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The light emission mechanism of a fluorescent lamp is much different than a black-body radiator. There are three primary steps leading to light emission in a fluorescent lamp:
1. Electron impact excitation of mercury atoms by an electrical discharge
2. Ultraviolet (185 nm and 254 nm) light emission by the excited mercury atoms
3. Visible light emission by ultraviolet stimulation of a phosphor coating
The phosphor material is deposited on the inner wall of the tubular glass lamp and its chemical composition determines the light emission properties of the lamp.
Two predominant types of phosphor coatings are used. Low-cost phosphors are employed to achieve “cool white” area lighting in cargo bays and ships. The emission from a cool-white lamp is shown in Figure 1. The NIR emission signature of the fluorescent lamp is different than an incandescent lamp; however, there is a significant amount of NIR emission from the fluorescent lamp that will cause interference with a night vision device. For a description of tubular filters for T-12 and T-8 fluorescent lamps, download this PDF (172 KB).
Figure 1: Cool-white fluorescent lamp emission
The cool white phosphor is adequate for area lighting. However, a longer lifetime phosphor mixture is needed for high performance lamps used in display applications (AMLCD backlights). Hence, a more robust tri-band phosphor mixture was developed for these applications. The tri-band phosphor mixture exhibits three primary spectral features: blue (450 nm), green (545 nm) and red (611 nm) as shown in Figure 2.
Figure 2: Tri-band fluorescent lamp emission
A fluorescent lamp with this robust phosphor mixture has a distinct NIR emission signature. The spectral peaks near 700 nm and 850 nm are troublesome and require additional filtering when used in proximity to a night vision device. Depending on the working environment (airborne or ground/mobile) and goggle type, several different Nightshield filters are available for tri-band fluorescent lamps, for example:
Laptop full-color LCD Class B: Download PDF (102 KB)
Ground mobile AMLCD (covert laptop): Download PDF (76 KB) |
