I'm trying to expose a silkscreen with photo-emulsion. I have an LED light (90 w) that is equivalent to a 450W - 500W incandescent light. The light is mounted 18" from the screen. I'm trying to determine the exposure time. Any ideas on how to calculate this? (I don't have a step wedge or extra screen).
There are far too many variables involved with burning the screen for anyone to give you an accurate answer. Burn time can depend on the type of emulsion, mesh count of the screen, exposure light, your film medium (transparency? paper & mineral oil? vellum?), and your exposure setup.
The best way for you to find out is to test yourself. You say you don't have a step wedge, but you don't really need one. I have used a piece of thick cardboard in the past to test exposure time. I started by burning the whole screen for about 7 minutes. I then covered about an inch of the emulsion with the cardboard. Every 30 seconds, I would cover another inch of the emulsion by sliding the cardboard forward.
Once the screen was fully covered by the cardboard, the test was complete. I washed out the screen to find which exposure time produced the cleanest result.
This method does unfortunately waste a screen, but hopefully you have the equipment to re-coat it yourself.
My biggest concern is your light. You say it is an LED light equivalent to a 450W - 500W incandescent light, but are you absolutely certain it can be used to burn a screen? Remember, emulsion reacts to UV light. I am certainly not an expert on LED lights, but as far as I know they do not emit UV light at all.