Fluorescent Lamp Mortality

Lamp mortality defines how long the average lamp is expected to last. It is expressed as RATED AVERAGE LIFE in hours of operation. See Figure 5.12.



Figure 5.12. Lamp mortality curve.

Average rated lamp life is when 50% of the large sample batch tested have failed and 50% are still operational. This is different to the time when the lamps are replaced, which is usually well before 50% failures occur from a batch.
Several factors affect fluorescent lamp life. Longer burning hours per start will extend lamp life. Lamp life is shortened by improper lamp current, improper voltage to the ballast, or improper cathode heating.
Average rated lamp life is based upon 3 hours per start for all fluorescent lamps. See Figure 5.13. Increasing burning hours per start from 3 to 12 hours increases lamp life, by 50%.



Figure 5.13. Typical life increase vs. burning hours per start

Overvoltage to the ballast will cause high tube current shortens lamp and ballast life. It also causes preheat lamps to start like instant start lamps - shortens lamp life.

Undervoltage to the ballast causes low tube current - makes lamps flicker, causes uncertain starting and reduces light output. Undervoltage to the ballast can cause preheat starters to recycle - shortens starter and lamp life.

Colour Rendering
Colour rendering defines how the light from the lamp affects the colours of objects being illuminated. Fluorescent lamp colour depends upon the phosphors within the lamp. Fluorescent lamps offer a range of colour rendering performance, but will not match incandescent lamps.