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 users needs. High pressure sodium and mercury lamps are very limited in the color and quality of light they produce. The colors they generate are often unpleasing or inappropriate for many applications. This helps explain why the use of metal halide lamps continues to increase dramatically each year around the world.
The color of light sources is a complicated relationship deriving 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 lamp color is to determine the associated Kelvin temperature. For example, if a retail store wants accent lighting to blend in with warm halogen incandescent lamps, they may choose a Venture MP 100/C/U/27K which has a Correlated Color Temperature (CCT) of 2700 Kelvin. This "temperature" is not simply an arbitrary number, but is correlation with actual thermal temperature. Anyone who has seen a piece of metal being heated will notice that as the temperature of the metal increases, the color of the metal changes. This is a rough explanation
of how the CCT of high intensity discharge and fluorescent light sources is measured. 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. The higher the number, the cooler the lamp. The lower the number, the warmer the lamp.
Spectral Energy Distribution
When we look at a light source, we "perceive" a single color. In reality, we are seeing literally thousands of colors and hues of colors made up of a combination of different wavelengths of light. These different combinations, and the relative intensity of various wavelengths of light, can be used to determine a light sources CRI.
Color Rendering Index (CRI)
In general, CRI is an indication of a lamps ability to show individual colors relative to a standard. This value is derived from a comparison of the lamps spectral distribution compared to a standard (typically a black body) at the same color temperature.
Incandescent lighting is the only light source that follows a true black body curve. Other sources (i.e. metal halide) are rated with a Correlated Color Temperature (CCT). 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. However, this guideline can be misleading with certain lamp types because a high CRI sometimes makes different colors easier to distinguish, but standard colors may appear different than they actually are. The following is an example of how the color "white" may appear under different light sources.
|Metal Halide||Metal Halide||White HPS|
|CRI - 65||CRI - 90||CRI - 90|
|White is||White is||White is|
Color Shift and Variation
Different colors are achieved in metal halide lamps by using different arc tube designs and by introducing various chemicals inside of the lamp arc tube. New lamps need to have these chemicals "burn-in" for approximately 100 hours before they reach their optimum color and light level. This is why new lamps can sometimes be unstable or vary in color.
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 in 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. However, if new replacement lamps are introduced into the group of lamps, color differences may be more noticeable, because the new lamps have not aged and shifted with the remainder of lamps. This is just one of the reasons why many users of metal halide lamps prefer to group relamp as opposed to spot relamping. There are many other advantages to group relamping as outlined in the section on group relamping.
The Different Colors
The advantage of the many colors of lamps offered by Venture Lighting is that they can be used in virtually any lighting application. Outlined below are the various colors currently available:
|27K - 2700 Kelvin||Often used as accent lighting to blend in with fluorescent 2700K applications.|
|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.|
Blue, Green, Magenta, Pink, Yellow. Special applications where color is needed without light loss due to filters.