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HID
Lighting
Negative resistance and why you
need a ballast to limit current
In a high-pressure lamp, if current increases, the
arc gets hotter. This tremendously increases the concentration
of ions and free electrons, making the arc that much
more conductive. The conductivity of the arc increases
enough that the voltage across the arc usually stays
about the same or even decreases if the current is
increased.
In a low-pressure lamp, a variation
of this causes the same thing. If you double the current,
you usually roughly double the concentration of excited
gas atoms and free electrons. The concentration of
ions must match that of free electrons but each excited
atom is bombarded twice as much by free electrons
(remember, there are twice as many electrons around
for an excited atom to see). The average kinetic energy
of the free electrons must decrease so that ion concentration
is also only roughly doubled. To get slower free electrons,
the electric field in the discharge (and voltage across
the discharge) must decrease.
In either case, it is not a good
idea to connect the lamp directly to a voltage source.
Once the lamp starts conducting, increasing current
will increase the lamp's conductivity, allowing more
current to flow. This process does not level off until
one of the following happens:
- A large fraction of easily ionizable atoms are
ionized,
- The concentration of ions/free electrons is so
high that more of these somehowimpairs mobility
of free electrons,
- The power supply's or wiring's limitations limit
the current.
At this point, the current is usually
around or over 100 amps or so, and will likely blow
fuses/pop breakers, and is certainly not good for
the lamp.
The term "negative resistance"
refers to a decrease in voltage across the lamp resulting
from an increase in current through the lamp.
Why the light from many discharge
lamps makes red things look dull?
Mercury lamps, most metal halide
lamps, most sodium lamps, and "cool white",
"white", and "warm white" fluorescent
lamps have a shortage of red and green light in their
spectral output. These lamps also have a surplus of
yellow and/or orange-yellow. Since red plus green
looks yellow, taking away red and green and adding
yellow do not affect how the lamp's color looks. Nearly
all yellow objects reflect red, orange, yellow, and
green. Increasing yellow output and decreasing red
and green does not change how yellow objects look.
However, red objects generally reflect mostly just
red light. With the shortage of red light, these look
darker. If they are not pure red in color, they will
not only look darker but also less red in color. Unphosphored
(clear bulb) high pressure mercury lamps are especially
bad at this, since they make very nearly no red light
at all. This is not a problem with most compact fluorescent
lamps, most 1-inch diameter 4-foot lamps, and other
fluorescent lamps that have "rare-earth"
phosphors. These phosphors, unlike those in older
formula fluorescent lamps, produce a strong, narrow
orangish red spectral band and a strong, somewhat
narrow, slightly yellowish green one, with little
in between. Under these lamps, reds usually look near
normal, or slightly orangish, or slightly excessively
bright. Greens often also look slightly brighter under
these lamps than under old formula "cool white"
and "warm white" fluorescent lamps.
Why photos taken under discharge
lamps often look blue-green?
The problem here is the fact that
film and human eyes have different spectral response.
Human eyes are quite sensitive to the short wave end
of the red range of the visible spectrum, but not
to the long wave end. Most color film responds about
the same to shorter and longer red wavelengths. Most
of the red light from fluorescent lamps, metal halide
lamps, sodium lamps, and phosphored mercury lamps
is of shorter red wavelengths. These lamps do not
emit much of the longer red wavelengths. This maximizes
red sensation by the eye for a given amount of actual
light. Producing less-visible longer red wavelengths
detracts from maximizing luminous efficacy of the
lamp, so this is minimized. Therefore, lamps make
a surplus of red wavelengths to which the eye is more
sensitive than film is, and a shortage of the red
wavelengths to which film is more sensitive than human
eyes. This results in the film seeing red less than
human eyes do, and this makes photos look blue-greenish.
Arc and glow discharges explained!
Electrons normally don't just move
or flow from a conductor into a gas. Something has
to make this happen. Explained below are ways for
this to happen.
In a glow discharge, positive ions
bombarding the cathode dislodge electrons from the
cathode material. There is a substantial electric
field near the cathode that accelerates ions toward
the cathode to make this happen. The whole process
tends to complicate itself, resulting in a double
layer of glow around the cathode, thin dark spaces
underneath and between these layers, and a more substantial
dark space between all of this and either the anode
or the main body of the discharge, whichever comes
first. In neon glow lamps, the anode is so nearby
that no main discharge body occurs. "Neon"
signs are longer, so a main discharge body occurs.
