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Technical Bulletin 13-12/86

Exposure Control - Gray Scale or Microlines?

Recommendation: A gray scale must be used to accurately control negative plate exposure for four reasons:

1. To indicate the proper curing of the photosensitive coating for press life.

2. To accurately control the dot size on the plate.

3. To minimize the effect of halation caused by overexposure.

4. To reduce the effect of dirt caused by underexposure.

Microline targets will not reliably provide for proper exposure control and consequently should not be used as a substitute for a gray scale.

Plate Exposure Control

Our recommendation is that microline test targets only be used as a tool to measure resolution and to function as a process control indicator. The microline test targets are not to be used to control exposure. They are not sensitive enough to changes in exposure time, particularly with the Viking™ plate line, to be a reliable exposure control device. As an example, a 50% dot on a Viking plate will change 1/2% to 1% for every gray scale step increase when contacted to hard dot film. However, the microline targets will not change perceptibly for 2 or 3 gray scale steps and consequently the use of microlines as an exposure control device could result in:

1. Inadequate curing of the photosensitive coating which could result in a loss in press life.

2. Undetected changes in dot size on the plate.

3. Halation caused by overexposure.

4. Pinholes caused by underexposure.

Use of Gray Scales - Procedure

1. Make quality assurance tests a habit.
a. Validate correct exposure every time a new package of plates is opened.
b. Once a week confirm exposure to detect possible power or equipment changes.

2. Exposure time, one of the variables in the printing cycle, is critical for all light sensitive graphic arts products. Correct exposure will assure the performance you expect from these high quality litho plates.

3. The gray scale is usually positioned on the gripper edge, out of the printing area, and exposed with the flat. It is placed under the maximum layers of film. Remember, the gray scale is a measuring instrument. It should not be considered a layer of film. Any Mylar or similar overlay material used in stripping should also overlay the gray scale.

4. Record light source usage. Metal halide diazo type bulbs are the recommended light source for exposing litho plates. Metal halide diazo type bulbs used over 1000 hours will continue to expose plates but may not provide enough actinic light to activate the polymer molecules, resulting in suspect plate performance. A record of light source usage will show when exposure bulbs should be replaced. The most efficient bulb for plates output mainly in the range of 385-425 nanometers.

5. A gray scale (sometimes called a transparent Sensitivity Guide) is a strip of film having either 10 or 21 steps of progressively greater density. The film has a density of approximately 0.95 at Step 7. It is used to measure the amount of actinic light reaching the surface of the plate.

6. Recommended gray scale reading, for maximum press life on Viking™ plates:

Viking™G1, G2, S2, S2A	Solid Step 4
Viking™ GMX	Solid Step 3

7. To increase or decrease gray scale reading, expose another plate and use the multiplication factor from the chart below.

To increase exposure by...	1 step	2 steps	3 steps	4 steps
multiply by	1.4	2.0	2.8	4.0

To decrease exposure by...	1 step	2 steps	3 steps	4 steps
multiply by	0.7	0.5	0.36	0.25

To Increase Gray Scale By: .25 Step More .50 Step More .75 Step More 1 Step More 1.25 Step More 1.50 Step More 1.75 Step More 2 Steps More 2.25 Steps More 2.50 Steps More 2.75 Steps More 3 Steps More

X Original Exposure By: 1.09 1.19 1.30 1.42 1.55 1.69 1.85 2.01 2.20 2.40 2.62 2.86

To Decrease Gray Scale By: .25 Step Less .50 Step Less .75 Step Less 1 Step Less 1.25 Step Less 1.50 Step Less 1.75 Step Less 2 Steps Less 2.25 Steps Less 2.50 Steps Less 2.75 Steps Less 3 Steps Less

X Original Exposure By: .92 .84 .77 .70 .65 .59 .54 .50 .46 .42 .38 .35

Copyright 1996 Imation. All rights reserved.