Nu Arc Consistency

[Jim Noel]

I have a recently acquired NuArc which appears to work fine but presents problems with consistency of printing times. I have searched the archives for answers, but came up empty.
I have run a series of tests with Pd and VDB utilizing calibrated step wedges.
When testing 100 units produced a density of 2.17. Later,probably about 30 minutes I printed a negative with significant HL density of 2.01. I printed it at 90 units and it was at least a stop too light. The next print was for 125 units and still a little light in density.
100 units take approximately 10 minutes. I read in an old post that Sandy King's unit produced 500 units in 10 minutes,(This was when he was using a NuArc) .
The bulb is essentially new. Input voltage ranges from 115-118 V on a dedicated line.
Have others had this problem with consistency?
Might it help if I open it up and adjust the integrator?
Will it be more consistent if I can significantly reduce the printing times?

Thanks for your input.

Jim

[Jason Greenberg Motamedi]

Have you cleaned the lens on your integrator? In the past when I have had inconsistency problems I found the lens on the integrator was dirty.
Also, make sure that your integrator is not moving around. On my model it is pretty easy to move the direction of the integrator, which can have a massive effect on printing time.

[ghostcount]

From Sandy King Post #8:

"Of course you can make one unit on the NuArc equal whatever length of time you like, but I personally like to calibrate so that one second of time equals one unit of exposure. If nothing else establishing the rough equivalence makes it easier to exchange exposure data with other people.

How to do it? First, unplug the NuArc and then remove all of the screws on the top. Then, carefully lift the top and move it back out of the way. Be careful when doing this to avoid damage to the power cord of the fan.

You can adjust exposure by two different mechanisms. At the very front of the unit, in the middle, is the light sensor. You can adjust it by moving it up and down so that it sees more or less of the hole that looks down to the bulb, or you can adjust it by opening or closing the filter on the front of the sensor. If you want to adjust to one second of time equal one unit of exposure, first turn on the unit and allow the bulb to warm up for about a minute. Then open (or close) the filter (or adjust the sensor away from or toward the opening to the bulb) until time and units match. There is no point in perfect eqjuivalence so don't obsess too much about perfection in carring out this operation."

[Jim Noel]

Thanks Jason. I am going to open it up tomorrow and the first thing I will do is check for cleanliness before I look into any adjustment.

[Jim Noel]

Thanks Ghostcount. I will look into some adjustment after I check for dirt.

[Michael Mutmansky]

I have had and used a few different older exposure units over the years (Amergraph, Nuarc, FL homebrews, etc), and I found that the integrator in all of them is never very consistent unless the conditions for the calibration are the same from calibration to exposure.

This is because the integrator probably doesn't have the same sensitivity curve as the printed material, but it is close, so most of the times, the error isn't too great.

However, all of the lamps have specific warm-up characteristics that affect this, and I recommend that you make a few exposures to get the unit hot (and the lamps, too) before doing any calibration. Then, don't ever start out the day printing an important print first-up. Make some initial exposures (dummies, possibly), and then once hot and in the same conditions as the calibration, you should get somewhat comparable results.

If, however, you really are getting inconsistent results from one exposure to the next, you probably have a faulty or possibly dirty integrator. That's when you need to start looking at head surgery to see if there is a problem.

I recall there was a vacuum tube part on the older Nuarcs that could lead to integrator problems when it got old (if I remember correctly). I don't know the details for that, because mine was newer and didn't have it, but a friend did, and got a replacement part from Nuarc to do a repair.


---Michael

[Jim Noel]

Thanks Michael. Every insight helps.

[Kimberly Anderson]

Speaking of integrators, my 26-1K does not have an integrator on it that I can see. The exposures are EXTREMELY consistent and I also see the exposure times changing as the lamp warms up...so *something* must be seeing the light as it is getting brighter.

[Michael Mutmansky]

The 26-1K integrator is behind the lamp in the reflector. There's a hole in the reflector, ans the integrator is there. If you take the lid off the top, you will see it right there in the center. Hard to miss.

Every exposure unit I have used will accelerate as the light source warms up. Some will slow down as the exposure increases beyond the warm up phase, too, presumably because of the spectral response.

My point is that you shouldn't do the calibration on a cold lamp. Run it at least a few cycles before calibrating to get more consistent results.


---Michael

[Jim Noel]

I opened up the NuArc this morning and found the integrator so dirty I don't know how any light was getting to the sensor. It is now clean and I will run more tests this afternoon or tomorrow.
For others who may be looking for the integrator it is near the front of the unit, black and about the size of a stack of 5 quarters laying on their side. I was surprised to find no direct light path from the lamp at which it is aimed, but apparently it picks up light from the various openings in the bulb housing.
I am looking forward to more predictable results and thank all who responded for their help.

Jim