Since these operate on AC, each end has a significant
dark space only half the time, so these regions are
a bit dim rather than dark. There is generally a natural
current density in the cathode process, generally
around a milliamp to .1 amp per square centimeter,
depending on the gases involved, the pressure thereof,
and the cathode material. A glow discharge at this
intensity is a "normal glow". Decreasing
the current causes the cathode's glowing layers to
cover only part of the cathode. In this case, the
glow often moves around, causing a flickering effect.
If the current is more than enough to cause the cathode
to be covered with glow, (or if the glowing layers
are forced into a thinner layer of space than they
normally use), abnormal glow results. The voltage
drop of the cathode process (this voltage is known
as the "cathode fall") will be higher than
normal. This causes ions to bombard the cathode harder
than usual. This increases "sputtering",
or dislodging of cathode material atoms. Sputtering
effectively "evaporates" cathode material
and often causes darkening of the lamp's inner surface.
Sputtering occurs more easily at higher cathode temperatures.
It is generally recommended to neither have significantly
"abnormal" glow nor significant temperature
rise in the cathode, and especially not both of these
combined.
The cathode fall of normal glow
is usually 50 to 90 volts for neon, argon, krypton,
xenon, or mixtures including significant amounts of
any of these gases. Some metal vapors may have somewhat
lower cathode falls. Nitrogen and some other gases
have high cathode falls usually near or even well
over 100 volts.
The cathode process in most HID
lamps and fluorescent lamps is the thermionic arc.
In this process, at the proper high temperature, some
material in the cathode fails to keep a grip on its
electrons. Therefore, electrons simply flow from the
cathode to the gas. The cathode fall is usually around
10 volts, and the heat dissipated in this process
keeps the cathode hot enough to let electrons flow
from it to the gas. The current density at the cathode
process of a thermionic arc is generally in the tens
or hundreds of amps per square centimeter of active
cathode surface, but can occaisionally be as low as
around an amp per square centimeter if a heat source
other than the arc heats the cathode.
Another arc process is the cold
cathode arc. In this process, ions bombard the cathode
material and dislodge electrons from it. This seems
similar to the glow discharge, but the effect is quite
different. The current density in the cathode process
is usually hundreds or thousands of amps per square
centimeter. The cathode fall is usually near the ionization
potential of the cathode material or the main active
gas ingredient, whichever is lower (for a minimum)
to twice whichever is higher (for a maximum). Substantial
sputtering may occur, especially if the cathode is
hot. Cold tungsten is usually reasonably tolerant
of this, permitting the use of this process in xenon
flashtubes.
An arc is often not entirely thermionic
nor cold-cathode, but one of these processes is usually
dominant.
If a hot-cathode lamp is underpowered,
the cathode is not as able to emit electrons by the
thermionic process, and significant cold-cathode arc
process may occur. This can cause excessive sputtering.
Starting a hot-cathode lamp also results in some of
this as the cathode warms up. Overpowering a hot-cathode
lamp can simply overheat the cathodes. Because of
this, it is generally advised to start fluorescent
and HID lamps as infrequently as practical and to
neither overpower nor underpower them. This makes
it difficult to dim fluorescent and HID lamps significantly
without being hard on their cathodes. There are some
special dimming ballasts for some fluorescent lamps.
These dissipate power into the cathodes to maintain
a workable thermionic process when these lamps are
dimmed. It is recommended to only dim fluorescent
lamps with appropriate ballasts, and to use these
dimming ballasts only with the lamps they were designed
to safely dim.
What do high-pressure sodium
lamps have?
One thing these lamps have is a
mixture of mercury and sodium, rather than just sodium.
If only sodium was in these, the voltage across the
lamp would be excessively low. Making the arc tube
longer to increase voltage drop would also increase
the watt-per-centimeter loss (explained below in section
8). A higher sodium vapor pressure would also increase
the voltage drop, but would broaden the sodium's emission
band to the point that much of the spectral output
is nearly infrared. This detracts from maximum most-visible
light output. Also, a mercury-sodium mixture conducts
heat less than pure sodium vapor. This reduces thermal
conduction of energy away from the arc (The watt-per-centimeter
loss). Another thing: Hot sodium is very highly chemically
reactive. Some of the sodium is lost as the lamp ages,
either permeating through the arc tube or chemically
becoming part of it. Therefore, a surplus of sodium
is included in the arc tube. The sodium vapor pressure
is controlled by the temperature of the "amalgam
reservoir(s)" of the arc tube, where any unevaporated
mercury and sodium reside. Proper lamp operation depends
on the amalgam reservoir(s) being at or near a proper
temperature.
