Light meters (also known as radiometers) allow you to check the curing effectiveness of your curing lights quickly and efficiently by measuring the amount of blue light (450 - 500nm) your curing unit is emitting. It doesn't really matter how bright your light appears. As a matter of fact, many bright-appearing curing units emit light outside the curing range. If the blue wavelength light is not strong, no amount of brightness will cure light-sensitive materials thoroughly. You can do your own scrape tests, but that is time-consuming and wastes composite. It is much easier to use a light meter.
Undercuring can result from lack of power in the blue light output. This means that a soft uncured layer can be left at the bottom of restorations. The real danger is that this lack of thorough curing is likely to go unnoticed.
Light meters probably do not accurately compare one light to another. However, light meters are useful when comparing individual lights for power loss. We suggest testing a light when it is new and then retesting it every week. If the power has declined more than 10% from when it was new, change the bulb (if halogen) and retest the light. In addition, check each of your tips. Some tips may have diminished capacity to get the curing power from the light to the restoration. Keep your tips "clean" resin adhering to the ends of the light tips can reduce the amount of light irradiating the restoration.
Our tests show that the variability in light meter readings underscore our recommendation not to use a meter reading to compare lights, but only to monitor individual lights for degradation of their curing effectiveness. In addition, this point can be further emphasized by our finding that even different units of the same light meter can give different readings.