Performance consistency is a major challenge for LED lighting applications and market acceptance. An LED’s output tends to vary in color (i.e., the color temperature changes) over time. Units for the same processed batch can also experience these variations.
Consequently, the LED industry has developed a “binning” methodology that makes it easier for users to select the LED source of choice. It should also be noted that LED light fixtures are integrated with electronics and sensors to continuously adjust output color and flux for greater consistency.
To measure color shift, the light industry uses the color rendering index (CRI) metric. CRI measures the amount of color shift that objects undergo when lighted by a light source. This is compared with the color of those same objects when seen under a reference light source of comparable color temperature. CRI values generally range from 0 to 100.
Unlike other light sources, LEDs can produce more light output by simply pumping in more power in the form of higher current. But there’s a limit to this approach. Eventually, the additional power pumped in generates more heat, which in turn reduces LED lifetimes below those of incandescent bulbs.
For many years, this has been a major technical hurdle that’s only recently been solved with the use of aluminum-based substrate materials and better heatsinking capability. LEDs have a potential lifetime of 100,000 hours compared to 10,000 hours for an incandescent bulb and 50,000 hours for a fluorescent light source.
White light for general illumination is usually obtained from an LED by one of two methods: by applying a phosphorous material on top of a blue or ultraviolet emitter or by accurately mixing the LED’s red, green, and blue colors in the right proportion. In either case, the correct materials, their composition, and their placement within an LED’s package are crucial to determining performance parameters.
For LED lighting users, it’s important to note that LEDs are not one-to-one replacements for conventional light sources like incandescents. An LED employed in general illumination is usually part of a light fixture that contains not only the light source, but also reflectors, lenses, a power supply, and electronic control circuitry.
When comparing an LED light source with other types of light sources, users must consider a relevant metric, such as the total cost of ownership of the light source’s lifetime. This includes labor, capital costs, and maintenance. Users also must consider the amount of energy savings over the light source’s lifetime.
Another factor is the concept of “useful lumens.” This is the amount of lumens needed to satisfy a specific application without wasting any light output. The concept is related to the application of a light source. Traditional light sources radiate light in all directions, more or less simultaneously. With their integrated lens systems, on the other hand, LEDs emit light in a narrow arc usually between 1108 and 1508. The more directional the light source is, the less light is wasted for a specific application like desktop lighting or parking-lot surface lighting.