Why do aging sodium lamps sometimes
cycle repeatedly on and off?
The sodium vapor pressure is controlled
by the temperature of the amalgam reservoirs at the
ends of the arc tube. As the lamp ages, the ends of
the arc tube get darkened, and they absorb light.
This makes them hotter. Therefore, the amalgam reservoirs
get hotter. This increases the sodium vapor pressure
in the arc tube, leading to different electrical characteristics.
When this effect becomes excessive, the arc in the
arc tube goes out. The arc tube must cool before the
vapor in it is thin enough to restrike an arc.
Aging sodium lamps sometimes repeatedly
turn on and off as the ends of the arc tubes overheat,
then cool off once the arc goes out. If a high pressure
sodium lamp repeatedly turns on and off, replacing
the bulb with a new one is usually all that is needed.
Thermal Conduction from High Pressure
Arcs, the Watt per Centimeter Loss
When energy is dissipated into an
arc, it largely leaves the arc by three mechanisms:
1. Some is used by the cathode and
anode fall mechanisms getting electrons from metal
to arc and vice versa. Nearly all of the energy here
ends up heating the electrodes. The anode fall is
not always significant, the cathode fall usually is.
2. Thermal conduction removes energy from the main
body of the arc. This ends up heating the arc's surroundings
and any container or arc tube.
3. Whatever energy enters the body
of the arc (not lost in electrode falls) and not thermally
conducted from the arc is radiated.
Of course, it is desirable to minimize
(1) and (2) and to maximize (3).
The electrode falls are generally
a fairly constant voltage. Designing the main body
of the arc to have more voltage across it (higher
voltage drop) and use less current reduces the electrode
losses. However, there is a limit to practical arc
voltages, since higher voltages may require complicated
equipment to supply them, and also higher pressure.
The thermal conduction loss is a
major loss in many high intensity discharge lamps,
especially ones of lower wattages. This loss varies
with arc temperature, gas and vapor type, and is largely
linearly proportional to the length of the arc. However,
this loss usually does not vary much with the arc's
diameter nor with the gas pressure. Often, especially
in mercury vapor lamps, the arc temperature is surprisingly
constant, and this leads to a surprisingly constant
thermal conduction loss from the arc, in watts per
centimeter of arc length. This loss increases if the
arc tube size and/or gas pressure are great enough
for convection to be significant, and the nearly constant
degree of this loss applies to typical general purpose
HID arcs that are many times longer than they are
wide. The loss is different for the nearly spherical
arcs in some special HID lamps. For typical mercury
vapor lamps, the thermal conduction loss is generally
around 10 watts per centimeter. For high pressure
sodium lamps, this loss is less constant but generally
near 10 watts per centimeter. This loss can vary with
the ratios of the mercury-sodium mix since sodium
vapor conducts heat more than mercury vapor does.
For metal halide lamps, this loss is less constant
and generally greater (in watts per cm.) due to convection
in the short, wide arc tubes that are filled to a
very high pressure. The watt/cm. loss could be reduced
by:
A. Using a shorter arc. This requires
a higher pressure for the same arc voltage. Also,
the parts of the arc tube within one tube radius of
the electrodes are subjected to being darkened by
evaporated/sputtered electrode material, so it may
not pay to have an arc length shorter than a few times
the arc tube diameter. Reducing the arc tube diameter
would help this, but a skinnier arc tube will get
hotter from the same watts of heat per centimeter.
All of this combined impairs the design of economical
miniaturized HID lamps.
B. Fill the arc tube with a less
thermally conductive material. Such materials have
larger and/or heavier molecules. Heavier molecules
move more slowly, larger size ones don't go as far
between collisions. This favors use of mercury and
xenon as HID lamp ingredients. Low-heat-conductivity
gases and vapors should be gaseous at reasonable arc
tube temperatures, chemically stable or inert at all
temperatures from below freezing to the arc temperature,
and not have major infrared or ultraviolet emission
lines that detract from efficiently radiating visible
light. This largely disqualifies polyatomic substances
and the vapors of heavier alkali metals. |
