A fluorescent lamp consists of a sealed glass tube. The tube contains a small amount of mercury and an inert gas, like argon, kept under very low pressure. In these electric-discharge lamps, a fluorescing coating on the glass-called phosphor powder-transforms some of the ultraviolet energy generated into light. Fluorescent lamps also require a ballast to start and maintain their operation.
Early fluorescent lamps were sometimes criticized as not producing enough warm colors, making them appear as too white or uncomplimentary to skin tones. A cool white fluorescent lamp has a CRI of 62. But today there are lamps available with CRIs of 80 and above that simulate natural daylighting and incandescent light. They also are available in a variety of CCTs: 2900 to 7000 (about CRI and CCT).
The "T " designation for fluorescent lamps stands for tubular-the shape of the lamp. The number after the "T" gives the diameter of the lamp in eighths of an inch. The T8 lamp-available straight or U-shaped-has become the standard for new construction. It also commonly serves as a retrofit replacement for 40-watt T12 lamps, improving efficacy, CRI, and efficiency.
In some cases, T10 and T9 lamps offer advantages over both T12 and T8 lamps, including higher efficacy, higher CRI values, a wider selection of CCTs, and compatibility with several ballast types.
Then there are T5FT fluorescent lamps. These lamps produce maximum light output at higher ambient temperatures than those that are linear or U-shaped.
Linear fluorescent lamps often are less expensive than compact fluorescent lamps. They can also produce more light, are easier to dim, and last longer.
Cold cathode fluorescent lamps are one of the latest technological advances in fluorescent technology. The "cold" in cold cathode means there is no heating filament in the lamp to heat up the gas. This makes them more efficient. Also, since there's no filament to break, they're ideal for use in rough service environments where a regular lamp may fail. They are often used as backlights in LCD monitors. They are used in exit signs too.