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A Primer on Light Bulb Technologies

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#1 ·
A Primer on Light Sources (Light Bulbs).

This primer covers the basics of various light bulb technologies, particularly those used in motor vehicles. This primer appears as a series of short articles I wrote that were published on:

Lighting Articles.

The topics are presented in the following order:

1) Lighting Terminology
2) Incandescent Technology
3) Halogen Technology
4) Xenon Metal Halide (Automotive HID) Technology
5) LED Technology
6) Neon Technology

I am a mechanical engineer with experience designing light bulbs and also experience at another company designing the head lights and tail lights that the light bulbs plug into.

-- missile_man

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1) Lighting Terminology

The human eye is a very complex device. The manufacturers of light bulbs have developed means of measuring light that are meaningful to them, but are not intuitively meaningful to laymen. Below is a glossary of common terms.

Brightness - See also "Lumens" and "Candlepower."
The term "brightness" is understood intuitively, but it is a complex phenomenon. What we perceive as being brighter or dimmer can be affected by a lot of things. Rather than delving into the numerous scientific complexities of this term, it will suffice for this section to allow the reader to make use of the intuitive meaning of brightness.

Candlepower
Just as for "lumens," candlepower is a measure of brightness that is adjusted to the selective sensitivity of the human eye. (See "Lumens.") A beam pattern is never completely equally bright in all directions. The brightness of light at a specific point is measured in candlepower. The total amount of light present in a beam pattern is measured in lumens.

CBCP
Center Beam Candlepower. Some data sheets for light bulbs list the CBCP. This is the brightness, measured in candlepower, at the center of the beam pattern. A spot light and a flood light can emit the same total light (the same lumens), but the spot light will have a much higher CBCP than the flood light.

CRI
Color Rendering Index. An important factor about color from light bulbs is how other colors look when illuminated by the light bulb. An incandescent bulb has all the colors of the rainbow, so all colors will be rendered properly if viewed under an incandescent light bulb. The Color Rendering Index (CRI) for incandescent lamps is a perfect 100. Other lights do not fair so well. One of the worst is a low pressure sodium bulb, which emits a nearly pure yellow color. Some well designed fluorescent bulbs have a CRI of 80, which is considered to be very good.

Fill Gas
Some light bulbs have a vacuum inside, but most have some kind of gas inside (the fill gas). The fill gas cannot contain any water vapor nor any oxygen, or else the filament will fail in seconds. The most common fill gasses are: Nitrogen, argon, krypton, and xenon. Nitrogen is used in incandescent bulbs because it suppresses arcing between the coils of the filament wire. Argon, krypton, and xenon are all “inert,” which mean that they do not react easily with other substances. Argon is the most plentiful in our atmosphere, so it is the least expensive. Xenon is the least plentiful and the most expensive. Of the three, argon has the lowest density, and xenon has the highest density. The higher the density, the more slowly the tungsten filament evaporates, and the longer the bulb’s life. Thus, xenon is the most desirable fill gas, but it is the most expensive.

Light
The sun emits a wide range of frequencies of radiation. Those frequencies that can be detected by the human eye we call light. Infrared radiation and ultraviolet radiation are not visible to the eye. Most light sources produce both light and invisible infrared and/or ultraviolet radiation.

Light bulb
The term "light bulb" is used here to refer to the light source, which is the glass envelope and all its innards.

Lamp
The term "lamp" is used here to refer to the light bulb plus all of the hardware surrounding the light bulb that positions, reflects, and focuses the light. In the light bulb manufacturing industry, however, the light bulb is called a "lamp," and the assembly that holds the light bulb is called a "fixture."

LPW
Lumens Per Watt. When any device uses electrical power, that electrical power is converted to some other form. An electric light converts some of the electric power into light. The rest of the electric power is converted to heat (infrared light and the warming of the light bulb's parts) and some ultraviolet light. Different light bulb technologies have different efficiencies. The efficiency (the term "efficacy" is used by scientists) is measured in lumens per watt, or LPW. (See "Lumens.") For two bulbs with the same brightness (lumens), the one that uses the least power (watts) will cost less to use.

Lumens
The human eye is more sensitive to some colors than it is to others. To account for this variation, scientists measure light in "lumens," which takes the amount of radiation (in watts) that is present and adjusts it to the human eye's selectiveness. For example, if we were in a room illuminated only by 100 watts of invisible radiation (such as ultraviolet), the light bulb's output is zero lumens. The human eye is more sensitive to green and yellow than to blue and red. Five watts of pure green light has more lumens (appears brighter) than five watts of pure red light.

