Qualities Of Light Sources

      In practical terms, light sources can be discussed in terms of the qualities of the light they produce. These qualities are critical to the result and must be understood when choosing the source for a lighting plan.

How Light Is Generated

      Most natural light comes from the sun, including moonlight. Its origin makes it completely clean, and it consumes no natural resources. But man-made sources generally require consumption of resources, such as fossil fuels, to convert stored energy into light energy. Electric lighting is superior to flame sources because the combustion of wood, gas, and oil produces pollution within the space being illuminated. Moreover, electricity can be generated from natural, nondepletable sources of energy, including the energy generated by wind, hydro, geothermal, and solar sources.

      How an electric lamp operates determines virtually everything about the light created by it. The common incandescent lamp generates light through the principle of incandescence, in which a metal is heated until it glows. Most other lamps, however, generate light by means of a complex chemical system in which electric energy is turned into light energy where heat is a side effect. These processes are usually much more efficient than incandescence—at the cost of complexity and other limitations. For instance, a fluorescent lamp generates light by a discharge of energy into a gas, which in turn emits ultraviolet radiation, which is finally converted to visible light by minerals that “fluoresce.” This process generates light about 400 percent more efficiently than incandescence and is the reason fluorescent lamps are promoted as environmentally friendly.

The Spectrum of Light

      The spectrum of light is seen in a rainbow or from a prism, and it includes all of the visible colors. We tend to organize color into three primaries (red, green, and blue) and three secondaries (yellow, cyan, and magenta). When primaries of light are combined, the human eye sees white light.Historically, using a filter to remove colors from white light generated colored light. Blue light, for instance, is white light with green, and red removed. Filtered light is still common in theatrical and architectural lighting.However, most nonincandescent light sources tend to create specific colors of light. Modern fluorescent lamps, for example, create prime colors of light (red, green, and blue) that appear to the human eye as white light. Other lamps, such as low-pressure sodium lamps, create monochromatic yellow light. While most lamps are intended to appear as white as possible, in some cases lamps are designed to create specific colors, such as green or blue.However, the intent of most light sources is to produce white light, of whose appearance there are two measures:

      1. Color temperature, which describes whether the light appears warm (reddish), neutral, or cool (bluish). The term temperature relates to the light emitted from a metal object heated to the point of incandescence. For instance, the color temperature of an incandescent lamp is about 2700K, appearing like a metal object heated to 2700° Kelvin (2427° Celsius or 4400° Fahrenheit).

      2. Color rendering index (CRI), which describes the quality of the light on a scale of 0 (horrible) to 100 (perfect).

      All white light sources can be evaluated by color temperature and CRI. Color temperature is the more obvious measure; two light sources of the same color temperature but different CRI appear much more alike than do two light sources of similar CRI but different color temperature.Natural light is generally defined as having a CRI of 100 (perfect). Color temperature, however, varies a great deal due to weather, season, air pollution, and viewing angle. For instance, the combination of sun and blue skylight on a summer day at noon is about 5500K, but if the sun is shielded, the color of the blue skylight is over 10,000K. The rising and setting sunlight in clear weather can be as low as 1800K (very reddish). Cloudy day skylight is around 6500K.When choosing electric light sources, it is generally best to select source color temperature and CRI according to the following table. Note that even if daylight enters the space, it is usually not a good idea to try to match daylight with electric light, as daylight varies considerably.

Point Source, Line Source, or Area Source

      Light sources vary in shape. The three basic shape types are point sources, line sources, and area sources. Each radiates light differently, thus causing distinctive effects.

Ballast or Transformer

      In order to operate correctly, many electric light sources require an auxiliary electric device, such as a transformer or ballast. This device is often physically large and unattractive and can create an audible hum or buzz when operating.

Lamp Size

      The physical size of the lamp affects the size of the luminaire and, in turn, determines how some sources might be used. Small, low-wattaage lamps permit small luminaires, such as undercabinet lights and reading lights; large, highpowered lamps, such as metal halide stadium lamps, require a large luminaire, both for heat and for the reflector needed to aim the light properly.

Voltage

      The electric power needed to operate a lamp is measured first by voltage. In the United States, the standard voltage services are 120 volts, 240 volts, 277 volts, and 480 volts. The standard 120-volt service is available in all building types; 240-, 277-, and 480-volt services are available only in large industrial and commercial buildings. Service voltage varies from country to country.Many types of low-voltage lamps, operating at 6, 12, or 24 volts, are used throughout the world. Transformers are used to alter the service voltage to match the lamp voltage.

Bulb Temperature

      The bulb of a lamp can get quite hot. The bulb temperature of incandescent and halogen lamps and most high-intensity discharge (HID) lamps is sufficiently high to cause burns and, in the case of halogen lamps, extremely severe burns and fires. Fluorescent lamps, while warm, are generally not too hot to touch when operating, although contact is not advised.

Operating Temperature

      Fluorescent lamps are sensitive to temperature caused by the ambient air. If the bulb of the lamp is too cool or too hot, the lamp will give off less light than when operated at its design temperature. Most other lamps give off the same amount of light at the temperatures encountered in normal applications.

Operating Position

      Some lamps produce more light or have longer lamp life when operated in specific positions with respect to gravity. Metal halide lamps are especially sensitive; some versions will not operate unless in the specified position.

Starting, Warming Up, and Restarting

      Some lamps, especially incandescent, start operating as soon as power is applied, but most other types, especially discharge lamps, like fluorescent and metal halide lamps, require the lamp to be started by a high-energy pulse. The lamp warms up gradually, first glowing faintly and then, after a modest period, giving off full light. If then extinguished, fluorescent lamps can be restarted right away, but most HID lamps, like metal halide lamps, must cool considerably before restarting, potentially causing several minutes of unwanted darkness. Obviously, these considerations can dramatically affect design when safety or security might be compromised by a long warm-up or restart time.

Dimming Characteristics

      Dimming is the process by which lamps are operated at less than full light, often as an energy-saving or mood-creating method. With incandescent lamps, dimming is simple and inexpensive, but with other types, dimming can be considerably more complex, and, in some cases, not advisable.

Energy Efficiency

      The energy efficiency of a light source is called its efficacy and is measured in lumens per watt. Like miles per gallon, the higher the number, the better. Low-efficacy lamps, like incandescent lamps, are less than 20 lumens per watt. Among good colored light sources, metal halide and fluorescent lamps can achieve up to about 100 lumens per watt; distorted color sources, like lowpressure sodium lamps, presently achieve almost 180 lumens per watt.