Start with the requirement
The most common mistake is buying a light without knowing what it has to achieve. Every space has a classification under recognized standards, and each classification comes with recommended light levels. Those levels, or the owner's own requirement, are the starting point for any purchase.
Without them, there is no way to tell whether a luminaire suits the space. Many sites must also meet a minimum light level, so getting it wrong can create safety and liability problems.
Check the local rules on light pollution
Many cities and regions now have bylaws and guidelines that limit light pollution and light trespass. Fixtures that break them can be ordered down after they have been paid for and installed. Find out which rules apply before you choose, and ask for evidence that the design complies.
Measure light, not power
Watts measure power, not light. Efficacy, the lumens produced per watt, varies widely from one luminaire to another, so two fixtures of the same wattage can give very different results. Set the light level first, then look for the luminaire that meets it with the least power.
Lumens at the source are only part of the story. What counts is how many of them reach the task surface: the road, the parking lot, the work area. A fixture that sends half its output into the sky or onto the wrong area wastes it, while one with fewer lumens and better optics can do the job with less.
Design it, test it, verify it
A pilot project feels reassuring, but it cannot show how long a luminaire will last, how its output will fall year by year, or how it will perform alongside hundreds of new fixtures with their own overlap and reflections. Results often change once the full project is built.
A photometric lighting design can. It models the actual site, fixture positions and conditions, and shows whether the required levels will be met and maintained. Without one, you are guessing.
Do not rely on manufacturer claims alone. Ask for test reports from accredited third-party laboratories, made to recognized methods such as those published by the IES and the CIE.
- An LM-79 test report from an accredited laboratory
- The IES file used in the design
- Lumen maintenance data (LM-80 and TM-21)
- A photometric design of your own site
Plan for the real site
LED drivers are electronic and more sensitive to power problems than the magnetic ballasts they replace. Surges, transients, inrush and low voltage that older fixtures tolerated can damage new ones. Look for several levels of surge protection.
Temperature, vibration, moisture, dust and corrosion all affect efficacy, lifetime and how fast output declines. A luminaire that performs well in one setting may fall short in another, so make sure its ratings match the conditions where it will be installed.
Judge cost over the whole life
The real cost of lighting is not the fixture. It is the energy and maintenance needed to run it for years. Compare lifecycle cost, what it takes to buy, power and maintain each option over its life, rather than the purchase price.
Efficiency and longevity drive the return. A luminaire that uses less energy to meet the requirement and lasts longer often costs more upfront and less overall.