Hi... I was wondering if there are any technical people out there who could help me with my exposure problems.
I use a home made unit using a brand new 1000W metal halide lamp from a hydroponics set-up. The exposures I am getting are taking way too long so I bought myself a UV-A light meter to take readings. The readings I have obtained are as follows, after allowing a good half hour to warm up:
0.26 mW/cm² reading above lamp (31") through the 6mm glass top.
0.32 mW/cm² without glass, same height.
1.22 mW/cm² a reading outdoors on a bright sunny day at mid day for comparison.
As you can see the glass has some influence but not a lot, I don't think, and compared to the sun the readings are a lot less. My exposure times are around 10 mins using Ulano 925WR coated 1+1. I have read on the forums of others using metal halide lamps at 1KW and achieving 20secs. I would be happy with 3 mins but can't get anywhere near that. Is it possible that these UV readings are way down on a commercial unit. Is the hydrponics lamp not producing the same UV power as a lamp specifically made for exposure units at the same 1KW. If anyone has any advice on those readings I would love to hear it, as I just can't figure it out. Thanks a lot.
My exposure times with my 1kw MH light are 20 seconds. I do not, however, use a hydroponics or grow-light. My light is a commercial unit (OLEC).
The engineers at OLEC told me to put my light 85% of the diagonal measurement of the screen away from the glass. That is considerably less than 31". Even though you expressed the thought that the glass is not influencing the time much, as I see it, it is increasing your time by nearly 25%.
Try bringing you light closer (to only 85% of your screens diagonal measurement), and find some glass with no UV filter.
Nick... thanks for the reply. The glass I am using is just ordinary, plate glass, as far as I know it doesn't have any UV filtration in it. The differences in the readings with and without glass amount to around 21%, but would that not be the case with any glass? Any glass is going to have some effect on UV transmission but it wouldn't suggest the difference between 10 mins and 20 secs. I guess what I really need to know is the difference between something like your Olec 1KW and my Hydroponics 1KW in terms of UV output.
As far as the distance is concerned, my screens are 23 x 28 with a diagonal of 32", so at your recommendation of 85% of the diagonal that would mean around 27". Do you think moving my light 4" inches closer from its current 31" would significantly improve my exposure times or should I look for another lamp? Thanks for the advice.
I am ignorant with respect to hydroponics/grow-lights, so I cannot with any authority condemn or recommend them. I do believe I've seen other posts on this board and other boards from screen printers telling us how they use them successfully, but again, that doesn't necessarily speak about the bulb you are using. I have seen other posts telling us of 10+ minute exposure times, but they were for the most part, Halogen light.
It is necessary to specify glass with no UV filter when you buy it. It is common for glass to have a UV filter in it to prevent fading of furniture and carpeting. It may not be necessary to go to the extent that I did on my unit, but what I have is called Starphire. It is tempered, absolutely clear with no seeds or inclusions and no UV filter. My piece with a small hole drilled in it for the vacuum fitting (my light is overhead shining down, not underneath shining upward) was nearly $300.00 USD.
Another item I cannot speak to is the emulsion you are using. I use Murakami TXR (pure photopolymer).
I am very aware that in the industry you will constantly find the recommendation to put your bulb 150% of the diagonal measurement of the screen away from the glass, and at 31" you are not even that far away. Since I felt the engineers at OLEC knew more about their product than I did, I asked them about distance and they were very insistant about the 85% distance.
I'm assuming your bulb is mounted in some type of reflector so that all the light is being directed toward the screen. Since you appear to have the capability to measure the UV light, I suggest you make a grid on paper or some large transparency. Perhaps with the grids lines at 1" spacing, and measure the UV in each and every square on the grid. Move the light 4" closer and measure again. Continue to move the light closer until you "measure" a UV "hot-spot" where the readings are considerably higher in one area than the remainder of the grid. At that point, begin to back the light away until the readings just even out.
Once you've accomplished this, then try exposing a screen with a proper step wedge test and see what you get. It might just be necessary to replace your light bulb with a different type.
Thanks Nick.... very good idea about using a grid to test the output. I will definately give that a try, sounds like a good way to work out the correct distance. Also didn't know about specifying a glass without UV filters, so I will look into that as well. Much appreciated.
Exposure Problem UV Power Output 
