especially for down lighting, the spot light “edge”. If your lighting representative is unsure, speak directly to the manufacturer.
- Components: Not only should all components be of top quality, but the manufacturer needs to independently validate the specifications provided by the supplier for any given component and ensure cohesion between components provided by different suppliers. Ask your lighting representative how the manufacturer tracks the supply chain and tests the components. Again, speak directly to the manufacturer if your lighting representative is unsure.
- Composition (materials): As far as the optical components are concerned, Poly-Methyl-MethAcrylate, or PMMA, is the most common material used to produce a good optical surface finish (it’s also called Perspex or Plexiglas). That said, where the PMMA comes from and the level of impurities matter, as the loss figure ranges from 0.1dB/com to 0.7dB/com—the difference between 5% and 50% loss for some optics designs. Ask your lighting representative or manufacturer whether the PMMA is “optical grade”.
While optics, components, and composition are not available on the box or packaging, they have a big impact on the quality of light and should be factored into any purchase decision. As a matter of fact, even assembly can affect light quality (the better the optical efficiency, the more sensitive it is to correct placement). So be sure to look beyond the specifications—as they alone do not tell the whole story.
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Sarah Bailey is the Manager of Marketing at Leapfrog Lighting. She has an interest in all things SSL, the environment, and, being a Canadian, hockey. She is an active participant in several LED lighting groups on LinkedIn, where many of Leapfrog's blog posts are discussed.