The Fluorescent Lighting System

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Section 3: All about Fluorescent Lamps

Because of their versatility, fluorescent lamps come in many sizes, types, shapes, colors and light intensity. Some are designed for operation in cold locations, while others generate large amounts of light at the expense of service life.

This section discusses topics specific to the fluorescent lamp. Because efficiency of a fluorescent lighting system is controlled by the lamp and other components of the system, efficiency is discussed in Section 8.

A one paragraph summary of the theory of operation of a fluorescent lamp can be found in Section 1, while the theory of operation of each component in the entire Fluorescent Lighting System can be found in Section 2.

Color and Special Purpose Light Fluorescent Lamps

In addition to general lighting use, many fluorescent lamps are produced for decorative purposes. Lamps that produce, red, green, blue and other colors are available. From most vendors, green and blue lamps appear normal until illuminated. Bright phosphors exist that generate green and blue colors, so a tube with untinted glass is used with these phosphors for blue or gree light colors. In the case of red and some of the other colors, a phosphor that produces a bright light is chosen (possibly just one of the white phosphors) and then the glass is tinted to the desired color.

In addition to primary colors, several shades of "White" are marketed by the fluorescent lamp manufacturers for general lighting use. These tend to fall into three shades: Warm White, Cool White and Daylight, although some manufacturers have multiple shades or grades of each, adding the word "Deluxe", "Designer" or designations.

Of course, there is no "white" color of light. Humans perceive white as a combination of several frequencies of light, which can be simplified to just a few primary colors, such as the red, green and blue used in television. A blend of phosphors is used to generate what is considered to be a white light. Depending on the blend, different shades of white can be produced.

The Warm White lamps have more red and orange light, and are marketed towards use in residential and other areas desiring light similar to what is produced by incandescent lighting. Grocery stores will sometimes use these lamps only in the meat and bakery departments to improve the look of these products, while using Cool White lamps elsewhere in the store.

Cool White lamps are the "normal" fluorescent lamp color, with more green in the produced light. This is the most commonly-used lamp color by far for traditional fluorescent lamps. Compact Fluorescent Lamps (CFLs) are still targeted at the home, so multi-lamp packages for consumers tend to come only in "Warm" or "Soft" shades of white.

Daylight lamps have a blue tint and are supposed to mimic the color of light reflected from a clear sky. These tend to have a "cold" appearance.

In Compact Fluorescent Lamps (CFLs), Soft White is really a variant of Warm White, which appeared starting around 2003. This color had much more red in it than Warm White, with Kelvin values in the 2,600 to 2,850 range. By 2007, consumers appear to have rejected the color, as Kelvin values this low are rarely seen on the shelves of the major retail stores. Kelvin values this low have not been seen in non-CFL lamps.

Bug Light

This is the fluorescent lamp equivalent of the incandescent "bug light". A tinted glass produces an yellow-orange light that is less attractive to flying insects than other light sources. It is designed for use on porches or other outdoor areas where light is needed but insects are not desired. A brighter, full-spectrum light must be located nearby to give the insects another light source that can attract them away from the area where the bug lights are located. Bug lights don't work on all flying insects, and won't work at all if they are the sole source of light in the area.

Plant "Grow" Light/Aquarium Light

The typical plant lamp generates both a red or near-red light, and a blue light, appearing purple or pink in shade. However, chlorophyll responds to only a few frequencies of light, so phosphers are selected that target these frequencies. In particular, plants are able to utilize red light centered at 642nm and 662nm, and blue light centered at 430nm and 453nm. While phosphors for blue light exist that cover the needs of chlorophyll, there are no phosphors for red that precisely cover the frequencies that chlorophyll wants.

Because the blue wavelengths available for phosphor are more compatible with what the plants actually want, having more blue phosphor in the mix probably produces better plant results, but determining what the mix is for a particular lamp likely requires actually viewing the lamp type in operation since the phosophor blend information is usually not published.

Black Light

The Black-light lamp (also known as "Black Light Blue") uses a specially tinted glass similar to the cobalt tinted glass used in chemistry applications, and a special phosphor to generate long-wave ultraviolet light while blocking the short-wave ultraviolet and most visible light. Such tubes are dark purple in appearance and are used in a variety of applications. Long-wave ultraviolet radiation is not immediately harmful, unlike the output from a germicidal lamp. Long-wave ultraviolet light is not visible to the human eye and the light that can be seen from a black light blue lamp is a small amount of violet visible light that manages to get through the filter glass.

Although stores will frequently attempt to sell a fluorescent Black Light lamps with their own fixtures, fluorescent Black light lamps may be directly substituted for the same size "white" fluorescent lights in existing fixtures. The lamps may be purchased separately, and are available in a variety of sizes, including Compact Fluorescent Lamps. These are handy for lighting indoor areas using existing fixtures when having parties or other short-term activities.

