Exposing a screen, directing ultraviolet (UV) light toward an emulsion coated screen is the singular purpose of an exposure unit. Many sources exist, and each has its place in the mix. Let’s talk about all your options for exposing your screens.
Sunlight
There is no better source for UV light than the sun. And
even in the biggest shop, it’s good to know how to expose a screen
using sunlight. You never know when you might have to improvise a
little. Basically, you can sandwich: 1) glass 2) film positive 3) screen 4) foam covered in black cloth or a t-shirt, and 5) a board the same size as your screen frame. Expose to sunlight and you’re good-to-go.
Unfiltered Blacklight
Not to be confused with blacklights that
are used to make posters glow, unfiltered blacklights will look like
traditional fluorescent tubes. These lights are very close to the glass
on such exposure units. While the least expensive of the professional
units (or homebuilt units) unfiltered blacklight exposure units have
their drawbacks.
The advantages are an inexpensive unit, and
compact (tabletop) as well. If you are building a unit like this
yourself, you can substitute grow lights for unfiltered blacklights.
Quartz Halogen
Backyard or work lights are often quartz halogen lights. This is a
light source often used in homebuilt exposure units, set up just like
sunlight exposure with the quartz halogen bulb suspended above the
screen.
This is a pinpoint light source just as the balance of the
exposure units we will discuss. Pinpoint light means all the light
emanates from a single bulb. It is much easier to hold fine lines and
halftone dots with pinpoint light as you will not experience the
undercutting of your images with light coming toward your art and screen
from all directions.
Mercury Vapor
Street lights are mercury vapor lights. The important thing to know
about mercury vapor is something you’ve probably already witnessed when
street lights flicker on at night and get brighter and brighter over 10
minutes or more.
These lights come on slowly and build in
intensity, which means the light needs to be turned on and warmed up
before you can expose a screen. There will be a shutter door shielding
the light while you place your film positive and screen on the glass in
the proper position.
A vacuum will commonly be a part of a unit
like this. The vacuum will activate and pull the screen mesh tight
against the glass of the exposure unit. The unit is then turned on,
opening the shutter door and activating a timer. When the time expires,
the shutter door will automatically close.
Metal Halide
Metal halide light sources operate much like mercury vapor and will
be found on higher end exposure units. Smaller units will have 1000 watt
metal halide bulbs and larger, more expensive units will be as high as
6000 watts or more.
You will see this light source as either a
cabinet unit or as a free standing exposure unit with the light set up a
measured distance from the vacuum table. Just as often on higher end
machines, an electric eye on the unit will read the light intensity (in
lumens) and time the exposure by this light intensity rather by counting
seconds. Bulbs will lose intensity over time. A unit that reads lumens
will allow for this loss and always give you the exact same exposure.
Best Option
Your best option? The highest wattage bulb you can
afford, and again if it’s in your budget, a vacuum attachment that will
hold your screen and artwork tightly against the glass. But as I
constantly say about equipment and techniques in this industry, any
exposure option will do the job for you. The difference will simply be
in efficiency and productivity in your shop.