A 21-step gray scale (from Dainippon or Stouffer) is used to determine exposure rates (most plate manufacturers supply an information sheet providing the correct step when exposing their plates). The scale is used to test lamp intensity, which should be undertaken periodically. Over time, lamps will degenerate and should be replaced (at approximately 750 to 1,000 usage hours.). If not, exposure consistency will wane with the consequence that plates are no longer able to hold fine detail. To compensate, exposure times need to be increased.
The scale is processed similar to that of a film negative. Process the plate as per your usual practice. The exposure rate should be your normal rate for when processing type. Other rates of exposure, such as those for solids and halftones, are determined correctly with the scale as well. Note that for accurate readings, plate material should be brought to room temperature and the lamps should be warm from previous exposure.
When processing is completed the plate will show step levels of gray. Some of the gray levels will have washed out and others will not have. One or two of the steps between the solid and washed out steps may exhibit slight surface corruption. The first step that does not show corruption indicates the exposure intensity of the lamps. Manufacturers have slightly different specifications for determining correct exposure. Toyobo, the manufacturer of the Printight plate, recommends a solid 14–15 for a .038" thickness plate and 15–16 for a .060" on the Stouffer scale. Other manufacturers have slightly different specifications. If the reading is lower than the recommended reading, it may be time to consider replacing the lamps as they have reached the end of their useful life.
Replacement lamps need to meet the manufacturer’s specifications for your machine. For consistent exposure, it is recommended that all lamps be replaced at the same time. Before replacing your lamps, you should inspect the lighting quality of the existent lamps. Pull the lamp drawer out, turn on the machine, and set the exposing timer for a short duration. Wear eye protection when viewing lamps under exposure. If a lamp is flashing (at start up or shut down), is slow to start, or inoperable, note its numerical position on the drawer, as there is usually an electrical problem not necessarily the lamp that is causing this problem.
Shut the machine off and disconnect the electrical cord. Pull the drawer out, either entirely or just as far as you need to. Remove all the lamps before installing the new lamps. The old lamps should not exhibit arching at the pins or oddities in coloration such as darkening at only one end of a lamp; an indication of a problem in circuitry (which will also shorten the life of the ballast). Gray or brownish bands near both ends of the lamp, however, is normal with lamp aging. Circuitry problems are usually the result of a poor connection either between the lamp pins and the lamp holder contacts, or between the ballast leads and the lamp holder terminals.
Some ultraviolet discharge lamps have a reflector side. This coated side should face down (to the vacuum table). When inserting the lamp make sure the direction of the bi-pin is correct. There are positioning guides (an embossed mark on the metal end cap) that will guide you. The lamp is turned 90 degrees and locked in place. Note the bi-pins must be secure and exactly parallel in the lamp holders. The positioning guide will help you discern if the lamps are correctly locked in place. It should be in the exact center position.
Check the lamps by reconnecting the electrics, starting the machine, and running a short exposure (with the lamp drawer open). If one or more of the lamps is not illuminated, note its position on the drawer and shut the machine off, disconnect the electrics and begin inspecting the connections. You can determine if a lamp is defective by switching its position with another lamp. This will tell you if it is the connections rather than the lamp itself that is the culprit.
Usually the problem is merely a loose connection to a lamp holder, especially if it is a quick-wire or pressure lock terminal rather than a screw terminal. The latter two are easily fixed simply through adjustment. Quick-wire terminals may pull loose when changing out the lamps so you will have to push on each of them to make sure they click back in place.
If the terminals check out okay, the wiring is otherwise correct, and you still have a problem with a non-functioning lamp, the ballast connected to this lamp position may be faulty and need to be replaced. If your platemaker is an older model and you are experiencing continual ballast failure, your ballasts are reaching the end of their life cycle and you should replace them all.