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 Comparing Light Sources
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- Comparing Light Sources
The performance of a lamp or type of lamp can be judged according to several different criteria. The most commonly used are efficacy, color rendering, color appearance and lumen maintenance.
- Efficacy
Types of lamps have differing abilities to convert electrical power into light. The quantity Different Of light emitted (measured in lumens) is divided by the power input to the lamp (measures in watts) to determine the lamp's efficacy. This is expressed as lumens per watt.
- Color Rendering
Color rendering is the degree to which a light source shows the true color of the objects it illuminates. This is measured on a color-rendering index, rated from 0 to 100. A traditional fluorescent lamp, for example, rates 54 on the scale, while incandescent lamps rate 100.
- Color Appearance
The apparent color of the light emitted from a light source is known as appearance. It is generally defines in two different ways. One is the broad categorization into warm, intermediate and cold. The other is by color temperature, measured in Kelvins (K).
- Lumen Maintenance
Incandescent lamps fail before there is any serious decline in the amount of light they emit. Other sources, such as fluorescent lamps, show depreciating light output during their lifetimes. These facts are important in knowing when lamps should be replaced. With incandescent lamps, it is normal to replace individual lamps as they fail. For other types of lamps, it is possible to define a lumen maintenance curve (light output against time) to predict when output will have declined to a point where the lamps need to be replaced, so that each lamp can be changed when the probability of failure reaches a certain level.
- The Color Of Lamps
One of the most important attributes of metal halide lamps is their ability to provide crisp white light in a variety of different color temperatures to accommodate user's needs. High pressure sodium and mercury lamps are very limited in the color of light they product. The colors they can generate are often unpleasing or inappropriate for many applications. In addition to this, the "quality" of color from these two sources is very poor compared to metal halide lamp. This helps explain why the use of metal halide lamps continues to increase dramatically each year in the world. The color of light sources is a complicated relationship from a number of different factors, including Correlated Color Temperature (CCT), Color Rendering Index (CRI), and spectral distribution.
- Correlated Color Temperature (CCT)
The first factor in choosing a color of lamp is to determine what Kelvin temperature is desired. For example, if a retail store wants accent lighting to blend in with warm lamps, they may choose a MH100/3K which has a Correlated Color Temperature of 3200 Kelvin. This 'not simply an "temperature" is not simply an arbitrary number, but has a correlation to actual thermal temperature. CCT is defined as the absolute temperature (expressed in degrees Kelvin) of a theoretical black body whose chromaticity most nearly resembles that of its light source. From this standpoint, the CCT rating is an indication of how "warm" or "cool" the light source is.
- Spectral Energy Distribution
When we look at a light source, we "perceive" seeing a single color. In actuality we are seeing literally thousands of colors and hues of colors. The combination of different wavelengths of light makes up the color we see. The different combinations, and the relative intensity of various wavelengths of light, can be used to determine a light source's CRI.
- Color Rendering Index (CRI)
In general, the CRI is an indication of a lamp's ability to show individual colors relative to a standard. This value is derived from a comparison of the lamp's spectral distribution compared to a standard (typically a black body) at the same color temperature. Incandescent lamps are the only light source that follow a true black body curve. Other sources (i.e. metal halide) are rated with a Correlated Color Temperature. The CCT, however, does not provide information on the quality of color. For this, a Color Rendering Index (CRI) is also necessary. In general, the higher the CRI rating of a lamp, the better different colors will show.
- Color Shift And Variation
Different colors are achieved in metal halide lamps by using different are tube designs and by introducing various chemicals inside of the lamp arc tube. New lamps need to have these chemicals "burn-in" before they reach their optimum color and light level. This is why new lamps can sometimes be unstable or have variation in colors. This, however, stabilizes after lamps have burned-in for approximately 100 hours. As metal halide lamps age, chemical changes occur in the lamp causing shifts in the chromaticity of the lamps. Different lamp designs shift in different ways and different lamps from the same group may shift different amounts. Generally, over economic life lamps will shift 200K to 300K in color temperature. After economic life a lamp may change as much as 500K to 600K. As a group of lamps ages together in a facility, the lamps will generally shift at the same rate causing very little color variation from lamp to lamp.
- The Different Colors
The advantage of the many colors of lamps offered by TLEA is that they can be used in virtually any lighting application. Outlined below are the various colors currently available.
3K-3200 Kelvin:
Used as a primary light source for retail applications.
3700 Kelvin:
Coated lamps. Used where a "softer" metal halide light source is desired.
4000 Kelvin:
Used in general lighting; factories: parking lots, warehouses.
5K-5500 Kelvin:
Daylight lamps: horticulture, aquariums, high color definition.
Special Colors:
Blue, Green, Magenta, Pink, Yellow. Special applications where color is needed without light loss due to filters.
- The Life Of Lamps
When purchasing a new light source or replacement bulb, lamp life is a very important consideration. The first thing to consider in understanding "lamp life" is that there are two very different and distinctive terms that describe life: "rates life" and "economic life".
- Rated Life
Rated average life for metal halide lamps is defined as: A value of lamp life expectancy based on laboratory tests of representative lamps, burning at rates volts, on an approve system, operating with a burning cycle of 10 hours per start. The "average life" is determined when the median (50%) of the lamps initially installed are still operating. Various operating conditions can affect the average life of lamps. One of the most important factors is burning position. Universal lamps can be burned in any position. Published "rated life" for universal lamps are based on the lamps being burned in the vertical position. "Rated life" for lamps burned in the horizontal position is 75% of the published rating for the vertical application. Lamps, like most electromechanical devices, have a shorter life the more they are turned on and off, Unless otherwise noted, lamps are rated on a burning cycle of 10 hours then turned off). At a burn cycle of 5 hours per start, the approximate lamp life is 75% of "rated life". AT 2 1/2 hours per start, the approximate life is 50% of "rated life". Other factors can also affect lamp life; high or low operating voltages, marginally operating control devices (ballasts, capacitors, etc.) extremely high operating temperatures, and various other factors. Combinations of these factors can multiply the reduction in "rated life".
- Economic Life
"Economic life" is a much better description of actual lamp life. In the simplest terms, "economic life' is the hours of operation a lamp is designed for to give; the optimum light output, aesthetic quality, and economical energy consumption. The "economic life" of lamps is generally defined as 60% of the lamp's rated life. This is a much more accurate indicator as to when lamps should be replaced. "Rated life" only indicates when half of the installed lamps will no longer operate. "Rated life" does not account for the lumen depreciation, color shifting, and loss in efficacy that always occur as lamps age. To consistently provide a quality lighting system, you must not only consider the lamps that fail, but the lamps that continue to operate. As lamps age, they undergo many changes. The most important of these is lower light output (lumen depreciation). This occurs even though the lighting system continues to consume the same (or sometimes slightly more) electricity. The color of light (CCT) that lamps generate also changes or "shifts" with age. This is rarely noticed during the "economic life" of lamps but often creates problems in the last 40% of "rated life". During the last 40% of "rated life", color shift may accelerate. It is during this period that more spot replacements are needed for failed lamps. The still functioning original lamps often appear to be a different color than the newly installed replacements. This spattering of colors, combined with lower light levels during the later part of "rated life", is unappealing and can be counterproductive to the work environment.
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