1. It shows when the bulbs over your tank have color shifted and need to be replaced.
2. You can quantitatively compare different bulbs and lighting technologies. There is no more guessing which marketing plan to believe or which Internet lighting guru to trust. You can see for yourself numerically what you are thinking of putting over your tank.
The two most popular probes are irradiance meters that measure light output in terms of lux or lumens, and quantum meters that measure light in terms of Photosynthetic Photon Flux Density (PPFD).
Irradiance meters measure the actual intensity of the light falling over a specified area. They are generally less expensive than quantum meters, but are also of less use for coral aquaculture. Irradiance meters focus on the total light present, which takes into account light radiation outside of the photosynthetically active range. This technology is better suited for use in other applications such as photography.
Quantum meters measure the photosynthetically active portion of the visible light spectrum. They are more expensive on the average than irradiance meters, but are not astronomically priced. They can be purchased for roughly the same price as a bulb replacement. An ideal quantum sensor would give equal emphasis to all photons between 400 and 700 nm and would exclude photons above and below these wavelengths.
Generally speaking, a light source with a PPFD reading of 100-200 is suitable
for keeping low light corals. A reading of 200-500 should be able to sustain
most corals that are found in tanks today. The most light demanding corals will
benefit from lighting in the 500-1000 PPFD range however the upper end of this
range can be extremely harmful to deeper water corals. These are very general
guidelines, and are meant to be a rule of thumb rather than a hard rule. We
still recommend acclimating corals to intense lighting schemes regardless of
how much light they are accustomed to in the wild.
