Sunlight has been around for as long as any of us remember. However, in the interest of protection from the weather, and the ability to continue functioning separate from sunlight, we have produced various forms of artificial light. Candlelight uses a wick that is saturated with wax, the wax burns prolonging the life of the wick and producing some meager light. Oil is also used to provide similar light through a wick. Gas is similarly used to provide light by by an open flame (possibly enclosed by a fibrous material that increases light output). While these have drawbacks, they are still available when the necessity arises.
With electricity, came novel ways to produce light. One way is to produce an arc, wherein the process of electricity flowing across an open space produces an intense bright light. These are sometimes used as intense spotlights for sky illumination. A similar arc can be caused through a low pressure gas, wherein the gas is electrically conductive (as in Mercury vapor), it becomes ionized and allows a passage of current. One other way is by a heated metallic element wherein a flow of electric current causes it to glow red hot and in the process produces light. While there are other novel ways of light production, this exposition will cover most general and a few specific characteristics.
An open flame produces a heating of the gases involved in combustion. This heating causes an emission of low intensity gamma radiation that we detect as light, and infrared radiation that we detect as heat. The low intensity gamma produces a smooth light emission curve.
An open arc produces light by a different mechanisms. As the voltage across two nearby conductors increases above the necessary value to jump the gap, electricity begins to flow using air as a conduction medium. The current flow causes ionization of the air space surrounding the conductors. Ionization involves removal of one or more of an atom's outer electrons. When electrons are reabsorbed by the atom, gamma emission occurs. Since it takes a finite amount of energy to dislodge the electron(s) from orbit, when the electron(s) returns to orbit the gamma emission is also a finite amount of energy. The finite and discrete amount of emissions emitted from an ionized gas can be detected and identified by a spectrometer, which is able to clearly identify which exact frequency region it occupies in the light spectrum. In this fashion distant stars can be identified as to what elements they contain. Since the content and distribution is a mixture of Nitrogen, Oxygen, and other trace gases, the light emission from an open arc using air is a smooth curve accentuated by sharp peaks corresponding to the de-excitation energy levels for the various atoms' electrons.
An arc that is enclosed within a space can have radically different light emission characteristics. One such use is with Low Pressure Mercury Vapor, or High Pressure Sodium Vapor. While the latter are used widely for street lighting, the former are used extensively for indoor lighting. When the presence of conductive gases is limited to one element, the light emissions will be specific to that element's particular excitation / de-excitation characteristics. Mercury vapor, for instance, emits a substantial amount of Ultraviolet light. This can be used to advantage by applying in HVAC use to sterilize an air stream, because ultraviolet light produces DNA damage. Since Ultraviolet light is not kind to humans, the emission from a Mercury vapor light is modified by a Phosphor that is coated on the inside of the tube. This phosphor becomes excited by the ultraviolet emissions and produces secondary emissions which are more pleasant to the eyes. Nonetheless, the initial spectral emissions are not fully masked.
Light produced by a heated filament is characterized by low level gamma emission and infrared energy.
A peculiarity for electric lighting, is that there will be components in the light output that reflect the nature of the source. If Alternating Current is the source, there will be a ripple in the light output whose frequency is twice that of the source. Since each cycle of alternating power has two pulses of energy, one positive and one negative, the light source gets two bursts of power input. Since the light output of any electric source tends to diminish rapidly after the source is turned off, the overall light output will be characterized by "bumps" followed by "valleys". If the energy input is large enough these variation will be small, but there nonetheless. Due to the eye's persistence, or the inability to detect a light change if the time period is too short, anything faster than about 30 cycles will seem to the eye as continuous light. With a 50 Hz power source the ripple will be 100 Hz. With a 60 Hz power source the ripple will be 120 Hz.
There are additional peculiarities with Mercury vapor fluorescent lighting, in that 1) it needs a increase of voltage for it to function, since 120 volts is insufficient to start the initial current flow, and 2) the geometry of the light bulb lends itself to line rather than a point output. These bring Electric and Magnetic considerations into the picture. An additional concern is that ionization and de-ionization are also sources of Radio Frequency emissions.
Under development . . .
-------------------------------------------
Sal La Duca
Building Biologist
BS,
BBEC/BBEI,
CIEC
FCC Licensed
792 Green St.
Phillipsburg NJ 08865 USA