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Lighting

The Nature of Light

Page 6:  Lighting

Originally, light bulbs operated by incandescence, with a glowing filament that mostly emitted infra-red. Modern lighting operates by luminescence, including fluorescent tube lighting, compact fluorescent bulbs, and light emitting diodes (LEDs).

 
 
Figure 6.1:  Fluorescent tube ceiling light fixtures (left) and light bulbs (right).

Fluorescent tube light bulbs generate ultra-violet (UV) light internally, which is absorbed by the light bulb coating (a luminescing material) and re-emitted outward (luminesced) as visible light. There is no way to know by casual observation whether the light bulb is leaking UV light (which is harmful), because UV light is not visible to photopic human vision.

Fluorescent tube light bulbs last longer if they are not turned off and on frequently. Because of this, the bulbs are often left turned on, to reduce replacement cost (to replace the bulbs less often). In some buildings the fluorescent tube fixtures are wired to not be turned off, other than to turn off the lights for the entire building (or entire floor of a multi-story building).

Compact fluorescent light bulbs are smaller and may not be degraded as much by being turned off and on. However, there are reports that compact fluorescents may be more likely to leak UV light because of sharp bends in the glass. But again there is no way to verify UV leakage in practical installations without special equipment (sensors that are prohibitively expensive).

Figure 6.3:  Compact fluorescent light bulb.


Light Emitting Diode (LED)

Light emitting diodes (LEDs) are solid state semiconductor chips that work on the photoelectric principle in reverse (emitting light instead of receiving light). They could actually receive light instead of emitting light, but not as efficiently as emitting light.

Originally LEDs were used to emit red light directly from the semiconductor material. Later, new semiconductor materials were found to emit other colors.

None of the LEDs can directly emit white light. LEDs were developed that would emit white light by closely spacing red, green and blue LEDs together. However, the spectrum of each of those three colors was narrow, providing poor chromacity (see right side of Fig. 1.2 in Lin and Liu).

Then a technical breakthrough made blue LEDs much less expensive. Luminescence materials (referred to as “phosphors”) were developed to be used in combination with the blue LEDs to produce light that is more white than the previous white LEDs, but with possible leakage of too much blue.

Figure 6.4:  Closeup of a blue LED with phosphor coating.

Early versions of this phosphor-based (luminescing) “white” LED leaked too much blue, without much immediate notice because human photopic vision is not very sensitive to blue light (cannot readily distiguish that there is too much blue light even though the excess blue light is harmful). This is the type of LED that is now becoming widely used.

Figure 6.5:  Spectral distribution of the consumer-grade LED of Fig. 6.4 (see also Fig. 21.8 in Schubert).

Figure 6.5 shows the spectrum of light transmitted from a blue LED with phosphor coating to produce “white” light. The spike on the left is blue light leaking through the phosphor coating. The hump on the right is light emitted by the phosphor. Notice that much more blue light is transmitted than other colors. The excess blue is not readily detected by human observers because human vision under bright light conditions (photopic vision) does not easily detect extra blue light.

“[Luminescent] materials often produce light weakly. The emitted light is normally overwhelmed by the considerably more intense light of the illumination. For this reason, in both microscopy and ophthalmology, filters are employed.”
— 
Basic Sciences in Ophthalmology, p. 35
“white-light LEDs degrade over time primarily through bleaching of phosphors so that they no longer efficiently absorb blue light. This shifts the color temperature of the device over time, with a corresponding change in the color-rendering index but, more importantly, an increasing blue emission from the device with time.”
— 
Molecular Vision (Open Access)

Attempts are being made to make this type of LED less blue, by developing new luminescing materials (phosphors). But luminescing materials degrade with time and use. It is not yet known whether new materials will block the excess blue and be able to continue to do so during life of the bulb. Affordable sensors need to be developed to monitor these lights in the field.


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