Fluorescent lamps are available in a variety of colors for decorative use as well as numerous shades of white for general illumination. The predominant white shade recommended for most office areas is cool white. Other shades of white used for various purposes include daylight, deluxe cool white, white, warm white, and deluxe warm white. Daylight and cool white provide a crisp, cool, businesslike atmosphere, and the warm whites find application in restaurants, homes, theaters, and similar areas. The appearance of colored materials will be better under a fluorescent light that contains a high component of the same color. Blue backgrounds improve with the cool whites and daylight. Deluxe warm whites strengthen oranges and yellows. Fluorescent lamps require ballasts to limit the current and to supply proper voltage to start and to operate the lamps. For general lighting purposes, the ballasts also contain a capacitor to improve power factor. The NEC© requires that all indoor fluorescent fixtures  (except  those  with  simple  reactance  ballasts) incorporate  ballasts  with  thermal  protection.  The thermal protector isolates the ballast and fixture from the circuit in the event of overheating. As a result, damage  from  fires  and  from  leaking  compounds  should be reduced. There are small fuses available that can be installed in the fixture to provide this protection for existing  ballasts. MERCURY LAMPS Mercury  lamps  (fig.  5-77)  have  the  best  maintained light  output  because  the  electrodes  operate  at  a relatively   cool   temperature,   resulting   in   less evaporation of the metals and oxides. The clear mercury  lamp  has  a  better  lumen  maintenance  than those  with  phosphor  coating.  Long  average  life  (16,000 hours and up) is a primary characteristic of most mercury lamps. There will be a different economic life for mercury lamps at each installation, depending on lamp mortality, power cost, equipment and wiring costs, frequency of replacement and cleaning of lamps, and other factors. Mercury lighting is one of the most economical  means  of  lighting  high-  and  medium-bay industrial  areas,  particularly  in  areas  where  color rendition  is  not  critical.  Small  wattage  lamps  have  been introduced,  and  it  is  anticipated  that  future  designs  will see a more widespread use of these in low-ceiling nonindustrial areas. An objectionable characteristic of mercury lamps is the time required to reignite (several minutes) after a momentary loss of power. CAUTION While the lamp bases are the same size as incandescent  lamps,  mercury  lamps  must  never be’used  to  replace  a  burned-out  incandescent lamp because a ballast must be used with mercury lamps. METALLIC-VAPOR LAMPS Metallic-vapor lamps resemble mercury-vapor lamps in appearance and have similar uses. The initial efficiency and control are better with metallic-vapor lamps  than  with  mercury-vapor  lamps.  In  regard  to disadvantages, the rate of depreciation is much greater with metallic than with mercury, the cost is higher, and the life expectancy is shorter. High-pressure  sodium-vapor  lamps  are  used  for highway interchanges, parking lots, and high-bay industrial areas. They are the most efficient of the light sources in general use in number of lumens produced per watt of electricity. The relative cost is high, the life ratings are not well established, and a special ballast is required. The color is slightly yellowish OVERALL ILLUMINATION Walls, ceilings, and surroundings are an important part of the overall illumination system since they redirect light to the working area. The most efficient lighting system is obtained when the fixtures are new and when the walls, ceilings, floors, and furnishings of Figure 5-77.—Mercury lamp. 5-44