In general, fluorescent Black Light Blue lamps cost two to four times what a good quality "white" Fluorescent lamp of the same size costs, at least through lighting supply or the larger chain hardware stores.

I recently bought two four-foot Black Light Blue lamps for under $10 US each and a $10 US two-bulb four-foot "shop fixture" for $30 US total, all from a chain home improvement store. At the national chain electronics store next door, they were selling a single two foot Black Light Blue lamp with its own fixture for $40 US, producing 1/4th the amount of light at more cost than what I put together myself. Speciality stores and drug stores usually charge much more for the same lamps and are really poor choices for shopping for any kind of lamp (unless it is an emergency), so shop around.

Comparison of Lamp Colors

Color Name Soft White
Warm White
Cool White
Plant or Aquarium
Black Light Blue
Typical Color Temperature (in Kelvin's) or approximate Wavelength (in Nanometers) 2,600K
647nm to 700nm 585nm to 647nm 491nm to 575nm 424nm to 491nm 647nm to 700nm
424nm to 491nm
Phosphor wavelengths commonly used in lamps At least one phosphor from each of the Red, Green and Blue columns is used in these lamps. The ratio of the phosphors used determines whether the produced light is perceived as Warm White, Cool White, or Daylight shades. 611nm
(Red- Orange)
Mix of
(Information on printing color tables on color printers can be found here.)

The wavelength ranges shown for non-white colors are as stated in CRC Handbook of Chemistry and Physics, 52nd edition. The actual light wavelength(s) emitted by a given lamp varies depending on the phosphors used by the manufacturer. For comparisons of lamps with approximately the same color temperatures, see Section 8.

The Kelvin color temperature ranges shown are based on published values for T12, 40 watt fluorescent lamps produced by Philips and Sylvania.

For the warm white, cool white and daylight colors, a manufacturer may produce several different lamp models with the same category designation (such as "cool white"), but the color temperature and brightness frequently deviate between lamp models. The deviation may be caused by the desire to generate more light output which may force the selection of a less desirable mix of phosphors, to use less energy (which almost always means less light output) or it may simply indicate older or newer lamp phosphor formulas. In some cases, the choice of phosphors is dictated by standards bodies.

In recent years, some manufacturers have added a new specification to their near-white lamps, called the Color Rendition Index, or CRI. The higher the number, the better the light quality is supposed to be. In general, this means that the phosphors selected generate frequencies of light closer to what the cones in the human eye can most readily detect.

In general, just knowing the number of lumens that the lamp produces and the approximate frequency (measured in Angstroms or nanometers) or the color temperature (measured in Kelvin's) are the far more useful values.

For color and special purpose lamps, there is a wide deviation between manufacturers as to what frequency light is produced in a lamp of a given color, so mixing lamps from different manufacturers may result in several shades of the same color. Further, lamp manufacturers tend to not disclose lumens or other specifics on non-white lamps, so there can be a significant difference in the amount of light the lamps from different makers produce.


One other special application fluorescent lamp has no tint or phosphor on the tube at all. This is called the germicidal fluorescent lamp, and they can be found in sterilizers and EPROM erasers. (An EPROM, or Erasable Programmable Read Only Memory, is an older device for storing computer data indefinitely.) These lamps emit large amounts of short-wave ultraviolet light, and this light is quite hazardous to biological tissue and will kill organisms. Such lamps have stern warnings printed on the tube advising you to never look at the tube when it is operating. These tubes are clear and you can see right through them.

Photograph of a germicidal fluorescent lamp and the safety warning label on the glass
Photograph of a germicidal fluorescent lamp and the safety warning label on the glass.

The light produced by the germicidal fluorescent lamp is identical to that produced by a damaged Mercury-Vapor lamp that has lost its outer globe. Normally, such lamps have an outer globe that absorb the ultraviolet light produced, letting only a bright visible blue light escape the lamp. Some mercury-vapor lamps have a phosphor layer on the outer globe so that it produces a light with more of a white look. Mercury-Vapor lamps are supposed to automatically extinguish themselves if the outer globe breaks, but this doesn't always work.

Fluorescent Lamp Shapes and Sizes

The most common fluorescent lamps are a shaped as a straight tube. Depending on the lamp type, these tubes will have one or two pins that extend from each end of the tube, or may have a plastic tip with recessed contacts.

Straight-line tubes come in standard lengths and diameters. The most common diameter is 1.5 inches, and these are known as T12 lamps. Lamps up to 96 inches in length come in this diameter, with 48 inches being the most common size.

