Help me understand units of exposure

I think it's an interesting question at the very least, and despite that it may appear to be a case of barking up the wrong tree, I think it's garnered undue razzing.

If I can say that my tissue needs X mJ/cm² at normal sensitization; then everytime I encounter a new light-source, a new day where the sun or weather is different, I can take a reading (or several readings throughout my exposure) and multiply that number (mW/cm²) by the number of seconds to get mJ/cm².

How hard is that?!?

Then I have a specific target number that I'm striving for everytime. The sun has become my NuArc!

Assuming outright that this isn't worth investigating, I think, is somewhat presumptuous. I can understand your positionthough; I'm sure it's irritating to see an under-experienced newbie putting the cart before the horse....

Well my horse and I are perfectly happy in this arrangement!

I am sorry if I gave the impression that I was razzing you in any way. Just had a different opinion on the info's usefulness in comparing units of UV energy between different practioneers of carbon printing.

I don't care where you put the horse, but you it seems that you have tried to change horses in mid-steam (going from using the values to compare the exposure times of different practioneers to comparing it to only one's own tissues.)

I can see the usefulness when making one's own tissue. So it sounds like you need a UV meter -- and I guess with such a meter, one would have to assume that the the proportion of the various sections of the UV spectrum remain constant -- if the meter reads UVA, UVB and/or UVC, then hopefully the amount of the UVA (which is what we mainly use to expose with) stays proportionally the same relative to UVB and UVC. Otherwise one is not measuring what actually the material is sensitive to and one might as well use one of the cheap UV "meters" that warns one about sunburning potential -- accruacy will not be needed because one would not be measuring what one is using.

Vaughn

This meter (Solartech Model 5.0) measure UVA/UVB and has a peak sensitivity of 390nm (IIRC).

No Vaugh, you weren't doing the razzing I was referring to...

Ultimately I'm going to try to apply it to my own tissues of course, but trying to get some sense of other people's tissues only seems natural. There will be 2 limits; a tissue that requires the most UV and one that requires the least. I'd like to know what the spread is.

The many questions & concerns perfectly highlight why doing a bit of investigation with a UV meter is needed; at the very least, to find some answers for them.

Last time I take advice from Robert Frost...

Vaughn said:

...my exposure under a 750W Merc Vapor bulb is about 90 minutes ... Howard's is 6 minutes under a 1000W "multi spectrum" lamp

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Graphic arts metal halide arc lamps are specific to the type of material being exposed. Using the wrong lamp can result in long exposure times. The metal halide HID lamps for graphic arts/UV curing have special dopants added to produce light at wavelengths that are optimum for a particular material. Iron-doped lamps emit heavily in the 350-400nM spectrum and may be a good match to dichromated gelatine which responds best to 360-375nM. Indium and gallium doped HID lamps produce most of their output in the 400 - 450nM range and may not be a good choice.

If the mercury lamp you are using is made for general lighting then it is probably doped to produce most of its light in the visible spectrum and as little as possible in the UV.

Holmburgers, I wish you were paying a little more attention to what people say... I have little to add to what I have already written and to what Vaughn elegantly / subtly put in his last post; two simple sentence killed your nonsensical / pseudo-scientific approach I'm afraid. (I'm skeptical on the fact that you can notice that though; see #2 below...)

I would suggest that you:
1. Give some heed / consideration to what has been very clearly written before,
2. Slow down your overcharged left brain and bring some of your sleeping right brain into play, (Believe me, you'll be amazed by the synergy you'll be getting!)
3. I beg you, pour some glop and make a couple of test prints (as little as this will suffice to light the bulb - about what we're trying to explain to you - in your head!) before writing again; at least then you'll have an idea on what you're talking about. (Revisit #1 here...) I'm sorry but you really don't have a clue, and that's clear as crystal...

I won't continue to this foolery anymore; it got unnecessarily personal / ad hominem (and I'm not helping here, I must admit!), w/o providing any useful / meaningful information to others (been there, done that, and got my lesson; I absolutely have no intention to repeat...) - that's the actual purpose of this forum, right?

Anyway, good luck to you!

P.S. For sake of making you happy/comfortable: You did good by purchasing that UV meter, really... (Can't wait to read about your precious results / conclusions that you're going to generously share with us later!)

P.S. #2. Since I was accused of razzing, I thought I would better do justice to that...

See my thread Determining Exposure Time.

cliveh said:

See my thread Determining Exposure Time.