MSCP
Mean Spherical Candlepower. Some light bulb data sheets list the MSCP. This is a way to represent the total light in the beam pattern by assuming that the beam pattern is equally bright in all directions. If you multiply the MSCP by 12.57, you have the lumens. (See "Lumens.")

Note #1: The human eye can distinguish fine variations in color, but it cannot distinguish fine variations in brightness. An incandescent light bulb that is rated as 1000 lumens looks to be about the same brightness as one that is 850 lumens even if the two bulbs are in the same room. The dimmer bulb would need to be about 800 lumens (a 20% difference) before we would perceive that one is dimmer than the other. If the two bulbs are in two different rooms, then the dimmer bulb would need to about 700 lumens (a 30% difference) before we would perceive that one is dimmer than the other.

Note #2: The sun produces a continuous spectrum of light that spans infrared, visible, and ultraviolet light. What our eyes perceive as white sunlight is actually a mixture of all the colors of the rainbow. Different kinds of light bulbs produce different colors. There are no light bulbs that produce the exact same true white color we see in sunlight, though some technologies approach it.

Note #3: An important thing to understand concerning light bulb design is that there is no such thing as a free lunch. Light bulb manufacturers design light bulbs by selecting a compromise between three factors: lumens, watts, and life (in hours). A light bulb can be re-designed so that it produces more light for the same watts, but it will have a much shorter life. Some long-life bulbs have lower brightness, with the designer taking advantage of the fact that you can't see the difference. The same game can be played with bulbs that use less electricity. A 52 watt bulb uses less electricity than a 60 watt bulb, but your eyes can't see that one is dimmer than the other. If you are trying to decide between two different light bulb technologies, do the “life cycle cost” math, which is how much it costs over the life of the system. If you plan to use the lamp for several thousands of hours, you will tend to save money by using a more efficient light bulb technology because the cost of electricity dominates the sum in households, though not so in cars.

Note #4: Most light bulbs are sold by their wattage, not their lumens nor their efficiency. This is practical when comparing two light bulbs of the same technology. An incandescent bulb at 100 watts will produce more light than an incandescent bulb at 75 watts. The comparison is impractical, however, when comparing two light bulbs of different technologies. A halogen bulb at 75 watts may produce as much light as an incandescent bulb at 100 watts. For automotive bulbs, a 9006 will produce a certain amount of light no matter what technology is used because the light level is defined by law (FMVSS 108). When comparing two 9006 bulbs, it is usually the life of the bulb that distinguishes one from another.

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2) Incandescent Technology

When you talk about lighting, most people think in terms of the more familiar incandescent bulb. Incandescent lights consist of a fine coil (or "filament") of tungsten wire inside a glass bulb. Electricity passing through the filament raises its temperature until it glows, or "incandesces."

The bulb either has a vacuum inside or is filled with a mix of argon and nitrogen. If any water or oxygen is present inside of the bulb when it is turned on, the filament fails within seconds.

The pressure inside is slightly lower than the surrounding air when the bulb is cold (turned off). That's why they pop when you drop them. When the bulb is fully warmed up, the pressure inside is about the same as the surrounding air.

An incandescent bulb uses “soda lime” glass. This is a "soft" glass, and it cannot tolerate large differences in temperature. A drop of water on a hot bulb will make it shatter.

For comparison, automotive sealed beam headlights are made from a "hard" borosilicate glass, which is similar to Pyrex (TM). Borosilicate glass is more expensive than soda lime glass, but can withstand large, rapid changes in temperature. That’s why your sealed beam head lights don’t explode when rain water hits them.

Incandescent bulbs are inexpensive to buy, but they have a short life (only a few hundred hours) and use a lot of electricity. Incandescent bulbs noticeably darken over their useful life. The tungsten wire evaporates slowly, and the tungsten is deposited on the glass wall. The inside of the bulb slowly develops a coating of metal, which reduces the amount of light that can get out. The larger the glass bulb, the less noticeable the effect. Very small bulbs, like those used in automotive taillights and the even smaller ones in dashboards, develop a silvery appearance inside the bulb by the time they fail.

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03) Halogen Technology

Halogen bulbs are a variation on incandescent bulb technology. There are definite advantages and disadvantages to this type of lighting.

Halogen bulbs are more expensive than incandescent bulbs. Here is why:

One of the things that shorten an incandescent bulb's life is that the tungsten within the bulb evaporates and deposits on the bulb wall. If a trace amount of a halogen gas (such methyl bromide) is added inside the bulb, a chemical reaction takes place which removes the tungsten from the bulb wall and deposits it back onto the filament, extending the life of the bulb.

However, in order for that beneficial chemical reaction to take place, the filament needs to be hotter than for incandescent bulbs. The good news is that a hotter filament produces a whiter light and is more efficient (more lumens per watt). The bad news is that a hotter filament means that the tungsten is evaporating faster, so a denser, more expensive fill gas (krypton) and a higher pressure are used to slow down the evaporation.

