I am often asked what type of light source to use when making screens for screen printing. There are so many different commercial choices and when it comes to home or hobby screen printing, there are even more. But the right light source for your particular situation will help you make a good stencil easier than with the wrong light source. It is possible to make working stencils with many scenarios and knowing about emulsion is the first step.
Always remember that direct photographic emulsions for screenprinting are sensitive to UV light only. That means in order to get a good exposure you light source should have a high UV output. When working at home with standard tungsten bulb work lamps, it is a common mistake to use too low of a wattage. A 1000 watt work lamp will give you much better results than a 500 watt. I wouldn’t suggest using anything lower than that for a single bulb tungsten set up. These work lamps also come with a glass cover or “lens”. This should be removed as they often have a UV filter coating which will remove the UV light from the light sent through the glass. You can use them bare bulb style.
The tungsten work lamps will often have a longer exposure time depending on what type of emulsion you are using but they are a true point source light source which has a few benefits of its own. Most importantly it will be better with getting detail out on the stencil. A point source or single bulb exposure unit will produce greater detail easier than a multiple bulb light source. The down side is these types of lamps produce heat. The professional units which use metal halide bulbs with much higher wattage have shutters and cooling fans to prevent the bulb from overheating and to protect the emulsion coated screen from excessive heat. The quality of the light differs for these two types of exposure units as well. Metal halide units will penetrate the emulsion layer on the screen much better than any other unit and reduce or eliminate any “scum” or filming commonly associated with some light sources due to under exposure on the ink well side. A metal halide unit will also require very dense film positives.
Multiple bulb units with fluorescent industrial black lamps run very cool and output a good amount of UV light. They often will reduce your exposure time compared to a standard tungsten unit and the professional machines tend to be much less expensive than metal halide units. The fluorescent industrial black lamp exposure unit is a logical upgrade or first exposure unit when starting a screen printing business. The down side is that this type of machine will be problematic in producing detail. These bulbs will demand much better film quality to work with longer exposure times for fine detail or line work. It is very common for multiple fluorescent bulb exposure units to leave the ink well side of the emulsion coated screen slightly under exposed. That is the cause of filming or the clear “scum” that clogs the stencil when the screen dries. But this can easily be delt with by blowing out the water left in the mesh after washout with low pressure air.
In the end I would say if you have the money, get yourself a metal halide exposure unit. If you have only some money get a multiple bulb unit with fluorescent industrial black lamps. If you have no money you can get a 1000 watt work lamp and remove the glass. That’s my order of preference. At this point in my shop I have a multiple bulb unit with fluorescent industrial black lamps. It works very well but I always make sure to have very dense film positives.