With all arc lamps, one has a strong electric field extracting valence
electrons from atoms in a rarified gas. The field then attracts the electrons
in one direction and the rest of the atom, now a positive ion in the other.
Electrons in the atoms are raised to a higher energy state by collisions or
by recombination of an electron and an ion. The high energy state is usually
unstable and the electron will eventually drop to a lower energy state in the
atom, releasing energy in the process. Since the states reflect fixed energy
levels, the transition releases a fixed amount of energy. This translates
into fixed wavelengths. If, as is usual, there are more than one high and low
state, the permutations result in more than one emitted wavelength. These are
called emission lines and the radiative species used in lamps have from two
to four strong lines and often have several weak ones as well.
This is in contrast to a tungsten fillament or a carbon arc where the emitter
is a hot material with a continuous spectrum dependent only on its
temperature. All thermal sources that are available to the consumer put out
much of their energy in the infra-red where it is of little use by plants.
The emission lines of a single element may not be at wavelengths preferred by
plants or people either. Sodium, has a double yellow line with little energy
elsewhere. Lines can be broadened by temperature, pressure or applied
magnetic fields. The former two are used in the sodium lamps. The low
pressure sodium lamp is an orange shade. The high pressure sodium (HPS) lamp
is more pinkish but slightly less efficient.
Metal halide (MH) bulbs do that to a lesser extent but use enough different
elements to give a more balanced spectrum putting the strongest lines where
the plants need them for vegetative growth, in the blue area. The pinkish HPS
is better for encouraging flowering.
Unfortunately, the emitters in the MH are solid at room temperature and the
bulb must run for some time before they vaporize. All bulbs are filled with
an inert gas like argon or krypton which only weakly participates in the main
arc but which stays gaseous at room temperature and allows the initial arc to
start. As the emitters cook off, the light gets brighter and the color
changes. With use, the emitters escape through the bulb envelope and the
colors change. Eventually, the bulb must be replaced, primarily due to such
ageing. Secondarily, the impact of electrons and ions on the electrodes will
erode them. Initially, this is good because it increases the electrode
surface area. Later, it simply serves to darken the bulb.
The voltage required to strike the initial arc is much higher than that
required to maintain it. Fluorescent bulbs and HPS bulbs use an inductive
ballast to create a large initial voltage kick. Mercury and MH bulbs do this
to a lesser extent and instead have an extra electrode close to one of the
main ones. This starter electrode requires much less voltage to start the arc
but must be switched off quickly. Its small size would make it burn out if
the main electrode couldn't take over. For MH bulbs used only a few hours per
day, starter failure can force early replacement.
There are also oddball bulbs such as mercury lamps with phosphors to absorb
the mercury UV lines and reradiate them in the visible spectrum. These are
high wattage versions of the standard fluorescent bulb and while more
efficient than mercury are less efficient than MH. I am told that you can
find 175 watt versions of these bulbs which will fit in the standard "yard
light" fixture. The $30 cost of such fixtures would make such a combination
attractive for the small scale user.
Bob Cruder - bcruder@miaco.com