To identify the correct photoresist exposure I suggest you to use a "21 step Stouffer transmission wedge" like
1,
2, ...
I bought mine from
Cape Fear Press*
To my knowledge, the most used photoresists (like
ORDYL Alpha 930 I'm using) require an exposure between 6 and 9 of the Stouffer scale.
Some Soldermasks (like
Dynamask 5000) require a value from 12 to 15;
Dynamask KM instead require a value from 8 to 10.
If you get a wrong exposure value, it is enough to modify your exposure accordingly to the instructions on the first 2 links.
Using the Stouffer scale will help you to get the best results for the material you are using: you have only to ask to the photoresist producer what are the suggested exposure values.
Please remember that the efficency of your lamps will be different if they were heated before the use or not. Because of this, I suggest you to keep the distance from the lamps and your PCB
more than 15-20cm: your exposure will be longer but you will get a more uniform light distribution and (due to longer exposure) a more repeatable total light output.
I NEVER suggest to use an exposure less than 60 seconds: if your lamps are more powerful, put them at a greater distance.
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Regarding the wavelength required by typical photoresists, in general it is accepted from 350 to 450 nm, so your "insect killer" lamps are more than good enough.
Please DO NOT use UV lamps with a shorter wavelenght than 350nm (like UVB lamps or EPROM erase lamps): they can be really dangerous to your eyes.
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When you work with photoresists, I suggest you to use as an inactinic lightning device like
**broken link removed** (first link I got using google). The CFL ones like this
**broken link removed** are very good because they permits you to work with a very good illumination.
*) Just an happy customer: even if living in a different continent, I got it in a very few days (12USD).