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1. Originally Posted by wmlaven
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.
Bill,

This is why I stated that there is a range of acceptable distances. However, consider that the distance from the lamps to the print frame glass is not a significant factor in the speed of the printer. Most people think that the illumination (and therefore printing speed) will fall off to the square of the distance (similar to how flash illumination falls off). That is not correct in this case. In fact, with a large enough bank of lamps, the distance from the lamps to the glass is irrelevant (within limits).

This is related to the integrated illumination that a point receives, and is somewhat complicated to calculate, but you can be assured that the difference in exposure time for a 2" OC lamp bank at 3" vs. 2" from the glass will essentially be negligable. However, as you get closer, you run the risk of uniformity problems. That's why I recommended 3" and not 2". It may be safe at 2", but 3" is definitely safe, and there really is no penalty to the greater distance.

I know that this sounds like a bit of a contradiction from what I stated earlier about system losses, but it has to do with near-field calculations of light sources (or magnetic fields, or heat sources, etc.) that have a large dimension relative to the distance between the emitter and the receiver. It's called the 'parallel plate' or 'parallel plane' theory. It's applied in many ways, like in the design of magnetic fields in capacitors, as well as illumination engineering.

Essentially, as long as the distance between the lights and the glass remains small relative to the overall dimensions of the light bank, the average illumination on the glass will remain constant. Where this approximation fails is on the edges of the bank, but I think we've determined that we want to stay away from the edges as much as possible anyway, so we avoid that problem.

---Michael

2. Just a couple of more opinions.

First, about printing speed in relation to tube spacing and distance from printing frame. My own UV bank has the bulbs spaced 1.5" on center. With this spacing there is less than 1/4 stop difference in printing speed between bulb to printing frame distance of two inches and six inches. I settled on four inches assuming that I would get more uniform illumination at this distance but lose virtually nothing in terms of speed.

About type of tube. As William notes, there is a lot of discussion out there about the printing speed of various types of lights, say comparing BL, BLB, SA, Aqua, etc. and many Pt./Pd. printers report that the SA tubes print faster. However, an equally large number have found that the BL tubes print as fast or faster. I have personally compared printing speed of BL and SA tubes on at least four occasions and in every tests the BL tubes were actually slightly faster. Not enough to get really excited about, but nevertheless faster in every test. This was very controlled testing, i.e. using the tubes in the same fixture and carefully controlling all over the variables that go into printing. I had a conversation on this subject a couple of years ago with the folks at Edwards Engineering and they told me that their own tests had given the same results. However, the difference in speed between BL and SA tubes of the same wattage in fixtures of the same ballast is minimal, certainly not worth the expense of changing out one type for another.

The above observations applies only to Pt/Pd printing and the other iron-based processes, i.e. VDB, kallitype, cyanotype, etc. If you print with any of the dichromated colloid processes the BL tubes print quite a bit faster.

So my opinion is this. If you print with only Pt./Pd. or the iron-based processes the BL and SA tubes give virtually identical results, with some slight differences in curve shape. For printing with most of the other alternative processes the BL tube is clearly superior in terms of printing speed. In fact, for these processes even the BLB tube is much better than the SA.

Sandy

3. "So my opinion is this. If you print with only Pt./Pd. or the iron-based processes the BL and SA tubes give virtually identical results, with some slight differences in curve shape." (from Sandy)

Sandy and I have discussed our tests offlist on several occasions and we get different results. At several workshops, I brought printing units with both BLB and SA bulbs and we always found the SA bulbs faster by about a stop. We tested with new bulbs so bulb age wasn't a factor. Similarly, I had a friend test two untis with POP and she found the SA's faster and more contrasty. We were very careful in our testing to limit all other variables. God knows why the difference. In my 20 years of working with Pt/Pd I've found that it sometmes behaves predictably and sometimes throws one a curve ball. Perhaps a new bulb buyer could buy bulbs from an understanding store and explain they want to do only a few exposures for tests so they can return the set of bulbs they don't like and see what works for them waking up on their side of the bed.

In response to Michael's claims that the difference between 3 and 6 inches isn't that great, I guess I'm wrong. I've always copmpared greater distances, say 2" versus 14" (which is how I found one printer's light set up when I suggested a change) and with those margins the results were different.

4. I've read everything. I like Clay's response the best.

5. Originally Posted by Joe Lipka
I've read everything. I like Clay's response the best.
Joe,

Damn, if I had know the responses were going to be rated I would have tried harder!!

But for what it is worth I agree with Clay. Built the largest unit that fits in your workroom since you may want to print larger at some time in the future. The economics of building UV banks is that it may actually be less expensive to build a twelve tube unit of 48" tubes than a twelve tube unit of 24" tubes. This is due to the fact that the 48" size is much more commonly used in construction than the smaller size. But if printing speed is important you should definitely use electronic ballast. My tests indicate that units with electronic ballast print up to 25% faster than units with magnetic ballast, which amounts to more difference in speed than you would normally expect to see with different type tubes, SA, BL, BLB, etc. And, not all electronic ballast is the same either for that matter.

