For UV curing, we usually recommend using RUVA style lamps. This means that each lamp has its own reflector inside that runs the entire length of the lamp. The reflector is a special paint that is applied inside the lamp before the phosphors are added. The resulting lamp will only produce light on one side of the lamp, and all light that tries to escape toward the back of the light is instead reflected back toward the finish product, before it has a chance to exit the lamp. This is a much better style of lamp for UV curing, although regular HO style lamps *can* be used if you build some kind of reflector system (aluminum foil works ok for this).

So when using the RUVA style lamps, you have to be careful to not spread the lamps out too wide, or you will have "cold spots" (little or no UV) in between your lamps. The formula to figure out spacing is pretty straight forward: The maximum distance between the lamps should not be greater than the distance from the lamp to the item you are curing. In other words, if the item you are wanting to cure is going to be 6 inches away from the lamps, then the lamps should be spaced no more than 6 inches apart. If the item will be 12 inches from the lamps, then 12 inch spacing is the maximum, etc.

A word of caution: It is very tempting to try to save money by using fewer lamps, spaced widely, and just have the item far away from the lamps. Just remember that this will reduce the amount of UV more than you think. There is a law of physics called the "inverse square rule" that applies:

The output of the lamp is inversely proportional to the square of the distance from the lamp.

Simply put, the output drops almost exponentially as you move away. As an example: If 6 inches is the perfect distance, but you want to use less lamps, so you move the item away to 24 inches away, 4x the preferred distance. This means you must square the 4x distance (4x4=16) so the amount of UV now hitting the item is 1/16th as much as what was hitting it at 6 inches. You have just changed a 1 minute cure into a 16 minute cure. Granted, there is some overlap because there is more than one lamp, but you get the idea, that it drops more than just the 4x distance you created.

For most applications, 12 inches is fine (thus, 12 inch lamp spacing) but it isn't ideal. 6 inches is typically ideal for many applications. Longer distances will work (and some applications require it) but cure time will go up dramatically.