When optimizing a plate material, it can be helpful have an idea of what range of exposure times to test based on UVA lamp intensity. While the method described below will provide a “ballpark” value, all typical plate optimization tests should still be carried out for fine tuning and verification.

Reading Manufacturer Documentation

  • Look for the millijoule (mJ) or joule (J) requirement for each exposure. This is the target “dosage” of UVA to obtain proper exposure.
  • If the millijoule (mJ) isn’t listed, you can calculate it. You need to know three pieces of information:
    • The manufacturer suggested exposure time (seconds)
    • The manufacturer’s UVA intensity (mW/cm2)
    • Your machine's UVA intensity (mW/cm2)
      • Solve for TARGET mJ: [Manufacturer Time] x [Manufacturer mW] = [TARGET mJ]
      • Solve for TARGET sec: [Target mJ] / [Your Machine mW] = [TARGET sec]
    • 1 joule (J) = 1000 millijoules (mJ)

Example of Manufacturer Documentation:
Click to enlarge.

Reading UVA Bulb Intensity

  • Check that you are using a UVA light meter that has been recently calibrated, using the proper sensor for UVA, with fresh batteries. Ensure the UVA sensor is clean.
  • Warm up the exposure bed for 10 minutes.
  • Divide the exposure bed into a 9 to 12 space grid using tape (or imaginary lines).
  • Place the sensor in the center of each grid space and turn on the exposure unit. Allow the reading to stabilize before recording the result.
  • Record the readings and average them together to get the overall UVA output.
    • Note any readings that appear to be outliers, and troubleshoot as needed.

Equations to Calculate Estimated Platemaking Times

  • Equation to convert millijoules (mJ) to milliwatts (mW):
    • [mW] = [mJ] / [sec]
  • Equation to convert milliwatts (mW) to millijoules (mJ):
    • [mJ] = [mW] x [sec]
  • Equation to calculate seconds from mW and mJ:
    • [sec] = [mJ] / [mW]

All Printing Resources specializes in assisting customers with this process, as well as troubleshooting from prepress to the pressroom. APR’s Technical Solutions Group is equipped with the right tools and experience to help. If you would like more information, please contact me at [email protected].

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