Exposing Screens: Difference between revisions
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|17 x 23" | |17 x 23" | ||
|<strike>28.6"</strike> 17" <ref name="exposure-height">The signindustry.com site states that the minimum distance for the lamp should be the diagonal measurement of the screen. For 17 x 23" this would be 28.6 inches, which would require a 53 minute exposure (!). I figured if you measure a 90° triangle from the lamp to the table, it would be 1/2 that, but who knows if that's actually an effective formula for distance.</ref> | |<strike>28.6"</strike> 17" <ref name="exposure-height">The signindustry.com site states that the minimum distance for the lamp should be the diagonal measurement of the screen. For 17 x 23" this would be 28.6 inches, which would require a 53 minute exposure (!). I figured if you measure a 90° triangle from the lamp to the table, it would be 1/2 that, but who knows if that's actually an effective formula for distance.</ref> | ||
|<strike>53 min</strike> | |<strike>53 min</strike> 18-½ min | ||
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With the next screen I tried using my calculation for the distance (28.6 / 2 = 14 or 15"). I gave it a couple extra inches, which brought it full circle to 17". The equivalent amount of time would have been 16 min, so I arbitrarily tried adding 6 min for 22 min exposure. Too long, but the image washed out the same towards the edges as in the center, so the height might be fine. | With the next screen I tried using my calculation for the distance (28.6 / 2 = 14 or 15"). I gave it a couple extra inches, which brought it full circle to 17". The equivalent amount of time would have been 16 min, so I arbitrarily tried adding 6 min for 22 min exposure. Too long, but the image washed out the same towards the edges as in the center, so the height might be fine. | ||
3rd try: 500W/17"/ | 3rd try: 500W/17"/19 min which looks to be the winning combination. Maybe it could be even a little less time for the exposure. <<< yes. 18-½ min. | ||
4th try (& truly winning combination): '''500W/17"/18-½ min''' | |||
=== Other notes on exposure === | === Other notes on exposure === | ||
Revision as of 23:49, 18 December 2014
Process
- Place screen under lamp.
- Attach lamp to armature and arrange it such that it is the correct height above the horizontal surface.
- Tape down black paper to flat surface.
- Turn off lights in the room with the exposing table. Turn on red lamp.
- Place the screen on the black paper, flat side down.
- Place transparency on screen.
- Place transparency face down on scrap paper so that the image is backwards.
- Spray the transparency lightly. Spray it away from everything; spray mount is impossible to clean up.
- Wait 30-45 seconds.
- Gently press the image down on the "back" of the screen (the side that gets the ink applied to it). Looking at the back of the screen, the image should read correctly (not be reversed).
- Place the glass on top of the transparency. Try to arrange it so the edge of the glass lands beyond the area of the screen that has emulsion on it. Otherwise the emulsion under the edge of the glass gets exposed at a different rate from everything else and doesn't clean out neatly.
- Expose the screen.
- Turn on the lamp and start the timer.
- After time is up, turn off exposing lamp. It's ok to turn on overhead lights.
- Clean out emulsion from screen.
- Clean out the screen right away.
- Gently coat both sides of the screen with water.
- Go back and apply the highest pressure jet of water possible to the entire surface of emulsion on both sides of the screen. Clean off any bubbles. Continue applying jet of water until the emulsion is washed out of the image on the screen.
- Gently dry off the screen with a kitchen cloth or paper towel.
- Dry the screen.
- Let the screen dry thoroughly before doing anything with it.
- It's fine to have it exposed to light at this point.
- Fill in uncoated areas of the screen.
Exposure times
Speedball emulsion & 250W BBA No.1 Photoflood 500W photoflood (250W takes too long)
| Screen Size active area/inside |
Lamp Height | Exposure Time |
|---|---|---|
| 16 x 20" | ||
| 17 x 23" |
Process so far
I started with 17" from some website. I think I started with 17 min exposure with a 250W bulb, then bumped it up to 35 min, which I'm realizing now wasn't a long enough exposure because with enough spraying the exposed parts of the emulsion were coming out.
Doing even longer exposures didn't seem viable, so I got a 500W bulb.
It seemed like the center of the screens and the edges were behaving differently in terms of washing out the screen, so I tried to figure out what would be the appropriate distance for the bulb. One site says that it should be the same as the diagonal length of the screen (28.6" in this case). I was thinking that if the light spreads in a 90° angle from the bulb, then it should be 1/2 that (Pythagorean theorem).
Starting with 250W/17"/35 min > 500W/17"/17 min > 500W/28"/44 min. 44 min is too long, so I cheated it and tried 500W/25"/35 min. The exposed emulsion washed out.
With the next screen I tried using my calculation for the distance (28.6 / 2 = 14 or 15"). I gave it a couple extra inches, which brought it full circle to 17". The equivalent amount of time would have been 16 min, so I arbitrarily tried adding 6 min for 22 min exposure. Too long, but the image washed out the same towards the edges as in the center, so the height might be fine.
3rd try: 500W/17"/19 min which looks to be the winning combination. Maybe it could be even a little less time for the exposure. <<< yes. 18-½ min.
4th try (& truly winning combination): 500W/17"/18-½ min
Other notes on exposure
According to the article below with a point light, the height of the lamp should be the diagonal length of the screen.
If a correct exposure time is known, and the lamp needs to be adjusted, the new exposure time can be calculated using the inverse-square law [2].
The power of the light is equal to the inverse square of its distance from the subject.
E.g. if a light is 1 meter away from the subject, moving it 2 meters away will decrease its power by 1/4. (1 divided by 2 meters * 2 meters)
To determine the new exposure time, (new height/original height)2 * original exposure time.
- Exposing Direct Emulsions (signindustry.com)
Hints at formulas for determining lamp height and exposure time.
- ↑ The signindustry.com site states that the minimum distance for the lamp should be the diagonal measurement of the screen. For 17 x 23" this would be 28.6 inches, which would require a 53 minute exposure (!). I figured if you measure a 90° triangle from the lamp to the table, it would be 1/2 that, but who knows if that's actually an effective formula for distance.
- ↑ Inverse-Square Law and Lighting, tuts+