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Novice Seeks Exposure Assistance

 
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Old September 15th, 2011 Sep 15, 2011 4:01:29 PM -   #1 (permalink)
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Default Novice Seeks Exposure Assistance

I'll apologise in advance for taking up your time but I've trawled this forum in search of my result and found nothing specific to my exact needs.

For the last year, I've been exposing in my bedroom and bathroom by switching lightbulbs and drawing curtains and to date I've had excellent results. I've made a lot of shirts for small, local bands and made a little profit and am very slowly upgrading my equipment.

So far I have only exposed 43t A3 white mesh screens and I have recently purchased some 120t A2 sized yellow frames and am not sure how to amend my exposure setup.

If I explain what I have currently have, do you think after you guys have stopped laughing, you might be able to help me make adjustments?

So from top to bottom, I have...

LIGHT SOURCE - 200watt Clear Incandescent Lightbulb with Pie Tin Reflector!
PERSPEX SHEET - 12" away from light source
CLEAR FILM POSITIVE
SCREEN - A3 DIMENSIONS - 43t WHITE MESH - SINGLE COATING OF EMULSION ON EACH SIDE [SPEEDBALL DIAZO PHOTOEMULSION]
BLACK FOAM

Exposing for exactly 30 minutes gets me a perfectly cured, crisp, even stencil with an easy washout.

What I'd like to know is how I should adjust my light source distance and my exposure time to allow for my larger screens.

Before anyone says "experiment"... I am a bedroom enthusiast with basic equipment so this is not only very time consuming but expensive... I'm buying craft shop emulsion, not large professional vats so wasting materials is something I'd really much rather avoid.

Is there an equation I should be aware of? I've read of measuring diagonals of screens to determine distance but not determining exposure and I've come to the plateau in my knowledge.

I would be indebted to any one of you who could assist me.

Thanks in advance to anyone who can help, and sorry if this message is too long and boring with no punchline!

Ian
 
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Old September 16th, 2011 Sep 16, 2011 1:20:29 AM -   #2 (permalink)
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Default Re: Novice Seeks Exposure Assistance

1. I'd increase my exposure distance to 20", the diagonal of my screen to get a more even cure, but that will more than double your exposure time? Maybe one to compromise.
2. Your bulb is a poor uv source, which is why your exposure time is so great. Think about a better source.
3. You are going to need to experiment! Do this: https://www.t-shirtforums.com/t-shirt...s/t106506.html
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Old September 16th, 2011 Sep 16, 2011 2:29:49 AM -   #3 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Thank you for your assistance.

I shall try 20" for an hour as my first port of call.

Assuming no luck, I shall give your cardboard method a go. This is a step wedge right? I've read about that.

As for the light source, I shall upgrade to a 500w Halogen at my next opportunity despite having the fear of knowing I will have to re-calculate all of my exposure times.

I'll respond with an update of how it all went.

Thanks again,

Ian
 
 
Old September 16th, 2011 Sep 16, 2011 3:11:56 AM -   #4 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Actually moving to 20" is a factor of 2.75x !!!! You might want to compromise there?
Yes, it's a manual stepwedge, you can pick up a stouffer strip quite cheaply.
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Old September 16th, 2011 Sep 16, 2011 4:31:07 AM -   #5 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Cheers Dave,

Let me just embarrass myself and ask what may be an obvious question?

Increasing the distance of the light source by a factor of 2.75 theoretically takes my exposure time from 30mins to 82.5mins right? This of course is a really long exposure time so I need to definitely get a new light source!

I've recently bought a 400w Halogen lamp on eBay but this won't be in my possession for a while. Is it simply the case that if I'm doubling my wattage I can half my exposure time? I feel as though that's a little optimistic and that the shift to halogen will also play a part. Can you enlighten me?

Last edited by Patt Man; September 16th, 2011 at 04:40 AM..
 
Old September 16th, 2011 Sep 16, 2011 6:55:31 AM -   #6 (permalink)
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Default Re: Novice Seeks Exposure Assistance

PositiveDave is right on as usual with the exposure distance--well, the best value for one.

IIRC, a doubling of distance is a quadrupling of exposure time. (Light intensity fall-off follows an inverse square) I don't know if there's a compensation because of some other part of the process that's being referred to... but figured I'd throw it out there. (I guess I come up with close to 2.75 at the figures mentioned on a little scrap... You have a software laden calculator or something, Dave? )

As far as the power goes, as above, I believe you have to quadruple the power to halve exposure time @ the same distance...

Best of luck!

Last edited by ScreenFoo; September 16th, 2011 at 07:05 AM.. Reason: Power and time
 
Old September 16th, 2011 Sep 16, 2011 8:51:38 AM -   #7 (permalink)
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Default Re: Novice Seeks Exposure Assistance

20/12=1.666
1.666 squared = 2.78
Doubling the wattage may quarter your time but you know nothing about it's output in the uv! Take any glass out of the way, it absorbs uv. It may be putting out a lot more than twice the uv light.
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Old September 16th, 2011 Sep 16, 2011 10:56:47 AM -   #8 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Ahh, so doubling UV output *quarters* the time.... Thanks for the excellent info, Dave.

(been a loooong time since I used a separate vacuum frame/light combo)
 
Old September 16th, 2011 Sep 16, 2011 11:33:36 AM -   #9 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Excellent, excellent stuff gents, really helpful.

Another question if I may?

What is UV output measured in and how do I find this out when buying bulbs/lights etc?
 
Old September 16th, 2011 Sep 16, 2011 12:20:55 PM -   #10 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Quote:
Originally Posted by ScreenFoo
Ahh, so doubling UV output *quarters* the time.... Thanks for the excellent info, Dave.

(been a loooong time since I used a separate vacuum frame/light combo)
No, uv is culmulative - it's just the number of uv photons of the right wavelength hitting your screen - I was just making the point that you might lower your wattage and still put out more uv, you could double your wattage and have a longer exposure!
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Old September 16th, 2011 Sep 16, 2011 3:52:09 PM -   #11 (permalink)
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Default Re: Novice Seeks Exposure Assistance

I see your point now, I was wondering about that.... Oh yeah, I forgot to mention, 2.78 is definitely quite close to 2.75... I wasn't trying to nitpick or anything, just making sure I didn't have the wrong equation. (Or spontaneously forgot algebra )

For some reason it sticks out in my mind that for the same type of light, (i.e. metal halide bulbs with the same doping, of the same age, etc.) you have to quadruple power to halve time. I might just be "cross-linking" power theory.

If you want a really fun, very cheap, and somewhat less than quantitative view of the color characteristics of your light source, you should check this craziness out:

DIY Spectro « Topologic Oceans

Waaay too much fun. (could be useful too, depending on the photo junk you may have lying about)
 
Old September 17th, 2011 Sep 17, 2011 5:18:01 AM -   #12 (permalink)
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Default Re: Novice Seeks Exposure Assistance

Oh my.
That's an awesome link!
I like to play with lights... I know self-promotional links are against the rules but this is not-related to me trying to get work or sell anyone anything... you might find this interesting, or not.


So I've got the day off work and I'm gonna bite the bullet and do this step-wedge test thingy despite knowing that coating the screen just to experiment with it is a ballache!

What would you guys recommend as a time interval for moving the cardboard and when should I start?

Again, sorry to bore you but you guys really are giving fantastic insights and its greatly appreciated.

Ian
 






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