Problem with DMax and pictorico OHP

[periclimenes]

Hi All,

I'm trying to make a palladium print using a digital negative made from pictorico OHP media. In trying to find the proper exposure time using my UV light box, I notice that I can't get the black as printed through the media to get as dark as the black where there is no negative between the light and the paper. That is, the DMax is not nearly as dark when I have to go through the base. Even when I do a very long exposure in test strips, the "black" maxes out at a deep brown, and then begins to bronze. Where there is no negative material, it gets much closer to black.

Is this normal? Can anyone offer any insight/advice?

Thanks.

 

[keithwms]

I think Scott Davis (TheFlyingCamera) on APUG has direct experience with this, I'd ask him if you don't get an answer here.

I am wondering what kind of source you are using- what kind of spectrum it has, and whether the OHP is adequately transmissive in that part of the UV.

 

[periclimenes]

Thanks for the reply. I actually posted this on APUG first, and was told I should post here...

The pictorico definitely transmits the UV light, because I get a nice looking step tablet. It's just that the black isn't black enough.

My light source is a BioRad UV Transilluminator (originally intended to view DNA bands using Gel Electrophoresis). It's UV peaks at 254 nm, which is not-so-great for eyes and skin, but it's the only thing I have.

I have had the same problems in the sun, as well.

 

[keithwms]

I'll take some pictorico to the lab with me and see if I can get a spectrum.

 

[mkochsch]

Sun

Tried using the Sun yet? Overlap some of the pictorico onto your step wedge width-wise and see what the base tests with the DNA lights.
~m

 

[periclimenes]

I have tried to use the sun, and it looks the same as when I use my UV unit.

I gather this is not a common problem?

Tried using the Sun yet? Overlap some of the pictorico onto your step wedge width-wise and see what the base tests with the DNA lights.
~m

 

[Ben Altman]

I'll take some pictorico to the lab with me and see if I can get a spectrum.

That would be pretty interesting. I've noticed the same effect, though it's almost imperceptible with a longish exposure. I use fish-tank tubes, which are at longer wavelengths, but maybe the OHP is cutting into the most effective part of the lights' spectrum even so.

Ben

 

[Katharine Thayer]

Wait a minute. This is probably going to be a stupid question, but I'll ask it anyway because I think sometimes the more interesting discussions come out of such questions. So here goes:

I don't know anything about platinum, and maybe platinum is way different from gum, but in my experience with gum, if the print is properly exposed, the DMax under the border area of the film will naturally be somewhat lighter than the border area outside the film, due simply to the base density of the film, and this is true regardless of the light source (photoflood bulb vs sun, in my case.) I take it this isn't true for platinum?

The inkjet film I use most of the time has a very low base density, so the tonal difference between the area under clear film and the area under no film is slight, but still noticeable. But Pictorico has more base density and attenuates DMax more significantly IME, as do paper negatives. I've sometimes exploited this effect by making two or three stepped borders around an image with different tones depending on what's between the light and the gum: negative and glass, just glass, or nothing at all.

Katharine

 

[keithwms]

I have done the measurement and the base density of pictorico OHP is indeed appreciable in the UV, particularly from about 350nm down.

So, here is my preliminary theory. You can't get enough contrast (between Dmax and Dmin) if your light source has output below the cutoff of the pictorico. If you were dosing most strongly at 350 nm and up, or so, then you might be able to offset the overall exposure (by manipulating your curve) such that you can get truer blacks, but I don't think you can do that if you are dosing with a deeper UV source.

Like I said, preliminary theory!

 

[Ben Altman]

I don't know anything about platinum, and maybe platinum is way different from gum, but in my experience with gum, if the print is properly exposed, the DMax under the border area of the film will naturally be somewhat lighter than the border area outside the film, due simply to the base density of the film, and this is true regardless of the light source (photoflood bulb vs sun, in my case.) I take it this isn't true for platinum?

Katharine

Hi Katherine,

The only bad question is one that isn't voiced...