For shorter-length tubes, narrower diameters are more common, including the T8 (1" diameter) and T5 (0.5" diameter) sizes. In recent years, the T8 size has become popular in 36 and 48 inch sizes, lengths where previously only the T12 diameter was available.

The length and diameter of the tube enter into the calculation of how much power the lamp will consume and how much light it can generate.

Apart from the straight-line tubes, fluorescent lamps also exist in modularized shapes in which two or four tubes are interconnected (or may be a single tube bent multiple times) so that a single arc of electricity passes through all sections of the tube. These types of lamps usually are seen in some smaller fluorescent lamps designs that allow the lamp to be detached from the base (containing the ballast and starting circuitry), and the lamp can be replaced.

The "twist" tubes that are highly popular in most Compact Fluorescent Lamp (CFL) designs are permanently attached to the base (which contains the ballast and starting circuitry), so the lamp tube itself typically does not have any part numbers or other standardized details. The manufacturer of the entire assembly usually declares what amount of light the unit produces, the power consumed by the unit, and may or may not give an indication of the color of the light the lamp produces. More information on Compact Fluorescent Lamps (CFLs) can be found in Section 7.

Fluorescent Lamp Functional Types

Fluorescent lamps are made for use in three classes of fluorescent light fixtures: Pre-Heat, Rapid-Start, and Instant-Start. Certain size and shape lamps are only made for one or perhaps two of these operational systems. For example, the 96 inch fluorescent lamps are only available for use with Instant Start fixtures. The newer T8 32 watt 4' lamps are instant start and also use a solid state ballast.

Some size lamps can be used in both Pre-Heat and Rapid-Start fixtures. The details of fluorescent fixtures and the components used in the three starting systems are discussed in other sections.

Modern Compact Fluorescent Lamps (CFLs) are instant start or rapid start designs, with Instant being more popular. However, some CFLs also have the curious behavior of illuminating immediately when power is applied, but at a noticbly lower light level, and then the lamp takes some minutes to reach its ideal brightness. Some even are initially bright, then grow dim and slowly get bright again. This annoying behavior is one of the few remaining objections people still have to using CFLs in place of incandescent lighting.

Fluorescent Lamp Operating Life

When used with the correct ballast and starter, fluorescent lamps have a long service life. Most manufacturers list 20,000 hours for the most common fluorescent lamps, although some "super energy saver" and compact lamps have an operating life of 10,000 or 15,000 hours. However, all of these life expectancy ratings are not based on continuous use. Instead, they assume a pattern of modest use where the lamps operate several hours total per day and are started only a few times each day.

The most wear on a fluorescent lamp occurs when it is started (particularly if the ambient temperature is lower than 50F), so it is not surprising to find fluorescent lamps that are left on for extended periods end up lasting far longer than the 20,000 hour average life. Similarly, a lamp that is turned on and off dozens of times a day will probably fail before it reaches half of the estimated service life.

The highest power consumption of a fluorescent lamp and fixture occurs during the starting process, and it is generally accepted that each start of a fluorescent lamp shortens its operating life by about twenty minutes. Also, during the start and first five minutes of operation, the typical fixture uses about the same amount of electricity it would have consumed running normally for seven minutes.

The wear-and-tear of the starting process is one reason why manufacturers recommend replacing pre-heat starters when the lamps are replaced. An old or faulty starter can attempt and fail to start a lamp a hundred times a minute (seen as a flickering lamp), and can completely wear-out a new lamp in just a few days.

Because of the start-up issues, many manufacturers recommend that if a fluorescent lamp is installed in an area that is intermittently occupied, the lamp should be left on if it will be needed again within twenty minutes. By leaving the lamp on instead of turning it off and back on again in five or ten minutes, the lamp will last longer and will use less electricity, averaged over its entire life.

As part of efficiency campaigns, many commercial facilities now use timed motion sensors that control the building lighting in some or all areas. For the most efficient use of fluorescent lighting, these motion sensors should be set to leave the fluorescent lamps on for at least twenty minutes after no motion is detected, before turning the power off to the fluorescent lighting. More information on efficiency and operating life can be found in Section 8.

Related Topics

Section 4: Traditional Pre-Heat Fluorescent Fixtures (HTML) [NEXT]

Section 8: Fluorescent Lighting Energy Savings and Product Comparisons (HTML)

Return to The Fluorescent Lighting Reference Index (HTML)

[Copyright 2002,2004,2007 Frank Durda IV, All Rights Reserved.
Mirroring of any material on this site in any form is expressly prohibited.
The official web site for this material is:
Contact this address for use clearances: clearance at
Comments and queries to this address: web_reference at]

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