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3 Characteristics for Assessing Light Quality

The recent boom in cost-effective options for solid state lighting technology has led to major changes in the way we illuminate our buildings. Light emitting diodes (LEDs) have become a popular replacement for many different types of lamps, and facility managers and building owners are being introduced to a fundamentally different light source with its own set of properties.
Building occupants occasionally complain about brightness, color, and changes in room appearance after being exposed to LEDs. Effective energy management involves choosing solutions that strike the balance between quality of life and energy consumption so understanding how to minimize changing conditions is important. The study of optics is a deep dive into physics, and there are countless factors that go into the way light affects our lives, but three properties of light cover a large majority of them: brightness, color temperature, and color rendering. A diligent consumer can keep these properties in mind when purchasing LED lamps and ease the transition from older technologies.
Color rendering index (CRI)
CRI is a measure of how accurately an artificial light source displays an object’s color compared to its color under a reference light source, usually incandescent light. It is measured on a scale of 1 to 100, with a rating of 100 representing no color shift while lower ratings suggest that the color appears unnatural. For example, a low pressure sodium lamp that you see illuminating parking lots makes even the most vibrant colors dingy and dull due to a low CRI. A good rule of thumb is that a CRI greater than 75 provides good rendition while a CRI less than 55 provides poor rendition (Doty, 2013).
Not all lamps within a class of technology have the same CRI. Fluorescents range from 50-60 CRI for standard white fluorescents to 90-100 for specialty “High CRI” lamps. Similarly, a standard metal halide fixture might have a CRI of 60-70, but warm-colored metal halides can reach 80-90. Therefore, if you have T8 fluorescent tubes that have been in place for years, check the manufacturer’s technical specifications for that product rather than relying on a simple internet search for CRIs of that technology.
While CRI is a major component in describing the type of light a lamp produces, consumer studies have found low correlation between consumer rankings and CRI, indicating that a low CRI doesn’t necessarily indicate bad quality light.
Color temperature
Color temperature is the color of the light emitted by a light source, expressed in Kelvin (K) and ranging from 1,000 K to 10,000 K. Just as the CRI was developed in reference to an incandescent lamp, color temperature references the temperature of an ideal black-body radiator that radiates light which is the same color of the light source.
A major source of shock for homeowners when replacing incandescent lamps with LED technology is the shift from one end of the spectrum towards the other. Unfamiliarity is a major component of this response, but there have been numerous studies on the physiological effect of different colors, ranging from the effect on sleep to the effect on taste and smell. Lighting vendors and manufacturers have taken note of this sensitivity and offer explicit color temperature information on a wide range of products.
Finally, brightness is a source of contention when installing new lighting technologies in a facility.
Luminous flux, measured in lumens (lm), is the light output of a lamp and can be an accurate metric for an “apples to apples” comparison in cases when you’re retrofitting lamps of similar types. Given the high efficacy (lm per watt) of LED lamps relative to fluorescent lamps it’s very easy to over light a space if you’re only focusing on reducing energy consumption.
To reduce the chances of occupants being unhappy with the newly installed lights do a quick lighting survey, using a light meter and conducting interviews to evaluate how satisfied they are with the current light levels.
Some vendors and manufacturers claim that the light output of lamp x is equivalent to lamp y, and while these can be valid claims the interplay of color temperature and brightness will greatly influence an occupant’s perception of a new light source. Cool white light (6500 K) may appear harsher even if the luminous flux is comparable to warm white light (3000K).
When updating your lighting configurations with newer technologies such as LED, establish your baseline of “light quality” using the three characteristics described here. From there, pay careful attention to the characteristics of the new lamps and balance creating agreeable indoor conditions with the cost and energy savings.
For more information on these topics, visit the following resources:
·         LED Lighting Facts