LED versus Incandescent Lighting

As the debate over LED versus incandescent lighting continues throughout a variety of industries, it seems there is less to deliberate within the color measurement industry; namely for use in densitometers and spectrophotometers. There are 5 compelling reasons for choosing a densitometer or spectrophotometer that uses an LED light source.

• LEDs last longer. In fact, LED’s can last up to 10 times longer than incandescent light; which translates into an average incandescent lamp life of 5000 hours whereas the average lamp life of an LED is 50,000 hours. This is an important factor when considering the amount and level of service necessary to maintain your densitometer or spectrophotometer.
• LEDs allow multiple measurement illumination conditions as described in the ISO 13655 standard. Since no single LED is available to produce an acceptable light source for sample measurement, multiple LEDS are mixed in some fashion to provide an overall spectrum that covers the required UV and visual range. With appropriate circuitry and monitoring, those multiple sources can be adjusted to cause the correlated color temperature of the source to shift from M0 (correlated color temperature of approximately 2856 °K) to M1 ( daylight or CIE illuminant D50 ) or to M2 ( UV excluded) illumination conditions. Each of these responses can be created by simply changing the drive current profile to the multiple LEDS. To accomplish the same task using an incandescent source would require a series of filters and a method for mechanically moving those filters into the illumination path.
• LEDs are more efficient. LED sources generate light through electroluminescence. In that process, electrical energy is converted directly to photons. Incandescent sources generate light by heating a metal element to 2800 °K or higher. Incandescent light is generated from a principle called black-body radiation. As a result, the majority of energy is delivered to infra-red wavelengths outside the visual spectrum. Because of this, usable light as a fraction of input energy is lower. Additionally, an incandescent light can heat the measured sample if the illuminator is not designed in a way to block infra-red light. Sample heating could damage the sample or impact measurement accuracy.
• LEDs are rugged. LED sources have been designed for application to industrial high vibration environments. The packaging techniques allow the source to be tightly coupled to the relatively rigid circuit boards within the device. Incandescent sources present mounting challenges due to their high temperature and the associated thermal expansion of the glass envelope. Additionally, the filament is relatively weak when operating at temperature and will be damaged by excessive vibration or shock.
• LED instruments can have improved stability. As discussed above, incandescent sources emit light from a very hot filament. Once that filament reaches operating temperature, it tends to be quite stable over a typical 1 second measurement window. Today’s instrumentation is often required to measure almost instantaneously and measure for long durations during scanning. In those applications, LED sources are much better as they turn on very quickly and with proper circuitry, they can be managed to a very constant output over time.