With platinum - or silver - diginegs the usual workflow is to figure out the minimum exposure that gives a true black through the OHP and then construct and calibrate the process - ie the ink densities needed to get paper white and the desired gradation of inbetween tones, based on always using that exposure.
On my Ziatypes, that "true black" does often seem an almost imperceptible touch less dark than the black outside the OHP, although it may be just the difference in paper texture where the vacuum has mashed the OHP into the surface.
In my case I tend to stop the coating inside the edge of the OHP, so no "step" is printed. I had not thought of using the effect creatively. These possibilities for edge treatments are one of the things I like about hand-coated hybrids.

Ben

 

[Joe Lipka]

Basically, the higher the wavelength of the UV light, the less the opacity of the Pictorico film?

Sandy King did a big study on UV wavelengths available bulbs to be used in alternate processes. I can't remember where it is, offhand, but I bet that would be a help, too.

 

[Ben Altman]

I have done the measurement and the base density of pictorico OHP is indeed appreciable in the UV, particularly from about 350nm down.

Brilliant! And intriguing. Do you have samples of other OHP materials for comparison? Would be pretty interesting. If not perhaps others could send in a sheet of whatever they have? What about the glass of the vacuum or contact frame?

Ben

 

[Ben Altman]

I have done the measurement and the base density of pictorico OHP is indeed appreciable in the UV, particularly from about 350nm down.
....

Like I said, preliminary theory!

Also, thinking about this, the other part of the puzzle would be to expose test strips for Dmax at different wavelengths and see how the process itself responds.

Ben

 

[keithwms]

Yeah, I think the main issue is that if the substrate has a base density that is appreciably high at the UV wavelength of your exposure, then you can't possibly make your ink dense enough to get a good density range in the final print. But if your exposure wavelength is say ~350nm or so then you at least have a shot. I'm not saying it will work optimally, but I think it's clear that wavelengths below 350nm will not work with this substrate.

Anyway, yeah, I'd be willing to take spectra from whatever people send me. Glass, plastic, whatever. Minimum size must be ~1x1 cm. All I have right now is pictorico ohp and normal film. N.b. don't send me unwaxed papers, they are too opaque for me to get a good spectrum! But if there are other transparency materials to test then I can run scans on those.

Ben, indeed, I suppose I could also check absorption values across a test strip / density gradient printed on the material, and from that one could probably back out the kind of curve that is needed to get optimal contrast as a function of the exposure wavelength.

 

[keithwms]

Ben when I read your comment about the contact frame, that leads me to ask the original poster, what kind of glass is it? I am thinking that most normal glasses cut off sharply at 350nm, you need fused quartz to go lower. If you are using plain old window glass then maybe all this is irrelevant and it's your frame that is limiting you, since you are using such a deep UV exposure wavelength. That would explain why you get the same issue with sunlight as well.

Just another stray thought.... try printing without a glass frame and see if that makes any difference...

Basically, the higher the wavelength of the UV light, the less the opacity of the Pictorico film?

Below 350nm the pictorico is no longer transparent.

 

[Katharine Thayer]

Hi Katherine,

The only bad question is one that isn't voiced...

With platinum - or silver - diginegs the usual workflow is to figure out the minimum exposure that gives a true black through the OHP and then construct and calibrate the process - ie the ink densities needed to get paper white and the desired gradation of inbetween tones, based on always using that exposure.
On my Ziatypes, that "true black" does often seem an almost imperceptible touch less dark than the black outside the OHP, although it may be just the difference in paper texture where the vacuum has mashed the OHP into the surface.
In my case I tend to stop the coating inside the edge of the OHP, so no "step" is printed. I had not thought of using the effect creatively. These possibilities for edge treatments are one of the things I like about hand-coated hybrids.

Ben

Wow, I've been trying for half an hour to get back into this thread; it wouldn't let me type. But finally I shut my browser down and opened it again, and here I am, typing.

With gum, in my experience, there's no way to get around the difference in DMax introduced by the base density of the film. You can artificially create a situation where the area under the film border *looks* as dark as the area not covered by film, but the only way to do that is to overexpose the gum and not develop adequately to compensate. If you overexposed the gum, you'd need to develop it longer to adjust for the overexposure, and once you'd got the print tones right, the difference in DMax between the border areas would of course reappear. So there'd be no advantage to overexposing the print just to try to overcome the base density of the film; in the end, one must just accept that the DMax under the film is the darkest print value you can get with that pigment mix, and it's going to be lighter than the darkest print value where there's no film.

