CoolLight

“Green Spike” or Why Do Fluorescent Lamps Have Mercury in Them?

With all the talk about the green spike in the spectral energy distribution of fluorescent lamps and how that can affect us as media creators, I thought it relevant to include an excert from a publication by the National Electrical Manufacturer’s Association (NEMA) entitled “Fluorescent Lamps and the Environment”. This publication and a lot more valuable information can be found at the NEMA website devoted to proper lamp disposal called www.lamprecycle.org.

The only thing they didn’t talk about that I think is relevant to discuss is why is there a green coloration sometimes in video produced under fluorescent lamps? This is a result of the mercury gas in the lamp which produces significant light in the green part of the spectrum while it is being energized. Its not a coincidence that mercury was chosen actually FOR this property. It turns out that our eyes are most sensitive to the colors in the center of the visible spectrum–green in particular. So light that is heavily influenced by green seems brighter to us. It’s not much of a problem today with all the choices available in fluorescent lighting. See my article on Color Temperature and Color Rendering Index for more information on the subject.

An Adobe Acrobat PDF version of the publication can be downloaded from www.nema.org/lamprecycle/nemafluorfinal.pdf

They also have information on the proper disposal of fluorescent lamps too.

Why Do Lamps Need Mercury?
“Mercury is an essential ingredient for most energy efficient lamps. Fluorescent lamps and high intensity discharge (HID) lamps are the two most common types of lamps that utilize mercury. Fluorescent lamps provide lighting for most schools, office buildings, and stores. HID lamps, which include mercury-vapor, metal halide, and high- pressure sodium lamps, are used for street lights, floodlights, and industrial lighting. A typical fluorescent lamp is composed of a phosphor coated glass tube with electrodes located at either end. The tube contains mercury, of which only a very small amount is in vapor form. When a voltage is applied, the electrodes energize the mercury vapor, causing it to emit ultraviolet (UV) energy. The phosphor coating absorbs the UV energy, causing the phosphor to fluoresce and emit visible light. Without the mercury vapor to produce UV energy, there would be no light. A four-foot fluorescent lamp has an average rated life of at least 20,000 hours. To achieve this long life, lamps must contain a specific quantity of mercury. The amount of mercury required is very small, typically measured in milligrams, and varies by lamp type, date of manufacture, manufacturing plant, and manufacturer.”
“If lamp life is shortened, more lamps must be purchased to achieve the same length of service, and the number of lamps that generators must dispose will increase.”

How Much Mercury Do Lamps Contain?
“Based on a 1999 NEMA survey, the average four-foot fluorescent lamp contains about 11.6 milligrams (mg) of mercury. This number has been steadily declining as lamp manufacturers work to reduce mercury content to the minimum amount technically feasible without reducing lamp life. The average four-foot lamp today contains over 75% less mercury than the same lamp would have contained in 1985. According to the U.S. EPA, total global natural and manmade emissions to the environment are 5,500 tons. Manmade sources in the U.S. released 158 tons of mercury in 1995. For comparison, all of the lamps sold in the United States in 1999 contain only an estimated 13 tons of mercury, of which only a fraction will be released as an air emission. Proper lamp disposal or recycling will result in keeping almost all of this mercury out of the environment.”

End of Excerpt

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