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Strange Step Test Results

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Strange Step Test Results

Postby invayne » Mon Feb 06, 2012 8:23 pm

HI there,

I'm new to this forum- apologies that my first post is a plea for help but I'm really feeling desperate here.

I've been screen printing for a few years now but because of space and financial restrictions I've only ever used screen filler and drawing fluid. The only successful experiences I've had with photo emulsion have been using the equipment at my university.

However, I recently built myself an exposure unit to work with at home but I've been having a lot of trouble finding the correct exposure. I've done some simple ballpark testing but it always turns out almost partially under exposed and overexposed at the same time. So I printed out a step test chart and tried working with that but I got the same kind of results- half is underexposed and half is so overexposed that all the emulsion rinsed out. There is no happy medium. I worked with 30 second intervals between 30 seconds and 8 minutes.

The exposure unit (image below) is built using the frame and unit for overhead fluorescent lighting (like you see in office buildings that have those foam-tiled ceilings). There are two 32-Watt T8 24" Fluorescent Light Bulbs installed in the unit (the curved kind). Then I removed the decorative plastic filter that spanned the whole thing and replaced it with glass. The unit sits on its back, glass facing upwards. I've been using high-density photocopies on plain paper that I've oiled to make translucent (this worked perfectly for me in the past with other exposure units). I use a small sheet of glass to weigh the screen down as I do not have a vacuum installed. I then place a heavy black yoga mat over the unit during exposures to seal the light in.

When I completed the step test I was extremely confused to find that the areas that had burnt through were the areas that were exposed the least, and the areas that had some visible detail were the areas that were exposed the longest. I'm working out of a small apartment and don't have a hose so I've been rinsing in my bathtub with lukewarm water, but I don't think thats the issue. I'm worried the problem lies in my exposure unit, though I havn't a clue what it could be. If anyone has any ideas, tips, suggestions- anything- please let me know. I've included images below to illustrate some of my points.

To summarize:
Exposure unit: two 32W fluorescent T8 tubes in a metal unit
Print: oiled high-density photocopy
Screen: basic Speedball screen (unsure of mesh count)
Emulsion: Speedball photo diazo, purchased and mixed recently

IMG_1377.JPG
IMG_1377.JPG (215.75 KiB) Viewed 2758 times


steptestfinal.jpg
steptestfinal.jpg (110.43 KiB) Viewed 2758 times
Attachments
steptest copy.jpg
steptest copy.jpg (150.04 KiB) Viewed 2758 times
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Re: Strange Step Test Results

Postby d fleming » Tue Feb 07, 2012 10:22 am

Too much space between bulbs. Flourescent tubes should be unfiltered black light, not the kind used to light a room.
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Re: Strange Step Test Results

Postby invayne » Tue Feb 07, 2012 7:02 pm

Thank you so much for your help. I was worried about it being the space between the bulbs. However, the exposure unit I use at my school, I can't remember the brand or model name unfortunately, uses fluorescent tubes similar to mine- not black light tubes. Its a much different set up though, its got about 20 straight white tubes and the glass is higher- probably about a foot away from the tubes. Do you think it might have a chance at working if I raised the glass or should I simply scrap the whole thing?
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Re: Strange Step Test Results

Postby invayne » Tue Feb 07, 2012 9:53 pm

Apologies- just used the exposure unit at school and it is black light. I hadn't ever noticed that. Thank you!
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Re: Strange Step Test Results

Postby d fleming » Wed Feb 08, 2012 9:59 am

Glad to be of help. I have two exposure units with fl. tubes. One is a nice vacuum table for t-shirt frames, the other I made myself for larger frames Glass on both is 4-6 inches above bulbs. Larger unit I built for friend is on wheels to do very large frames, he uses reverse cut vinyl for positive then leans frame against wall and places light unit about a foot away to expose. Anything bigger than that we expose in sunlight.
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