Effects of vacuum and black light exposure units on the film positives
Submitted By: admin Date: June 11, 2011, 11:37:31 AM Views: 2979

If you are using an array of black-lights or other diffuse light source you may find that your ability to print fine detail is restricted, here's why:
This is a schematic diagram of an exposure unit.

The mask is in perfect contact with the flat screen. If there was only one small tube the system would approximate to a point source and the light would cure the screen so that the image is faithfully transferred to the screen.

Compare that to the effect of the light from the tubes at the extremes, they are going to undercut the mask and result in a loss of detail and closing of halftones.

If you do not use a vacuum the chances of good contact between mask and screen are nil, and that includes a piece of plywood and some bricks.
Once contact is lost the undercutting is made worse.

    * So point light sources are good, but if you have a point source then most of the energy will be wasted and result in long exposure times. Even commercial exposure units will make a reasonable compromise which leads to the exposure distance being about the same as the diagonal of your screen.
    * Vacuum units are essential for fine detail and halftone printing.
    * The screen in the area between point source and diffuse source will receive a variable amount of light which means that some may wash out other bits won't. You will have variable detail depending on your washout.
    * Over-exposure will make the problems worse because more of the semi-cured emulsion will resist washout.
    * I've shown a flat screen, screens aren't flat, they follow the contours of the mesh. Even with a good vacuum contact will not be perfect.
    * Light travels in straight lines - except when it hits anything, the first thing that happens is light-scattering so that even a perfect system would have some undercutting due to scatter. Tinted mesh absorbs uv light and reduces scatter.

Courtesy of Dave at Positivity LTD.

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June 29, 2012, 12:30:38 PM
I think this is an excellent discussion and I have been down this road many times.  This now becomes quite complex.  Light does not go through glass in a straight line.  Years ago I was able to witness an experiment set up by PPG Industries in their Glass Research Lab.  They wanted to purchase the best exposure unit and when they looked at how light was bent as it traveled through glass, they realized the imperfections inherent in the common exposure units . fluorescent tube or point light source.

Now in the production of a film positive in an inkjet or laser printer as well as in a conventional Graphic Art camera, the opaque or printing areas must be on the top side of the film.  When the film is contacted to an emulsion coated screen, capillary film screen or the mylar carrier of a pre-sensitized indirect film the closest contact possible, under pressure or vacuum is accomplished.  BUT, imagine if the pre-sensitized indirect film carrier is also an inkjet receptive surface. . then you could print your opaque image directly to the mylar carrier, expose the carrier side of the film to UV light, develop to film (wash off the unexposed areas) and attach it to the mesh.  The resulting detail of the stencil is far superior to printing or making a film positive on a separate piece of inkjet film and contacting it to the coated screen or pre-sensitized film.  Distance of the positive image to the light sensitive emulsion is critical.
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October 25, 2011, 06:12:15 PM
Dont forget the type of film used, I find can be even more important. A few decades ago i burned screen's with the sun. not the best case scenario in any stretch of the imagination but because of the type of film we used (ortho) it worked. try that with Vellum or other inferior type of positive. Last time I checked it also is a single point of light.  ;)
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July 07, 2011, 02:40:47 AM
Light does travel in a straight line, but not through mass, such as emulsion. (think refraction as light passes through water)

Often the explanation of light traveling in a straight line is the sharp line of a shadow on a clear sunny day. However, even on the clearest day you can still see fine in the shadows, and that is because so much light is reflected off air molecules and objects.

Same in an exposure unit, the light scatters no matter what light source. No light unit will ever perfectly have light traveling only

If your exposure unit doesn't already have black walls, make them matte black to reduce (you can only reduce, never eliminate) light scatter.

Get a lamp that is strong so your exposures are fast (but not too fast, but that is another subject.)  I've not tested it, but 5K seems right.

We have a 5K Olec and we get great results, better than what I see in my travels to other shops. We have the lamp at the distance from the screens that they recommend, which is also important.

The OLEC has a shutter and that is important, otherwise you are trying to calculate exposures with a bulb that is always warming up or fading out instead of full on or full off.

I've heard people explain that tube exposure units are better because they are closer to the screen. I don't know the science behind why not, but I see inferior results.

Also important is the rubber blanket and how the vacuum works to pull the film to the emulsion without distorting the screen and making good contact film to screen.

With all things in screenprinting, it is the system that makes for superior results, one part of the process taken in isolation cannot necessarily be considered "superior."  A crude example is that a very fast emulsion may make up somewhat for a weaker lamp. Emulsion, coating, distance from lamp to screen, vacuum, lamp, light meter, and mesh must all work together to get the results that you require.