Electronic Ballast
Revised: September 18, 2012 2
creating artificial light. The discovery of electric power and the possibility of transmitting it in a simple manner facilitated the development of modern lamps. Today there are nearly 6,000 different lamps being manufactured, most of which can be placed in the following six categories: incandescent, fluorescent, mercury vapor, metal halide, high-pressure sodium (HPS) and low-pressure sodium (LPS). Except for incandescent lamps, all of these light sources can be termed as gas discharge lamps. Fluorescent and LPS lamps operate on low-pressure gaseous discharge, and the mercury vapor, metal halide and HPS lamps operate on high-pressure gaseous discharge. The mercury vapor, metal halide and HPS types are commonly known as high-intensity discharge (HID) lamps.
The major characteristics to be considered when choosing a lamp are its luminous efficacy, life, lumen depreciation and color rendering. Luminous efficacy is the measure of the lamp’s ability to convert input electric power, in watts, into output luminous flux, in lumens, and is measured in lumens per watt (lm/w). The luminous flux of a light source is the electromagnetic radiation within the visible part of the electromagnetic spectrum multiplied by the sensitivity of man’s eyes to that part of the light from the source. The visible portion of the spectrum covers the wavelength range from approximately 380 nm to 780 nm (Figure 1). The life of a lamp is the number of hours it takes for approximately 50% of a large group of lamps of the same kind to fail. Failure means that the lamp will no longer light or that light output has dropped to a specific percentage value. Lumen depreciation during life is a characteristic of all lamps. This is a process of lamp aging, an important consideration in lighting design. Finally, there is the matter of color rendering. The lamp types do not provide the same nominal “white.” Their difference in spectral distribution can produce two effects within a lighted space. Some of the colors of objects within that space can appear unnatural or faded – reds can appear brown, violets nearly black, etc. Second, the entire space may “feel” warm or cool. For example, a mercury lamp, lacking in reds and oranges, makes a space seem cool, whereas an incandescent lamp, with deficiencies in the blue and violets, makes a space feel warm.