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Shedding some light on process illumination

May 16, 2005
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Illuminating the interior of a process vessel, tank or major pipeline is not rocket science, but it can lead to problems when what one might hear about selecting illumination sources is simply myth.Here are some facts to consider:

Myth: Lights of the same wattage produce the same amount of illumination.

Fact: Illumination varies widely among same-wattage lights, depending on the design and manufacturing precision of the instrument, especially when a spotlighting reflector is involved. First, to operate efficiently the reflector must be optically designed to produce the light distribution required by the application. Further, to maximize output, the bulb’s filament must be placed accurately at the focal point of the reflector.

This will ensure that the reflected light is directed outward in a parallel, spotlight pattern. Only the light emanating directly outward from the filament will be at an angle — exactly what is needed in many process applications. Similar considerations are needed when using flood lighting, where the light is deliberately directed outward in a less coherent pattern. In that case, the challenge is to get the most effective trade-off between effective light dispersal and operating efficiency.

Myth: The best measure of effective light output is stated in lumens.

Fact: Lumens determine how much light is being produced at the source, and as such it’s a good measure of the omni-directional output of an incandescent bulb. When a reflector is used to direct the light output in one direction, lumen ratings are not a good measure of effective illumination because they don’t consider the efficiency of the reflector and its relationship to the filament. In most process illumination applications the gold standard for measurement is a parameter that measures not just light output but actual illumination at a specified distance from the source. That parameter is the candela, more commonly called foot-candles. It tells you precisely what amount of illumination you’ll get in the area where illumination is required.

Myth: Bulb life expectancy is a minor concern when specifying illumination sources.

Fact: Bulb life is second in importance only to instrument design and light output capability when deciding on a process illumination source. When in doubt, ask the operators how much maintenance and downtime is involved in replacing a burned out bulb and do the math to convert those wasted hours to dollars and cents. In addition, one has to consider the likelihood that when a bulb burns out the operators will continue to operate the system without adequate illumination, risking significant loss of quality control.

Myth: Currently available options and features seldom affect the functionality of an illumination source.

Fact: Selecting the optimum options and features for a light source can significantly increase operating efficiencies and lower installation and maintenance costs. The most common of these are timers, remote switches and special materials of construction.

Myth: Given the same wattage, 24-volt and 120-volt lights produce the same light output.

Fact: A 24-volt light will produce significantly more light output than a similar 120-volt light with the same wattage rating. The proof is in the math: volts x amps = watts. So, with wattage held constant, amperage must be reduced as voltage increases. In other words: When wattage is held constant, amperage is inversely proportional to the voltage, meaning that using higher voltage involves more resistance which, in a filament bulb, involves a longer, thinner filament. But the 24-volt light with its thicker, shorter filament is much to your advantage. First, a thicker filament can burn hotter, producing a brighter, whiter light. Second, a shorter filament produces more light precisely at the reflector’s focal point, resulting in more efficient reflection and more useable light.

It is understood that a good engineer doesn’t know it all, but he or she knows where to find it and how to make it work. If you want to know more about process observation, go to L.J. Star’s website, www.ljstar.com, and click on “Technical Data.” You’ll find a wealth of factual information there, including more information on lights as well as other process observation equipment and techniques.

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