However, the higher fill pressure will break a bulb made of soft glass, so a more expensive hard glass is used instead.

Since costs rise when moving to a harder glass, typically the glass envelope is made much smaller. However, a smaller glass envelope around a hotter filament results in a glass envelope that will get much hotter. Since the bulb wall is hotter, it takes more careful engineering to design a head lamp with a replaceable halogen bulb because it is difficult to account for the higher temperatures.

Warning: When replacing any halogen bulb, do not touch the glass envelope with your bare skin. The salts in your skin oils penetrate and weaken the glass. The bulb not only has a shorter life, but when the bulb dies the filament doesn't merely burn out, but rather the bulb envelope shatters. The light bulb industry calls it a "non-passive failure."

Xenon-Halogen: Some newer halogen bulbs use expensive xenon instead of argon or krypton as the fill gas to improve the performance of the bulb. Some people call these new bulbs "xenon bulbs", but other light bulb technologies also use xenon, so the name "xenon bulb" is confusing. "Xenon-halogen" would be a more precise term.

Halogen-IR(TM): Halogen-IR(TM) is a halogen bulb with multiple thin layers of a special coating on the outside of the glass bulb. By carefully controlling the thickness of the layers, a selective filter results that is transparent to visible light but reflects infrared light. The infrared light that normally would escape as heat is reflected back onto the filament. Thus the filament is warmed by heat that would have escaped into the lamp's surroundings, which makes the bulb more efficient.

There are automotive H-IR (TM) bulbs, but the most widespread use of the technology is in outdoor flood lamps. A 60 Watt H-IR(TM) can produce output similar to a 120 Watt standard incandescent or a 90 Watt standard halogen.

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4) Xenon Metal Halide (Automotive HID) Technology

There is a whole family of HID (High Intensity Discharge) light sources, and the automotive versions are only a small part of that large family. In general, all of these HID sources are “mercury vapor” lights.

Mercury lights are bright, compact sources. Mercury vapor lights, and their metal-halide cousins, have been described as "lightning in a bottle." They are discharge sources like fluorescent, but the mercury vapor lights produce a blindingly bright continuous arc across the electrodes. The higher pressure of the mercury vapor light changes the light's output compared to the low pressures in a fluorescent bulb.

Mercury vapor lights tend to emit a large amount of ultraviolet radiation. In some mercury vapor bulbs, an additive (a "dopant") is mixed into the glass to block the ultraviolet light.

Mercury vapor lights are very efficient, and are used in commercial landscaping, roadway overhead lighting, warehouse and factory lighting, and sports field lighting. They have a life that is measured in many thousands of hours.

One noticeable characteristic of mercury vapor bulbs is a slow warm-up time. They emit only a very faint glow for the first couple of minutes, and do not reach full brightness for about ten minutes. If you quickly turn them off and back on, it takes several minutes again to regain full brightness.

Metal Halide:

The metal-halide bulb is a variation of the mercury vapor bulb. Chemicals categorized as metal-halides (such as lithium iodide) are added inside the bulb envelope to improve the color of the light. For example, lithium adds a strong red color to the mix of colors produced by the light bulb. The color rendering of metal-halide bulbs is impressive, and they have high efficiency. They tend to cost more than their mercury vapor cousins, but find their way into the same applications as mercury vapor lights. Like mercury vapor lights, metal-halide lights have a long warm-up time.

Xenon Metal Halide (automotive HID):

Some metal-halide bulbs use xenon as the fill gas. These bulbs are called, appropriately, "xenon-metal-halide" bulbs. One notable special application of HID is automotive lighting. HID lighting initially was found only on high-end luxury cars in Europe. They are now found on about half the cars sold in Europe, and are gaining rapid acceptance (in spite of their high cost) in the United States.

One obvious hurdle to adapting HID to cars was the very slow warm up time. For all practical purposes, vehicle lighting needs to be "instant on." Modifying the ballasts and the bulbs to tolerate such use has been an ongoing development process for HID manufacturers. Starting an HID bulb rapidly shortens its life. Also, the more often you turn the bulb off and on, the shorter the life. Fortunately, the life reduction occurs from cold restarts, not hot restarts. If your head lights are warmed up and you flash your head lights, you won't reduce the bulb's life noticeably.

Automotive HID life expectancy is about 2000 hours, which is twice the 1000 hours you'd expect from some high-end halogen bulbs, but is a much shorter life than non-automotive HID bulbs. Automotive HID bulbs, though, are more efficient and produce much more light than halogen bulbs. A 35 watt automotive HID bulb will produce more light than a 65 watt automotive halogen.

