What ya-all know about refrigerant dye?

I use them everyday....they may be in modern car ac systems, but they are not in old systems AFAIK. Any blacklight will work, if there is indeed any dye in the system.... leaks will show up as fluorescent green, under a purple (black) light.
If you think you have a leak, and there is no dye....you can buy the dye cartridges and add some....it will stain the refrigeration oil.... you need to run the system with the dye in there for a little while. Then just look for problem areas in the dark. HTH, Lefty71