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An LED produces light from electronic transitions in a layered, solid state semiconductor. Recently, organic light emitting diodes (OLED) have started to appear in the commercial market. These organic devices have not gained wide acceptance in rugged aerospace applications. Hence, the following discussion will focus on the semiconductor (non-organic) type LED.
LED lamps are available in single colors, for example blue, green, yellow-green and red. The efficiency and brightness of these LEDs have improved dramatically over the last 5 years, in particular in the blue spectral region. In fact, blue LEDs were unavailable a decade ago because the semiconductor alloy needed to achieve a blue wavelength was simply not fully developed. In a remarkable turn of events, new semiconductor chemistry based on gallium nitride enabled high efficiency blue LEDs to rapidly emerge from the lab into the marketplace. This new GaN alloy now yields very high brightness blue emission. In turn, the blue emission can be harnessed to generate a “white” LED color. The white emission is realized by applying a thin phosphor coating over the blue emitting LED.
Spectral emission curves for several single color LEDs are shown in Figure 1. Note the narrow (FWHM 30 to 50 nm) nature of the spectral emission bands. Red LEDs exhibit a very prominent secondary emission peak in the NIR that must be strongly attenuated to achieve compatibility with night-vision devices. NVIS LED Filters exhibit much higher visible transmission than NVIS filters designed for incandescent lamps.
For a complete overview of Korry Nightshield filters for LEDs, click here.
Figure 1: Monochrome LED emission spectra. The spectra are color coded and ordered from left to right in the following sequence: blue, green, yellow-green, and NVIS red LED emission.
The spectral emission curve for two white LEDs is shown in Figure 2. The prominent peak near 450 nm in both curves is associated with the blue LED “pump” radiation that is not captured by the phosphor coating. The emission throughout the rest of the visible spectrum is derived from the phosphor coating. The relative amounts of blue and longer wavelength emission depend on the phosphor coating chemistry and thickness. Hence the curve shapes will change depending on these variables. The LED industry uses the term “color temperature” to describe the spectral properties of a white LED. For LEDs, this term does not have a physical meaning, but is used to relate the LED color coordinates to the color coordinates achieved by a black-body source. The graph shows both a “high color temperature LED” as well as a “low color temperature LED.” The proper NVG filter will depend upon the white LED color temperature.
Figure 2: High color temperature and low color temperature white LED emission spectra.
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