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For those who do not wish to search:

Determining Exposure Time

I love the Brassai method of determining exposure time, as he claimed to time his night shots by smoking cigarettes.
“To gauge my shutter time, I would smoke cigarettes – a Gauloise for a certain light, a Boyard if it was darker.” Do others have a similar method of gauging long exposures? I have sometimes exposed salt prints by the time it takes to make and drink a cup of tea.

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Night exposures certainly have that sort of leeway! And with my long carbon exposures certainly have many minutes leeway where it makes no difference if I expose at 90 minutes or 95 minutes!

I wonder what kind of light he would have smoked a cigarette de chanvre for.

Vaughn

I had one of these in the prepress dept...

http://www.ebay.com/itm/UVP-BLAK-RAY-ultraviolet-Meter-model-j225-j-225-short-wave-uv-meter-/320773849872?pt=LH_DefaultDomain_0&hash=item4aaf9c8310

The way I used it was to turn on the lights in the photopolymer platemaker, a unit with tightly spaced rows of fluorescent bulbs, maybe a dozen 48 inch bulbs. They weren't blacklight, because they put out plenty of regular light, but they were special bulbs that put out a lot of UV. I'd touch the four bumps of the sensor against each bulb and run from one end to the other and watch the needle. If any bulb was dying, you'd never know by the white light because the bulb would look plenty bright, but the needle would take quite a dive.

So your meter, you can use it to make sure your light is performing as it was the last time you used it. Or you can judge the UV of daylight. Or you can go shopping for UV light sources and pick the source that gets you the highest readings.

So even though I always used material with a consistent response to light, I never "calibrated" any device in the shop to a particular setting based on a published speed of a plate or film. I always burned a Stouffer wedge with each plate - and adjusted exposure based on the results on the test strip. Nobody in the shop ever dared make even one plate or film without a step wedge on it.

Bill -- those were probably BL's -- just like the blacklight bulbs (BLB), but without the visible light filter built inside the tube (which makes them a bit more expensive than the BL's).

Vaughn said:

Bill -- those were probably BL's -- just like the blacklight bulbs (BLB), but without the visible light filter built inside the tube (which makes them a bit more expensive than the BL's).

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Sounds right, they burn cooler too. Overall, I think it was a low-priced unit but good enough for the photopolymer plates.

Other exposure units in the shop and platemakers had much better point source lights.

That makes sense -- the energy of the white light would just be absorbed by the tube instead of shining through. I was not satisfied with the softer carbon prints (contrast and sharpness) I got with BL tubes compared to the sort-of point sources -- Merc vapor lamp in my case.

You lost me there Vaughn. How would BL bulbs create a softer print than a point light source like a MH or merc vapor bulb?

BL (and BLB) bulbs are a very diffuse light source. It is like enlargers for silver gelatin printing where a diffusion enlarger will give a less sharp and less contrasty print than a condenser enlarger (everything else being equal).

A carbon emulsion is very thick (relative to silver gelatin) and if you imagine a small dot sitting on the top of a carbon tissue with light from a diffuse light source, the shadow cast by that dot ends up spread over a significant distance by light hitting it at a very shallow angle. See the diagram below (okay...it is a pretty rough drawing!LOL!)

But a point source (or close to it) the dot's shadow is straight down into the carbon emulsion with very little spreading.

I made a palladium print of Friday and accidently printed the neg upside down under a bank of BL tubes. Just the thickness of the film base was enough to make the image mush. Same idea.

Vaughn

I find my Nuarc to vary with the ambient temperature. 10C = fast exposure, high contrast underexposed, 15C = normal, 20C = very slow, low contrast and over exposed. So to get reasonable results I have to use different dichromate percentages and slightly alter exposure according to temperature. Took a while to work out.

I adjusted the spiral thing on the integrator to allow the max light to enter in an attempt to even out its inconsitencies but that made no difference except to how fast it counts. The perspex cover on the integrator has cracks from time and exposure to UV so I have considered removing it or replacing it with something else.

Loris Medici said:

Holmburgers, I wish you were paying a little more attention to what people say...

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Loris, this has gone too far.

I'm sorry that you find me so reprehensible.

"nonsensical / pseudo-scientific" and "...as little as this will suffice to light the bulb - about what we're trying to explain to you - in your head!", plus, "I'm sorry but you really don't have a clue, and that's clear as crystal..." and so on.

You've made all the worst assumptions possible, instead of giving me even the slightest benefit of the doubt.