Brightness is something I hear hobbyists talk about way to frequently when it comes to reef lighting, but there’s much more important things that cannot be measured by the human eye that should be considered when looking at reef lighting. Our eyes work much like a camera, which judges light based on relative light and less on actual output, so more scientific means must be used when figuring out what out photosynthetic pets need.
Cameras are the perfect comparison when talking about how our eyes work because they have a computer that works much like our brain to adjust to what we are actually seeing. When light enters a camera, the computer core automatically adjusts the shutter speed and aperture to get the best exposure onto the film. When light enters the human eye, the brain does the same thing to adjust the pupil and activate either cones (more detail, less sensitive) or rods (less detail, more sensitive) to get the right exposure. It’s these adjustments that make judging light impossible with the naked eye.
So let’s look at what happens when we look at a light. Let’s say we are looking at a tank in a dim basement lit by a single compact fluorescent. It’s dark in this room, so our eyes dilate and more rods are activated so we can see well. The tank appears very bright because our eyes are in their most sensitive mode. Now let’s look at a halide lit tank sitting in a brightly lit room with a lot of windows. Our eyes do exactly the opposite, constricting the pupils and activating more cones than rods, to get the fine details. The tank appears dim because our eyes are adjusted for ambient brightness in the room.
Different conditions like these make it impossible for our eyes to see without bias, so we must use more scientific means to come to a conclusion. Lux meters are a good place to start because they show an unbiased reading of just how bright a specific area is, but this still leaves out the fact that photosynthesis only happens in specific wavelengths. This is where the PAR meter comes into play. PAR meters are calibrated to read only photosynthetically usable radiation which is a measurement of the amount of photons generated that may be utilized in photosynthesis. One of the most commonly used in the hobby is the Apogee meter.
Next time you look at a tank and begin to think that it can’t possibly be bright enough to support coral, take a look around you and think about how your eyes are interpreting your surroundings. You may be surprised at just how dim a bright tank can get.