Adventures in Lighting: Light Sources 101: Understanding Incandescent Illumination
Tungsten rods with evaporated crystals, partially oxidized with colorful tarnish.
Purity 99.98%, with a high pure 1 cm3 tungsten cube for comparison.
Photo by Alchemist-hp via Wikipedia
With the wide variety of light sources available to digital shooters these days, a lot of factors go into choosing the right light. This sub-series of Adventures in Lighting will break down and explain the different kinds of illumination and their uses to help Digital Video readers make more informed decisions.
We’ll start with one of the most ubiquitous forms of illumination in our world: tungsten (incandescent) bulbs.
“Incandescent” means light from heat, and although the sun and fire are both incandescent light sources, the most common source of incandescent light is the traditional filament light bulb.
The light bulb, refined (but not invented) by Thomas Edison, features a base (traditionally a medium screw base), two electrodes, a filament coil (most commonly of tungsten) and a glass envelope (bulb) that encloses the filament in an oxygen-free vacuum. When current is applied, the electrons pass from the positive electrode through the filament to the negative electrode. The movement of electrons through the tungsten filament, along with resistance to that flow of electrons, causes the filament to heat up to the point where it begins to incandesce—or emit light from the heat.
Closeup of a tungsten filament inside a halogen lamp.
The two ring-shaped structures left and right are filament supports.
Photo by Planemad (Arun Ganesh) via Wikipedia
Tungsten, a naturally occurring metal, is a very brittle substance, but it has the highest melting point of non-alloyed metals and the second highest melting point of all elements on the periodic table. (Tungsten’s melting point is 3,695°K or 6,192°F.) This makes it perfect for the source of incandescence, as the standard tungsten filament can heat up to more than 5,000°F.
With that extreme heat, if oxygen were present inside the bulb, the filament would simply burst into flames and burn out. Instead, the bulb is filled with an inert gas, most often argon or nitrogen. Since the tungsten is a delicate and brittle substance, it will start to deteriorate through use, especially during the extremes of current when the light is turned on or off. Small particles of tungsten will fall off of the filament and collect on the coolest part of the bulb, which is the cause of the blackening you see on older light bulbs. When the filament loses enough tungsten and becomes too brittle, it will break and the bulb will no longer light.
The glass of the bulb—called the envelope—must also be able to withstand the intense heat inside the bulb. Common bulbs have a borosilicate envelope that is positioned far enough away from the filament to prevent melting.
Carbon filament lamps showing darkening of bulb with use.
Photo by Ulfbastel (Ulf Seifert) via Wikipedia
Many people misuse the term “halogen” with regard to incandescent lamps. Halogen refers to a specific class of bulb, not simply the standard light bulb. Tungsten-halogen bulbs (usually called halogen, sometimes called quartz-iodine) are filled with a halogen gas in addition to the argon or nitrogen—the halogen gas is most commonly iodine, though sometimes it’s bromine, chlorine, fluorine or astatine. When heated by the filament, this iodine gas captures loose particles of tungsten that fall off the brittle filament and redeposits them back onto the filament, which prolongs the life of the bulb.
This process requires the filament to burn at a higher temperature than the standard incandescent bulb and, as a result, also requires a much thicker and more robust envelope, which is generally made of quartz glass.
Since these bulbs burn hotter, they actually have a higher color temperature than standard household bulbs. While the standard bulb will burn around 2,600°K to 2,800°K, halogen bulbs will burn between 2,900°K and 3,200°K, making them more beneficial for use in photography. The thicker, higher-melting-point quartz glass allows there to be less space between the envelope and the filament, making tungsten-halogen bulbs much more compact than their standard cousins. Nearly all professional lighting instruments for film and video utilize halogen bulbs. Because of their intense heat, halogen bulbs require special fixtures to house them. Although some household fixtures utilize halogen bulbs, most do not.
Pros and Cons of Incandescent Light
A modern tungsten-halogen 200W bulb featuring
a thicker quartz envelope and iodine
gas to prolong the life of the filament coils.
The sun is an incandescent light source. Simply put, it is a great ball of fire that emits light from the burning gases. This light contains all of the colors of the rainbow: red, orange, yellow, green, blue, indigo and violet. An incandescent light bulb is based on the same concept; the light emitted from the heated filament is what is called “full spectrum,” meaning that it contains the full spectrum of colors of the rainbow. Although tungsten lights are a lower color temperature than natural daylight (which is combination of direct light from the sun and ambient skylight to raise the color temperature), they still contain all the colors and will render “true” colors to photographed objects (as long as the camera is set to the correct white balance).
Incandescent bulbs can be dimmed using standard dimmers. The color temperature lowers as the bulb is dimmed, but they require no special hardware in order to dim.
Incandescent bulbs are commonly available and inexpensive. They come in a wide variety of intensities (wattage), from as low as 5W to as high as 20,000W. The standard medium screw base is one of the most common fixture connectors on the market.
There is no problem with “flicker” with incandescent lamps—at any frame rate or shutter speed.
Comparing a standard tungsten 200W bulb (circa 1970) and
a modern tungsten-halogen 200W bulb (below).The larger ceramic base
on the modern bulb is necessary because of the heat generated.
The primary problem with incandescent illumination is that there must first be intense heat before there can be light. Somewhere between 80 and 90 percent of the energy used by an incandescent fixture is emitted as heat, while only 10 to 20 percent is emitted as light. This is where the incandescent lamp has been vilified in recent years by the environmental lobby. Incandescent sources are not efficient in their use of electricity as most of the electricity is lost to heat.
Incandescent bulbs are not the mustache-twirling bad guy that the environmentalists would have you believe, however. Their natural light is often more pleasing to the human eye and more suitable for photographic applications. The bulbs contain no dangerous gases and, except for sharp glass, are not a danger if broken.
All that heat being generated adds up, however. With enough incandescent sources burning bright, you can significantly raise the temperature of the space you’re shooting in. This can make the shoot uncomfortable for talent and even dangerous for children or animals. It can also make food photography very challenging. The lamps themselves heat up considerably from the bulbs inside, making them difficult to handle when they’re hot.
Although incandescent sources are inefficient in their use of electricity, alternative light sources—with the exception of arc lamps (old carbon arc or HMI sources)—cannot yet match the intensities possible with large incandescent bulbs.