Question About UV Light Source
Have done the required reading on Unblinkingeye and other sites. But have not found why most light source have 8-12 bulbs. Can you build a smaller unit, say with 4 bulbs, for nothing larger than 8x10? I plan to build one, but thought I would ask, since there are a few here that do alt. printing.
The bulbs I plan to use are 24 in., blb - or 20 watts and will be using mag. ballast in place of electric. Will I be wasting my time? Should I just go ahead a get at least 4 more?
p.s. - Yeah! I know, this is just starting something else...but using the sun for the Ziatypes just isn't working out. Darn clouds
I built a unit with 3 UV lights in a closet drawer set. The bottom drawer holds the lights and a power strip. The top drawer has no bottom and has a rack to hold an 8x10 frame. The lights are hooked to a Gralab and the whole thing is pretty light tight. I love it. I do Cyanotype and Van Dyke Brown and get times of 8 to 10 minutes. This is around the same time as our high end burner at my school.
Four bulbs would barely cover the area. You need to be able to get the image "centered" on the bank of bulbs. Need to avoid the ends of the bulbs in both directions. Go for six minimum, and then with 8 bulbs you can go at least to 11 x 14.
Definitely use more than 4 bulbs. You will want the coverage to be as even as possible, and if you are going to spend the time to do this anyway, I would recommend just doing it 'right' from the beginning.
Listen to Clay, he knows.....he has saved me a lot of money, if that is an incentive..
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Here's the issue with using too few bulbs: uniformity problems.
If you make a printer you generally want the lamps as close together as possible while still allowing room for your fingers to get the lamps out to replace them. Most people space the lamps about 2" OC (Whoops... that means 'On Center' for all you non-engineering geeks).
2" OC will result in a 1/2" spacing between the lamps, assuming you are using the standard T-12 UV lamps that are available. I wouldn't recommend going any further apart than that.
The reason for this spacing is that you generally want to place the print frame very close to the lamps to increase the printing speed of the unit. If they are spaced too far apart, you may see banding on the print from the non-uniform output associated with the lamp and the band between the lamps.
If you place the print frame glass about 3" or so from the lamps, a 1/2" spacing between lamps will not be visible in my experience, but you need to make sure that you have a good UV reflector behind the lamps (this goes without saying).
So, with 4 T-12 lamps, you cover an area of about 7-1/2" in width, but there is an additional problem. The area at the very edge of the printer will have less UV light than the middle, and so you will still have a uniformity problem.
This can be explained through fairly simple light integration or magnetic field theories, which are way too afield for this discussion. However, you can think of it this way: If you were to place your eye at the center of the location of the print frame, looking at the lamps, you will see a bunch of lamps in all directions, and there may be one directly in front, about 3" away.
If you place your eye at the edge of the printer looking at the lamps, you will see a lamp directly ahead, and more lamps to one side, but none to the other side.
This lack of lamps on both sides at the edge of the printer will cause less exposure in this area. This is an inherent property of every fluorescent exposure unit. The easiest way to avoid this is to make the unit with enough lamps that when an 8x10 is place under it, it still 'sees' lamps on both sides at the edge of the print.
This problem will occur at the sides of the lamps, but it can be even worse at the ends because there are some things about fluorescent lamps that can cause problems, one of them being that over time the lamps may have a tendency to get 'end wall blackening' in the last inch or two, which will severely reduce output. The lamps also inherently have reduced output in the last inch or two because of the cathode and anode that are built into the lamp. So basically, avoid the lamp ends if at all possible.
My recommendation when people design a UV unit is to make the unit at least 4" larger in every direction than the largest print you think you are going to make. It's really better to be 6" from the ends if at all possible. That's the best way to ensure that you will have good uniformity everywhere. So for 8x10, I would recommend at minimum a 16x18 exposure bank, preferably a 16x22 bank minimum. That equates to a 16x24 bank using 24" lamps, which equals 8 lamps 2" OC.
If you use 8 lamps, you may or may not be happy with the uniformity of the print if you happen to move up to an 11x14 image, because you are right on the edge of where things will begin to be noticeable.
