Alliance for Lighting Information


What is the Difference Between "Initial", "Mean" and "Maintained" Conditions?

by David M. Keith

The performance of lighting systems can be degraded by many different aspects of the system. Some of them occur from the moment the system starts working and remain the same over time, while others get worse - and better - depending on the maintenance of the system.

When the lighting system is first lighted, the ballast-lamp combination may only be providing 95% of the rated lumens for that lamp. The aiming of the fixture and the resulting lamp-position combination may reduce performance. However these changes to system performance will not change over time. When the system is new, the Light Loss Factor will probably be less than 1.0, but relatively high - around 0.90 for example. Such conditions are termed "initial", to describe the lighting system performance at "Time Zero" when performance is at the best it is expected to be over the life of the lighting system.

This is because the further degradation over time is cumulative, but typically reduced - occasionally or periodically - by maintenance. In other words, the lamp and luminaire produce less light as the lamp burns and as dirt seeps in. As time goes by, the losses due to lamps aging and dirt accumulating will increase, and system performance decreases. Eventually, without any maintenance at all, the lamp will fail and the light level will drop to zero.

Proper maintenance will regain the reducetions in output - up to 50% - due to lamp aging and dirt. Lamp replacement will provide the luminaire with the "nearly-rated" value for lumens that occured at the "initial" stage. Cleaning at the same time can restore most or even all of the losses due to dirt.

The maintenance cycle - the period between replacing the lamp and cleaning - corresponds to a particular combination of lamp and dirt losses. The longer the time allowed between maintenance, the lower the overall system performance drops. Predicting the duration of the maintenance cycle and the effectiveness of the cleaning are part of the lighting designer's tasks.

Information for developing such predictions includes the "lamp life" and "mean" lumen data provided by manufacturers. For some lighting systems, the "end-of-life" data would be appropriate, even though that data can be difficult to obtain.

The "maintained" level refers to the performance of the lighting system just before maintenance occurs, corresponding to the lowest level of system performance. The losses over the maintenance cycle will typically effect the maximum, average and minimum illumination levels - the quantity of light - but not the distribution or the value of any ratio of levels. "Maintained" illuminance levels may be as low as half of "initial" values, while uniformity ratios are unchanged.


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