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Discussion Starter · #1 ·
Deciding between these two options I would like some feedback on which of the two is the best choice for an exposure unit.

The light source seems to be the key to getting halftones so therefore I am asking in terms of which source would provide the best ability or is neither a good option? or both gives equal results.

I have attached images of the two LED light sources.
 

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Get the floodlight type.
Easier for beginners, and also better if you do halftones.
Get the most powerful you can find, especially if you buy a Chinese one.
 

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The most important (light) factors for halftones are:

1) A light source that does not hit the art from multiple angles, as that can undercut the art, effectively making the dots smaller on the burned screen than on the film. That's why "point source" light sources were used in high-end exposure units, rather than banks of fluorescent tubes. LED strips are the "new" FL tube, in that regard. A flood light, be it LED or halogen, is closer to the idea of a point source light. The sun is the ultimate point source light (in this particular solar system).

2) Having the light source the correct distance from the art/screen so that the entire area is covered as evenly as possible. There is a tradeoff between being closer and getting a faster exposure at the expense hot spots, of being farther back and getting a more even but slower exposure.
 

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Any suggestions?
Buy a real one from a US based shop.
It will be more expensive, but it will be CE certified and will probably last longer.
Here is an example.

also what is the general accepted distance for a "point source" light ?
The distance depends on the size of the screens you will be exposing.
Obviously the distance will also affect the exposure time, as the light spreads over a wider area.
 

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Discussion Starter · #9 · (Edited)
thanks for all the feedback, on the topic of halftones I read alot about vacuum tops being required to achieve halftones.

however I have seen many videos where that is not the case, for a DIY exposure unit using foam and a piece of 3/4 inch plywood attached by a locking pull latch could this create enough pressure or seal to get a quality halftone image?

like in the images
 

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I read alot about vacuum tops being required to achieve halftones.

however I have seen many videos where that is not the case, for a DIY exposure unit using foam and a piece of 3/4 inch plywood attached by a locking pull latch could this create enough pressure or seal to get a quality halftone image?
It can be done without vacuum
 

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There is "optimum," and then there is "good enough." Don't let the pursuit of perfection prevent you from doing anything at all. In the end, you are printing on a piece of Jersey fabric, not exactly a precision substrate. Lots of people do just fine with florescent tube and foam pressure exposure units, including some people I know from here. It's what I started with. The main shortcoming of my first unit was weak UV output.
 

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As an old guy, I started out in the '70's using carbon arc lamp, still the best, but bad for your health. Those made a tiny point source, like a miniature sun, and everything else since is considerably larger source, like Metal Halide and the like. But getting caught up in the LED idea, I bought a 200W COB, which is about 2" square, added the driver, and a heat sink to mount it to, plus a 12v computer fan attached to the heat sink to keep it cool, around $300 with the box and all. Works great. But, we went for 450 watt unit from Saati, and it's really good. As for distance from the glass, I seem to recall twice the diagonal of the vacuum frame not the screen. The light is brightest in the center, so if it's too close, the falloff around the edges will give you an uneven exposure. It should be back far enough so that the screen is getting as even illumination as you can make happen. Both the one I built and the Saati are 405 namometer, works well with the emulsions we use. We print single color up to 10 color sim process, the halftones we get are fine, after fine tuning the exposure time.

Steve
 

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Discussion Starter · #14 · (Edited)
With a single point light source the diagonal is this based on the screen size(ex. 20x24)?, the film size(13x19)? can someone give clarity on that point?

What if using tube or strip lighting whats the best light to glass distance in this case?

My issue is space so I am trying to design my exposure unit to also support my screen printer( not a traditional press...its more like if anyone is familiar with the silkscreennow blue diamond press you will understand my idea of putting the exposure unit and press together) so I really cant have a giant exposure unit in either width or height was hoping to find the best way to do this with it being a table top design.
 

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The diagonal is this based on the screen size(ex. 20x24)?, the film size(13x19)? can someone give clarity on that point?
The size of the emulsion coated area actually, because unused space is irrelevant.
Having said that, there is no standard distance ratio, because it depends on the light beam angle of the light source.
Rectangle Triangle Wood Slope Font

A wide beam angle does not mean you can use the full width. You can only use the 30% sweet-spot in the middle.