All that listed above is for a 3" spacing from the lamps to the print frame glass. You can use fewer lamps if you increase the spacing from the lamps to the print frame glass. You'll be fine with a spacing from about 2" to 4" from the lamps with the spacing listed. There's really no point in increasing the distance, as it reduces printing speed, and there's no point in trying to reduce it because you have to be careful with uniformity issues. That's why 3" is about ideal.
If you go to 6" away, then you could probably increase the lamp spacing to
4" OC and still have solid uniformity. However, be aware that your print times will be substantially longer (greater than 2x longer) due to two factors. The first is you are using a lot fewer lamps, and the second is there are 'system losses' in the unit, and as you move further away, they typically increase. This is another issue of light integration, and a bit too complex to explain here.
There are many ways to solve the UV printer problem, but I think that the best way is to do lamps spaced 2" OC and make a bank large enough to cover the largest print you want.
I also think that you should consider using 48" lamps if you have the room. It will probably not cost any more (it may actually cost less because the lamps and ballasts are more common), and you will have the room to get two 8x10 prints going at any one time. If you only are printing one image, you can use the very large sweet spot in the middle with impudence.
I also recommend electronic ballasts if you can get them, as they are better for the lamps, have higher efficiency, and produce less heat, which does have a negative impact on the output of the lamps. With 48" lamps, you can get 4-lamp ballasts, which will cut the cost difference down a bit. Make sure that all your lamps and ballasts are identical, because they will often vary in output, which could cause striping problems.
Oh BTW, can you tell I'm an illumination engineer by training?
If you have any other questions, feel free to post.
Michael's reply is very comprehensive. I would add only a couple things. First, I have found that the distance from the bulbs to the print frame can equal the distance from one bulb to another OC (in contrast to M's making the bulb-to-print distance slightly longer than bulb-to-bulb OC). One wants to watch for uniformity, but getting the print frame as close to the light source without banding (ie with uniformity) means faster print times.
Second, consider, too, the type of UV light you use. Many people obey the majority of Alt Process literature and buy black light bulbs when they may not be the best fit for the material they are using. You always want to match your light source to your print material. My experience, and that of others (plenty of posts on this topis on the web) is that Super Actinic (SA) bulbs result in faster print times and slightly more contrast with both Pt/Pd and POP. (SA's output most their light at 420nm rather than the 360nm of BLB's) I don't know how Van Dyke or Cyanotype respond to SA's, though someone out there does.
WOW!! Well I knew there was a reason to ask, just had never seen any "Good" explanation. The summary you just listed looks like a great article for the article section. Have done quite a bit of searching (readgogglee) and have not seen anything like what you just posted. Would say your training is an asset for us, because no where have I found this type of information, and so well explained - Even I understand .
Guess I will go and pick up the additional lighting and parts to use at least 8 bulbs, seemed like overkill for 4x5 negatives, but you have explained it very well. Wish I did have room for the 48 in. but right now, no room. Will see how this works out for me and if I move up to a larger format, or start doingsufficientt prints (read have more than one processing at a time) then I will want to go that route.
Thank You for such a concise explanation.
As a p.s. to my own post, the KISS philosophy (Keep It Simple, Stupid) works for UV lights as it does for much else. For years, I took my Pt/Pd dog-and-pony show on the road and taught workshops in California, Oregon and Washington, hauling printers, trays, etc with me. For cheap simple printers I bought these cheap little flourescent fixtures designed to be mounted under cabinets in kitchens for local light. I got them at Home Depot type stores for about $10 apiece. I'd yank out the tube that came with them and replace them with 15" Super Actinics (bought from a good light supply store or an aquarium store). I'd sit about 5 or 6 of these next to each other and the 8x10 print frame would lie on top of them (face down of course). The units have their ballasts built in and I'd just plug them all in a power strip and way you go. So, it cost about $125 for an 8x10 printer and while the bulbs were only 15 watts, it was 15 watts over 15 inches of bulb so the power per inch was the same as larger 20 watt bulbs. These little printers print as fast as the sun (we tested them) and as fast as most commercial units. And if you want to get really fancy, you set the lights inside a wooden box designed around the dimensions of your print frame and so when you put the print frame on top, you have no light leaks. It's so easy it's dangerous. No wiring, no special ballasts, nothing, And though these under counter units are made really cheaply, mine have kept burning like the Energizer Bunny for decades of teaching.