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.

the room are colored with a high-reflectance color. Lighting is, however, only one of many factors that make up the whole environment. While the highest lighting efficiency may be maintained in a completely white room, the psychological effect of such surroundings on the occupants may be less than harmonious. The use of color in the surroundings, even if it means sacrificing lighting efficiency, is necessary for the well-being of the occupants. Ceilings should have the lightest color, preferably an off-white. Shiny surfaces should be avoided, as they result in glare. As mentioned earlier, lighting levels start dropping immediately after the installation of the fixtures. Lamp burnouts and lamp depreciation contribute to this, but the principal cause is the accumulation of dirt. It is not uncommon to find lighting levels one half of the initial values after only a year or so of operation. The lighting maintenance program must include cleaning and painting of the walls and ceilings in addition to the fixture cleaning schedule.

When mounting any light fixture, follow the manufacturers' step-by-step instructions. The most important thing to remember when mounting any fixture or device is to ensure that all connections are both electrically and mechanically secure.

High-intensity discharge lamps (HID) will be discussed further in chapter 6.

TROUBLESHOOTING LAMPS As light sources are designed to operate most efficiently and economically at their rated voltages, special emphasis should be given to using lamps to suit the voltage of the circuit. Operation within the normal operating range is desirable; because both overvoltage and undervoltage operations have a determined effect on the life, efficiency, and economy of the light source. The effect on lamps operated over or under their rated voltage range is described below.

FLUORESCENT LAMPS Line voltage higher than the maximum of the ballast range will shorten lamp and ballast life. Line voltage below the minimum range will reduce illumination and may cause uncertain starting of some types of fluorescent lamps.

INCANDESCENT LAMPS Line voltage higher than the maximum lamp range will increase the light output but will shorten lamp life. Line voltage below the minimum range will extend lamp life but will reduce light output approximately 3 percent for each 1 percent in voltage drop.

MERCURY LAMPS Line voltage higher than the maximum lamp range will shorten lamp and ballast life. Line voltage below the minimum range will reduce illumination and may cause uncertain starting.

LAMP GUIDES The most common troubles encountered with lamp equipment, the probable causes, and the suggested solutions are contained in table 5-6.

MAINTENANCE OF LIGHTING SYSTEMS Lighting has a great influence on the quality and quantity of work as well as a direct bearing on employee morale. The necessity for periodic attention to the lighting system cannot be overemphasized. To prevent progressive deterioration of the system, personnel must provide regular maintenance and prompt repair of any deficiency. Maintain the required illumination intensity by keeping lamps, fixtures, and reflective areas clean and in good repair; by replacing defective lamps; and by keeping the voltage steady.

It is well known that dirt absorbs and masks light. The progressive decrease of light caused by accumulat-ing dirt renders periodic cleaning of lighting equipment a necessity. The frequency of cleaning depends largely upon local conditions. Fixtures in air-conditioned and air-filtered rooms may require cleaning only once a year. In an atmosphere that is heavy with dust and fumes, cleaning every few weeks may be necessary. The cleaning schedule for a particular installation should be determined by light meter readings after the initial cleaning. When subsequent foot-candle readings have dropped 20 to 25 percent, the fixtures should be cleaned again. Readings should be made with the light meter at the working surface with the meter reader in the position of the operator or person using the working surface.

Lighting equipment should be washed, not just wiped off with a dry cloth. Washing reclaims 5 to 10 percent more light than dry wiping and reduces the possibility of marring or scratching the reflecting surface of the fixtures. To clean removable glassware, reflectors, and diffusing louvers, immerse them in a solution of

Table 5-6.- Lighting Maintenance Chart

synthetic detergent cleaner and scrub them with a soft brush or sponge. When incrustation is not removed by scrubbing, use No. 0 steel wool to remove dirt film. Rinse in warm, clear water and dry with a clean cloth. 

CAUTION Do not immerse lamp base or electrical connections in the cleaning solution.

Glassware, reflectors, and diffusing louvers that cannot be removed should be cleaned as follows:

Wipe with a moist cloth or sponge, using a solution of synthetic detergent cleaner. When incrustation is not removed by sponging, use No. 0 steel wool to remove dirt film. Take care to ensure that shreds of steel wool do not touch the pin contacts or get into the lamp socket.

Wipe off excess moisture with a clean cloth. Clean fixture holders and stem hangers with a moist sponge or cloth dampened with synthetic detergent cleaner and wipe dry. Enameled, chrome, aluminum, or silver-plated reflecting surfaces that cannot be adequately cleaned and polished should be replaced.

Neglected lamp outages reduce illumination. If burned-out lamps are not promptly replaced, illumination may drop to unsafe foot-candle levels in a short time because of outages alone. In some cases, it may be satisfactory and more economical to clean lamp surfaces and fixture interiors only at the time of relamping. Each activity must determine whether cleaning is to be accomplished by electrical, self-help, or custodial service personnel.

Burned-out lamps are replaced on request. To prevent reduced illumination from lamp outages, do the following: Instruct employees to report burnouts as they occur.

Replace blackened or discolored lamps, even though they are still burning. Discoloration indicates the lamp is nearing the end of its useful life. Replace fluorescent lamps as soon as they begin to flicker. A burned-out lamp in a live circuit may cause damage to starter and ballast. Blackening at the ends of the tube adjacent to the base indicates that the lamp is near the end of its useful life. In general, replace with the same type, wattage, and voltage as that of the lamp removed. If frequent burnouts occur, the voltage rating of the lamps may be too low. Lamps of higher wattage than called for on lighting design plans should not be used.