Here is one way to design a LED light that will meet your planted tank requirements. Obviously, you first start with the tank you want the light to work with. Next, decide if you want the light to be suspended above the top of the tank, or sit on top of the tank. Suspending it is by far the best idea. Decide if you want to spend a little more money and be able to adjust the apparent color temperature of the light, which requires more LEDs and two LED controllers.
A very good PAR to design for is 60 micromols per square meter per second. This is medium intensity light - if you miss the target, you should still have either low-medium light or high medium light, both of which are very usable.
Pick the LEDs you want to use: Cree XP-G, which are high output, high power, and a bit cheaper than Cree XM-L, which are higher power, and higher output.
If you will suspend the light above the tank, you will want to use optics on the LEDs to focus the light to reduce the spillover light, and make better use of the full output of the LEDs. For lights which will be less than 2 feet from the substrate, 60 degree optics work well. For lights up to at least 36 inches from the substrate 40 degree optics work well. Decide which you will use.
It is best to run LEDs at less than maximum power to ensure that they will last for many years before “wearing out”. Also, the more power you run them at, the more likely you will need active cooling and a quality heatsink, which add to the cost, the complexity, and the noise. Pick a LED driver or drivers which can deliver the current you want to use.
Look up the lumen output the LEDs produce at that current from the pdf data sheets for the particular model LED you will use.
Write down these parameters:
LED model _______________
lumen output per LED_______ (L)
Height of light from substrate________inches (H)
Optic cone angle__________(probably 60 or 40 degrees) (ř)
Now make this calculation:
where “a” equals the spacing between LEDs in inches, and “n” equals the number of rows of LEDs to be used.
Assume one row of LEDs will be used, and calculate the spacing between LEDs ( the square root of the result above)
Alternatively, assume 2 rows of LEDs will be used, and again calculate the spacing between the LEDs.
Assume 3 rows of LEDs will be used, and again calculate the spacing between the LEDs
To decide how many rows of LEDs will be best, calculate the spread of the high intensity portion of the cone of light. That will be equal to 2 x H x tan ř/4. That spread should be about 2/3 to 3/4 of the front to back depth of the tank. If it is less than that, 2 or 3 rows of LEDs should be used, to expand the spread.
Now that you know the spacing of the LEDs, and the number of rows of LEDs, calculate the number of LEDs per row. The row of LEDs can end at about one half of the tank front to back depth from each end of the tank, to avoid spilling a lot of light at the ends of the tank. The length of each row will be the tank length minus the tank front to back depth. Divide that number by the spacing of the LEDs (a) to get the number of LEDs per row.
Look at the specifications for the LED drivers you want to use to see the minimum and maximum number of LEDs they will power. Based on that you can determine how many LED drivers you need. If you want to be able to adjust the apparent color temperature of the light you will have to use two colors of white, cool white and neutral white, for example, with separate dimmable drivers for each color. Make sure the number of LEDs of each color is not fewer than the minimum for the driver.
Electrically connect each color of LEDs all in series, up to the maximum the LED driver will power, and connect those to one dimmable driver. Do the same for the other color.
Pick a heatsink or heatsinks that will accommodate the rows and LEDs per row that you will use. If you are using an LED current near the maximum power for the LED model you are using, also figure out a cooling fan or fans configuration to keep the heatsinks cool.
Finally, determine how you will hang or otherwise support the light, and find the parts needed to do that.
Use of a “splash guard” is optional. Lights hanging above the tank do not need any splash guard. Lights sitting on top of the tank, with no glass lid on the tank, may need a splash guard, and will need one if you use an air stone in the tank.