A coated screen is exposed anytime that UV light is introduced to the environment in which the screen is exists. Exposure units are designed to enhance and control UV light waves so that the screen is burned quickly and accurately. Although it is possible to expose a screen using any source of UV light, it is strongly recommended that printers use some sort of professional exposure equipment in order to ensure a clean and accurate stencil.
Depending on the emulsion that you have chosen and the light source that you are using for exposure, burn times can range from a matter of seconds to many minutes. Most emulsion manufacturers should be able to provide you with approximate exposure times for the emulsion and light source you are using. However, other factors such as humidity and temperature can cause a need to raise or lower exposure times. Fortunately there are simple tests that can be performed to determine the exact exposure time that is appropriate for you circumstances.
Step Wedge Test
A step wedge test is a method of exposure wherein multiple exposure times are recorded onto a single screen in an effort to determine the best exposure time for current conditions. To perform a step wedge test, the following steps must be followed:
- Place a film positive onto a coated screen.
- Determine the manufacturers recommended exposure time based on the emulsion and light source you will be using.
- Determine the increments of time with which you will be exposing your screen. For fast burning emulsions you will use small increments (5-10 seconds), for slower burning emulsions you will use 30-60 second increments.
- Draw lines at the top and bottom of your screen indicating how many separate burn times you will use.
- Expose the entire frame for two increments less than the manufacturers recommended exposure time. For example, if you have chosen 15 second increments and the manufacturer’s suggested exposure time is 4 minutes, then you will start at 3 minutes and 30 seconds.
- Next, cover the first section of your screen with an opaque sheet so that the light can no longer expose that section.
- Expose the screen again for the increment of time you have decided upon.
- Continue steps 6 and 7 until the entire screen is exposed.
- Once the entire screen is exposed and rinsed you will most likely find that one section of the screen looks better than the rest. This section indicates the best exposure time for your conditions.
- *This test should be performed quarterly or anytime that you change a product or piece of equipment related to your exposure process.
Exposure calculators are film positives that have been printed with a series of images designed to help printers determine exposure time. Most exposure calculators employ a series of halftone filters that filter the UV light in an effort to gauge the performance of the chosen light source with the chosen emulsion. Printers need to have a general idea of what their exposure time should be before using an exposure calculator and should follow the instructions that come with the calculator to ensure success. Some calculators will also include information to help determine whether or not the mesh that is being used will support different levels of halftone information. This is a valuable tool for anyone that will be working with process or photographic prints.