It's easy to eyeball. Just switch it on and point it at the wall.
You will notice that as you increase the distance, the sweet-spot is getting larger but also less bright.
Larger distance = larger sweet-spot = less brightens = longer exposure times.

My issue is space so I am trying to design my exposure unit to also support my screen printer( not a traditional press...its more like if anyone is familiar with the silkscreennow blue diamond press you will understand my idea of putting the exposure unit and press together)
In that case the original idea of using LED strips may be better.
Just make sure the LEDs are spaced equally and then use the wall trick to find the optimal distance from the glass.
The optimal distance in this case is when the light is smooth without any gaps.
Wearing dark sunglasses when doing so will help.
 

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Discussion Starter · #16 ·
The size of the emulsion coated area actually, because unused space is irrelevant.
Having said that, there is no standard distance ratio, because it depends on the light beam angle of the light source.
View attachment 276661
A wide beam angle does not mean you can use the full width. You can only use the 30% sweet-spot in the middle.

It's easy to eyeball. Just switch it on and point it at the wall.
You will notice that as you increase the distance, the sweet-spot is getting larger but also less bright.
Larger distance = larger sweet-spot = less brightens = longer exposure times.


In that case the original idea of using LED strips may be better.
Just make sure the LEDs are spaced equally and then use the wall trick to find the optimal distance from the glass.
The optimal distance in this case is when the light is smooth without any gaps.
Wearing dark sunglasses when doing so will help.
I was curious I saw a video where someone used several(6) halogen lamps(300watt) for their exposure unit, they where the square lamp types..... is this advisable perhaps with LED lamps?

Im curious if the multiple lamps will create coverage issue with the burning process.
 

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I was curious I saw a video where someone used several(6) halogen lamps(300watt) for their exposure unit, they where the square lamp types..... is this advisable perhaps with LED lamps?
That's because halogen lamps don't emit much UV light, and 300w is a bit too low.

Many people start with a single 500w lamp but exposure times are very long.
That's around 5 minutes if you put it very close to the screen (ie 10 inches), and 15-20 minutes if you put it 40 inches away.


Im curious if the multiple lamps will create coverage issue with the burning process.
Using multiple lamps is possible, but you have to figure out the optimal spacing and distance from the screen.
Like I said... a single point light source is easier and better.
 

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sbremmer suggested a very important concept.
It is not as much about the type of light used in the exposure unit as about what wavelength the emulsion is most sensitive to.
With LED applications, the LED can be selected precisely to the wavelength of the emulsion for maximum efficiency. That can be anywhere between 390 to 410 nM. Most incandescent sources have output centered between 250 and 350 nM missing the target (emulsion activation wavelength) completely.
When the LED and emulsion wave length match the result will be an extremely short exposure time when compared to any other light source that does not match.
This dynamic may deliver results that will require you to bracket test to determine if point source of led panel is best for the half-tones you need.
PS: Place no faith in the product published wavelength. Place your faith in who is selling it.
 

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Discussion Starter · #19 ·
sbremmer suggested a very important concept.
It is not as much about the type of light used in the exposure unit as about what wavelength the emulsion is most sensitive to.
With LED applications, the LED can be selected precisely to the wavelength of the emulsion for maximum efficiency. That can be anywhere between 390 to 410 nM. Most incandescent sources have output centered between 250 and 350 nM missing the target (emulsion activation wavelength) completely.
When the LED and emulsion wave length match the result will be an extremely short exposure time when compared to any other light source that does not match.
This dynamic may deliver results that will require you to bracket test to determine if point source of led panel is best for the half-tones you need.
PS: Place no faith in the product published wavelength. Place your faith in who is selling it.
As in where you would purchase the item(ie. Amazon?) or the actual seller from say Amazon or Ebay?
 

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Most incandescent sources have output centered between 250 and 350 nM missing the target (emulsion activation wavelength) completely.
These numbers are wrong, but it is true...incandescent light sources don't emit much UV light.
But with enough wattage, the UV output is enough to do the job.
A 2000w Halogen tube for example (they are used for studio lamps etc) will emit around the same amount of UV light as a 10w UV LED.

For an entry level beginners unit, any UV light will do. Even grow lights work well.
 
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