Over or Underexposed???

Hi. Can anybody please help me with my delima. I just exposed my first polymer plate (boxcar kf152). I did it with a Stouffer gauge. After washing/rinsing the plate, I got a solid step 11, meaning the number 11 is not visible after the rinse. My exposure time was 4:00 min. According to the data sheet, they recommend 17-18. Now, please excuse my stupidity but how do you use the exposure correction table? So if I had 11, do I need to decrease my exposure time to achieve 17? On the correction table it says if you want to decrease by 4 steps you multiply original exposure by .25. When they said “Original Exposure”, does it mean my time (4 min). So 4 x .25 = 1 min??? Is this right? Oh yeah! I’m a BIGTIME newbie on this one so please help. :(

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Hey blackivory69,

You will see that 17 on your Stouffer gauge is a lot darker so therefore will take longer for the uv light to penetrate through to the polymer underneath it. In order for you to get to step 17 you will need to increase your exposure time. As the instructions that I can see only (http://www.stouffer.net/using21step.htm) go up to increasing by 4 steps i’d say you’re going to want to do another test exposure for 16 mins at least.

The strength of your UV light source will determine how long you’ll need to expose for so although someone may have told you to expose for 4 mins they may have been using a different exposure unit to you hence that why your one is requiring a significantly longer exposure time. I expose my plates for 15 mins. Good luck and don’t be afraid to use a bit of your treasured polymer material to test with. Once you have it dialled in to the right exposure you’ll be able to get consistent results.

Good luck


With photopolymer, unlike other applications, the solid means still standing, not washed out or corrupted.

This needs to be tested against every type or thickness of plate that you use. Each has its own specifications.


OK, you say step 11 is not visible, in other words it is washed out. Then I will assume that step 10 is visible. Referring to the Stouffer website referenced above, if you want to go up 4 steps from step 10, you multiply the exposure time by 4, so your 4 minute exposure is now 4 X 4 minutes or 16 minutes. This should get you up from step 10 to step 14. If you want to go up another two steps to step 16, you have to multiply your new 16 minute time by 2. 16 minutes X 2 is 32 minutes, which is what you need to get to step 16. If you want to get to step 18, you multiply your new 32 minute time by 2, which is 64 minutes to get to step 18.

This is theoretically how it works, and I won’t go into the theory unless you ask me to.

However, I would keep in mind that at long exposure times like 64 minutes, the theory might not hold up exactly, so you might get somewhat different results.

At this point, faced with such long exposure times, I would ask what exactly the light source is that you are using, and whether you can improve it by, for instance, increasing the intensity (like moving the bulbs closer to the plate), or changing the type of bulbs to ones with a better wavelength for exposing plates, or replacing the bulbs if they are old and used (and therefore have les intensity).

To make these tests, all the plate material you have to use is a piece big enough to put the Stouffer scale on, so you won’t have to use much plate material.

Their are various scales that can be used and usually plate manufacturers will provide the correct range for their plates for most of them. In most cases, it does not matter what you are using, what matters is that you get to the right configuration. You can’t do this by math, waste of time, only by testing. Even with the best platemaking machine on the planet you still have to test as bulbs wane over time and throw off the exposure rate.

Note that exposure times do have a certain amount of latitude but not by much, maybe a step one direction or the other.

Oh, also note that standards did change a while back. I have an old Stouffer scale that is a step below current.


We perhaps could agree that math will get you close. If the scale is good for anything at all, it is good as a guide to getting close to the correct exposure. From there, you are correct, you need to test with the type of work you hope to do, small lines and detail vs. big chunks of wide-line images.

I’ve been using the image that I made together with the Stouffer that gave me the step 11 and it’s doing fine. In fact I’m happy with it. But the data sheet (Boxcar) said 17-18. Am I gonna have a problem with this one in the long run? The shoulders are holding so far.
I will try to expose another plate to 15 min when I get a chance. How about the post exposure? I’m doing 10 min right now. Should I adjust it also? Higher? Lower?


The specs will only get you so far.

I just spent the last several days hunting down a nightware of a wiring problem, changing out ballasts, even ordering new factory spec tombstones (sockets) for Saturday next delivery at $120 for shipping. Plus, as long as I was at it, putting in a new brush and all new bulbs. Frickin expensive. But I had half a dozen clients desperately waiting for plates this weekend. Finished repairs at noon, did my tests most of the early afternoon. Started pumping out the plates, well, most of them. Too fried to get out the last batch, but that was okay with the client.

Upshot, since everything was changed, I went back to factory specs. That will get you just so far. Every machine or set up is going to have its unique hot spot, and for differing plates and thicknesses. You will have to adjust exposure and washout until you get what you need. It never ends up at spec.

If you are happy with what you got, stay there.

When I told my postal carrier on Friday afternoon about the next day delivery thing he said Chicago is shut down, over 500 flights have been canceled. I told him it was coming via UPS. The look on his face was worth it. Delivered at 9:30 am.



Post-exposure is generally 1.5 x to 2.0 x the initial exposure. It’s not that big a deal. Dry out times could be though, try to stay close to spec there. Plates can shatter like glass if left in the oven too long.