Light output can be measured in lumens, lux or foot-candles. Using lumens as the unit, lamps are typically rated as follows:

• HID lamp: 40,000 lumens falling by 10% in 6 months and 30-40% in first 3-4 years
• T-12 lamp: 30-40% lumens drop over lifetime
• T-8 lamp: 10-12% lumens drop over lifetime if at correct temperature
• T-5 lamp: 10-12% lumens drop over lifetime if at correct temperate

Output and longevity of different fixtures and bulbs depends on the temperature of the ceiling where they operate. Too high or low a temperature can drastically shorten bulb life. This is even a problem with modern T-8 or T-5 lamps. Look for lamps with:

• Constant output ballasts which put out the same amount of light regardless of temperature (within certain limits)
• A manufacturers warranty on the lamp/ballast system

Replacing lamps (bulbs) can be a huge problem with high ceilings, so lamp longevity is crucial. The labor cost to replace a lamp is typically higher than the lamp cost. Some companies swap lamps on all fixtures in an area simultaneously whether they’re burned out or not, on the assumption that the ones that are not will probably burn out soon anyway. This is called group re-lamping and can save significant labor costs.

Replacing Fixtures during a retro-fit can be very expensive in terms of labor and material costs. If the optimum positioning and spacing between the new fixtures does not match the positioning of the old fixtures, new hanging points must be installed, with associated wiring. A good lighting designer will think through these issues in detail.

Payback can vary from 18 months to several years, depending on a lot of factors. Good lighting has to take into account various factors including:

• Ceiling height which reduces light delivered to the work surface
• Enough lumens for the task(s) at hand, which depends on output and positioning
• Initial lumens which will decline over the fixture life. (40% for magnetic ballasts, 10-15% for electronic ballasts.)
• Light quality, including color index
• Dimming in response to daylight and occupancy levels

Ballasts: Most of the energy savings over the last few years have come from improvements in ballast design. These include

• Constant output – the ability to put out the same amount of light regardless of temperature fluctuations
• Dimmability – the ability to reduce light output (and energy consumption) in response to daylight and occupancy sensing