Sandy

6. Can we get a dumbed down version please. I have just read the thread for the third time and I am no where near being clear. Do I have this right?

use BLB or SA bulbs
Use more than four and go larger cuz you might want to go bigger later
Put them 3-6 inches above the printing frame

Mike mentioned some kind of ballast-what is wrong with just using the ballasts from a shop light or one of those under the cabinet ballasts that wmlaven was talking about. My head hurts.

7. Originally Posted by mark
Can we get a dumbed down version please. I have just read the thread for the third time and I am no where near being clear. Do I have this right?

use BLB or SA bulbs
Use more than four and go larger cuz you might want to go bigger later
Put them 3-6 inches above the printing frame

Mike mentioned some kind of ballast-what is wrong with just using the ballasts from a shop light or one of those under the cabinet ballasts that wmlaven was talking about. My head hurts.
Summary: There is no single Right Way to do this. Just a few wrong ones.

8. LOL - Mark, I feel your pain..and even worse, I requested this information from the individuals .. Michael, Clay and Sandy - you guys are great. William, thanks for your input as well. Now how do I explain to the wife that the darkroom needs to be larger so I can use a 12 light 4 foot light source? Maybe I can tell her it's a tanning bed.... Now correct me if the vision is off, but that is quite a bit of space. Guess I'll just have to stick with the 24 inch bulbs. Thanks guys, this was fun.

9. Originally Posted by mark
Mike mentioned some kind of ballast-what is wrong with just using the ballasts from a shop light or one of those under the cabinet ballasts that wmlaven was talking about. My head hurts.
There are two reasons I don't recommend the cheap magnetic ballasts. The first is something called ballast factor, which Sandy alludes to in one of his posts. Electronic ballasts will typically output more light with the same lamp, which makes printing faster. This in not always the case, but in general it is correct.

The second is energy efficiency. While I know that we're only talking about a few ballasts here, the electronic ballasts are much more efficient than the magnetic. It is part of my professional and personal beliefs to attempt to conserve energy wherever possible. I don't specify magnetic ballasts, and I don't use them personally in my residence.

Mike asked 'why', not 'how'. 'How' is easy, do 8 lamps, 2" max. OC, and put the print frame glass about 3" below (assuming max of 8x10, and may be suitable for 11x14 depending on your tolerances. Personally, I would do 10 for an 11x14 to be safe.). Put a fan in the unit. Shield the lamps from normal viewing angles. Put a bright white (not specular) metal reflector behind the lamps and tie it into the ground of the fixture. Wear safety goggles when exposing. Adjustments to these recommendations are acceptable within limits.

'Why' is much more complicated.

---Michael

10. Originally Posted by wmlaven
" At several workshops, I brought printing units with both BLB and SA bulbs and we always found the SA bulbs faster by about a stop. We tested with new bulbs so bulb age wasn't a factor. Similarly, I had a friend test two unties with POP and she found the SA's faster and more contra sty. We were very careful in our testing to limit all other variables. God knows why the difference.
Ahhhhh, I just noted that your comparison was SA to BLB, not SA and BL? If that is so then I would agree with you that the SA tube prints faster and gives more contrast than the BLB tube with all of the iron processes, though my tests indicate that the difference is only about 1/4 stop in both speed and contrast. The BLB tube is basically the same tube as the BL but it has a filter that cuts out all radiation above about 420nm, and as we know the iron processes have considerable sensitivity to radiation in the near visible.

As to reported differences in finding between the BL and SA tube I can not comment because there is no way of knowing the exact circumstances of the tests and many factors other than the light source itself can have an impact on printing speed. I only know what I personally have found in my tests, which could have some flaw though I have spent a lot of time trying to standardize them. But for anyone interested in doing this kind of tests here are some rough guidelines. Everyone knows the importance of keeping such things as exposure time and distance from the printing frame to the tubes constant and of using the same paper for the test, RH, etc. but beyond the obvious there are a number of other factors that must be considered.

1. Age of bulbs -- Must be the same because there is a significant reduction of light output as tubes age.

2. Same ballast -- Any tests of tubes should be done using the same ballast and the same fixture.

3. Tubes should be cooled with fan -- The output of tubes drops significantly as they heat up to around 95-100º F. This may have more impact on radiation with some types of tubes than on others.

4. The exact sensitizing mix of must be used. Palladiuim is much slower than platinum and even a couple of drops difference in the amount of platinum in the 2ml that is typically used for a 8X10 size print can have a big impact on printing speed. I did some tests of this recently for the appendix on UV lights that I wrote for the new edition of Dick Arentz' book on palladium and platinum printing and I was quite surprised to see what a big difference even minute amounts of platinum can have on speed.

I want to also point out that many of the original claims made for the faster printing speed of SA tubes was based on the use of high output (HO) and Very High Output (VHO) units, which are rated at from 2X to 4X more in wattage than normal output tubes. A regular output 24" tube is rated at 20 watts, while a 24" high output tubes is rated at 75 watts. And of course you need special ballast for the HO and VHO tubes.

In any event my own testing has found the Aqua, SA and BL tubes to be about equally efficient in printing with the iron processes, including cyaonotype, Pt/Pd, kalliype, VDB. For printing with dichromated colloid processes (gum, carbon, etc.) both the BL and BLB tubes print faster than the SA or Aqua.

Sandy

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