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# Thread: Help me understand units of exposure

1. ## Help me understand units of exposure

In my carbon reading I keep reading about people using a Nuarc machines and they list Units of exposure. Does this translate to time or is it solely a Nuarc measurement and cannot be translated for the non nuarc owners out there? I don't have a Nuarc and don't plan on getting one. (Wallet and Wife say no)

2. Check out post #5 from this thread -> Metering UV Exposures...

Greg (gmikol) says, "NuArc integrators just measure the UV output and slow down or speed up the counter. It does not give a calibrated measurement."

I'd like to understand this a little better myself. Namely, can we reliably convert the NuArc output number into mJ/cm²?

The nutshell of that thread I linked to is about using a UV meter, which outputs in mW/cm². If we multiply that number by seconds, we get millijoules/cm², and this is a measurement of total UV energy absorbed.

If the NuArc integrator gives a reliable number that, coupled with the aid of a UV meter, can lead us to a calibrated measurement, then we'd have a much better idea of the energy requirements for exposing carbon tissue.

3. Originally Posted by holmburgers
If the NuArc integrator gives a reliable number that, coupled with the aid of a UV meter, can lead us to a calibrated measurement, then we'd have a much better idea of the energy requirements for exposing carbon tissue.
Or you can expose a Stouffer step wedge and develop it to see how much exposure you are getting. The units, then, become an arbitrary number that you double or halve for every two steps on the wedge that you want to move.

4. Right, but these would be arbitrary numbers, as opposed to an absolute measure of the energy.

5. Originally Posted by holmburgers
Right, but these would be arbitrary numbers, as opposed to an absolute measure of the energy.
I think that's what they are.

In a practical sense, you can use them immediately that way.

If you want to know (at least approximately) how much energy a unit represents, you can work backwards from the characteristics of the material you are exposing.

6. You're right of course, and maybe that's all mark is asking. I'm hoping to know a bit about these NuArc integrator's for my own, ulterior motives...

7. You two are over thinking. I was wondering if a unit was = to a set amount of time.

1 unit= 1 min etc, hour, light year etc.. Looks like that is not the case.

8. A unit is sort of a combination of Time & Intensity.

A photocell monitors how bright the light is. You get more or less real time as the light gets "dimmer" or "brighter".

I spent a lot of time in prepress, where there were many different platemaking and exposure units. I seem to recall them being somewhere near a second, and different machines had different settings, so it wasn't standardized.

But again the time changes with intensity, so you get consistent results.

9. Well what good is an amount of time if the energy output is variable? I thought the whole point of integrators was that they counted up the UV energy that it received, specifically because the output of these light sources is highly variable, which makes a time measurement useless.

edit: In other words, what Bill said.

10. Integrators measure the UV energy received and output the measurement in an arbitrary manner. In other words, the numbers cannot be translated to even another NuArc integrator unit; they're sensor / integrator specific. Nevertheless, it's possible to calibrate the integrators to make their measurements match (where, additionally / optionally, 1 unit = 1 sec.), but that would be an exercise in futility because a.) Bulbs change output with age and voltage (each bulb at its own pace, depending on usage etc. ...) therefore you would need to calibrate often and b.) you actually don't need to know the absolute UV energy at all, why would you?

Regards,
Loris.

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