Even though the addition of metal-halides improves the color, the color is still strongly bluish, which is evident when you see a car with HID bulbs approaching you at night. The bulbs also slightly change color with age. If one bulb is replaced, both should be replaced to maintain a uniform appearance of color. Note, too, that the beam pattern may have a lot of colors in it when the bulb is first started (a cold start). When a warm HID bulb is turned off and allowed to cool, the metal halide chemicals condense and solidify in the bottom of the bulb. When the bulb is started cold, it takes time for the metal halides to heat up and evaporate and become part of the bulb’s fill gasses again. Once the bulb is fully warmed up, the color should be uniform.

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5) LED Technology

LED (Light Emitting Diode) lights do not produce light by heating a filament nor by creating a spark between two electrodes. Diodes are tiny electronic components made from two semiconductor materials. One material has atoms with an excess of electrons. The other material has a deficit of electrons (called "holes"). The contact patch between the two materials is called a "junction." With a sufficient voltage, the excess electrons cross the junction to fill the holes on the other side. As the atoms’ electrons fall into the holes, they give off very precise quantities of energy in the form of a photon (light).

The amount of energy in the photon dictates the photon's frequency, and the frequency corresponds to a particular color. Thus, LEDs emit a precise color (they are "monochromatic"). Most other light sources have numerous frequencies combined together. Not so for LEDs. If the end result that you are seeking is a specific color (other than pure white), then LEDs are highly efficient.

LEDs are known for producing very little heat, but that perception needs to be balanced by the fact that LEDs produce very little light. If you want a lot of light out of a small package and pack numerous LEDs into close quarters, then you will have a light assembly that becomes hot to the touch, hot enough even to melt the plastic lens if the light is poorly designed or if the light is used in an application for which it was not intended. LEDs also produce noticeably less light as they warm up.

LEDs offer some important advantages. Being electronic components, the LEDs themselves are immune to vibration. Assuming that the LED's circuit is properly protected from dirt and moisture, LEDs will last hundreds of thousands of hours. That is an extremely long life for a light source. Protecting the circuit and assuring a durable connection to the power source, however, are not trivial matters. Notice how many cars you have pulled up behind that have LED taillights with some of the LEDs not working.

LEDs have a long life. Some newer LEDs have sought to increase the light output by intentionally running the LED at an excessive voltage, which decreases their life. On the one hand, this is a smart compromise. The LED's life is so very long to begin with that decreasing that life even 100 fold still leaves you a long-life light source. A more serious drawback in over-driving LEDs is that they produce a lot more heat. If you double the light output, you more than double the heat produced. In some applications the over-driven LED designs make sense, but not for all.

In the 1990’s, white LEDs began to be marketed. The technology is advancing rapidly, but is still in its infancy. As a source of illumination (numerous LEDs packed together), and on a basis of light output (lumens) per dollar, most LED lights are expensive. Individually, white LEDs are among the most expensive of LEDs. Recall that LEDs are inherently monochromatic, and that white is a combination of many colors. Thus, white LEDs are a false white, and various approaches have been taken to create a color that our eyes perceive is “white.”

Advances are being made to improve the white LED's outward color (color temperature) and their ability to properly illuminate all colors (the Color Rendering Index). As the cost, efficiency, and CRI of the white LEDs improve, they will find an expanding role in our lives.

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6) Neon Technology

Like fluorescent, neon is a discharge source. The neon tube has electrodes at both ends, though they are different in construction. A fluorescent bulb has filament-like electrodes, but a neon bulb has sheet metal electrodes formed in various shapes, such as cylinders.

Both neon and fluorescent bulbs have a low-pressure gas inside, though the gas pressure in neon bulbs is about 50 times greater than that of fluorescent. Neon tubes are at about 1/15 atmospheric pressure, whereas fluorescent bulbs are only at about 1/750 atmospheric pressure.

Like fluorescent, most neon bulbs need a high voltage, so they need a ballast circuit. Unlike fluorescent, neon bulbs don't necessarily have to have mercury and phosphors, though many neon bulbs do have both.

The color depends on what kind of fill gas is inside of the bulb and the phosphors. Pure neon without phosphors will produce a fiery red color. An argon-mercury bulb without phosphors will produce blue light. An argon-mercury bulb can with phosphors can produce a variety of colors depending on the phosphors used. In some designs, the glass is tinted as well.

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#9 ·
You're welcome! I haven't worked for General ELectric since 1994, but somehow all of that stuff stayed in my head. I hope the post doesn't put too many folks to sleep. I have often joked that I know more about light bulbs than anyone in their right mind would want to know.
 
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