Editing Exposing Screens (section)

== Exposure times ==

Speedball emulsion & <strike>250W BBA No.1 Photoflood</strike> 500W photoflood ''(250W takes too long)''

{|
!Screen Size<br />active area/inside
!Lamp Height
!Exposure Time
|-
|16 x 20"
|<strike>25.8"</strike>
|<strike>39 min</strike>
|-
|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&deg; 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> 18-&frac12; min
|}

=== 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&deg; 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-&frac12; min.

4th try (& truly winning combination): '''500W/17"/18-&frac12; 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''' <ref name="inverse-square">[http://photography.tutsplus.com/articles/rules-for-perfect-lighting-understanding-the-inverse-square-law--photo-3483 Inverse-Square Law and Lighting], tuts+</ref>. 

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)<sup>2</sup> * original exposure time`.

==== Trial & error ====

Ok so with a yellow screen & 550W/17"/9 Min it seemed hard to get the emulsion out. I had to make 3-4 passes. At 8-&frac12; minutes, the emulsion came out pretty quickly with no issues with the exposed emulsion. 

Then I exposed a screen with white mesh at 18-&frac12; minutes and the exposed emulsion washed out. 

The signindustry.com site says that yellow mesh should take ''longer'' to expose. I don't know if the mesh color is contributing to this behavior at this point.

* [http://www.signindustry.com/screen/articles/2003-11-26-BS-ExposingDirectEmulons3.php3 Exposing Direct Emulsions] (signindustry.com)<br />''Hints at formulas for determining lamp height and exposure time.''