As I said sometime back in another thread, with a visual demonstration to illustrate the point, in my experience calibrating for DMax makes no sense for gum, because DMax (the darkest print value the pigment mix will print) reaches its limit even before the number of steps printed and retained reaches a maximum, so in my experience DMax is a necessary, but not sufficient, condition for determining the correct exposure. I prefer to go by the maximum number of steps printed and retained to establish the exposure.

Don't know if that's helpful, but I suppose its just a long way of saying yes, it was a stupid question, because platinum and other metal processes ARE very different from gum. Sorry I blundered in,
Katharine

 

[periclimenes]

After reading Keith's post about the absorption spectrum of pictorico (thanks, btw; i'm a physics guy, too) I decided to see what it would take to get a nice black out of my exposure unit. It took about 80 minutes, but it's possible. 50 minutes is not-quite black, but you can only tell when you compare it to a spot that was outside of the negative. I did a step tablet with a 50 minute exposure and here's what it looks like (note: I don't have access to a scanner, so this is a photo of it)

 

[keithwms]

I don't know, that looks like charcoal grey to me! I think there should be a much smaller difference between the blacks in the pictorico / no-pictorico areas. Also the whites look greyish. Do you agree? Or is it an effect of the photo?

 

[Ben Altman]

Anyway, yeah, I'd be willing to take spectra from whatever people send me. Glass, plastic, whatever. Minimum size must be ~1x1 cm. All I have right now is pictorico ohp and normal film. N.b. don't send me unwaxed papers, they are too opaque for me to get a good spectrum! But if there are other transparency materials to test then I can run scans on those.

All I have is Pictorico, too. Anyone else have sheets of other substrates they would be interested in characterizing? Maybe this is just academic, but you never know what you'll find until you look.

Keith, what is the vertical axis on your plot? Or to be practical, what do you consider to be significant opacity?

Best, Ben

 

[periclimenes]

Keith - Thanks for the reply. I'll scan it at work today and repost.

 

[keithwms]

Keith, what is the vertical axis on your plot? Or to be practical, what do you consider to be significant opacity?

The vertical axis is the absorption. It can be related to extinction coefficient etc. but basically, if you look at the mid visible region, you see that the absorption is quite low, but below 350 it just takes off and at absorption levels higher than ~3 or so my measurement is basically telling me that it's almost completely light blocking. There is a bit of light getting through but not much.

As for what I consider 'significant opacity,' that depends. One might think that it's just an issue of prolonging the exposure to get good blacks. But the underlying issue, with regard to the tone scale, is that at low wavelengths there will probably not be enough difference between the base absorption of the pictorico and the maximum density of the pictorico+ink to get a good, contrasty tone scale. That situation is what I would call 'significant opacity.' In that situation, I allege that it probably won't be possible to get good whites and good blacks with this substrate and inkset. The base density is so high that the contrast in transmission between ink / no ink isn't big enough.

I could be wrong, there could be superduper UV blocking inks that will do the trick. Perhaps a red ink or something like that might be able to do it. But I think my own approach would be to look for a higher-UV exposure unit, because in the fairly deep UV, pictorico is simply rejecting most of the watts you're throwing at your paper.

 

[keithwms]

Periclimenes, quick question, you are using a 254 nm unit, right? So it is a mercury lamp. That being the case, there are is the 365nm i-line, maybe you are getting some contribution from that unless it's really well blocked with a filter. That line might do a much better job dosing the paper for you. Can you check and see what filter is in place....

 

[periclimenes]

Here's a scan

How does it look?

I don't have many details about the exposure unit as far as filters and such. It definitely looks like it could be a series (8) of mercury lamps underneath a bluish glass filter.

 

[keithwms]

Looks better, but I think the tones can still be improved, and your exposure time is pretty scary, considering the source you're using.

Can you remove the bluish filter and see what happens?

N.b. please protect your eyes around your exposure unit! You know that already but I'll say it anyway

 

[periclimenes]

I'll see if I can pop the filter off... Thanks for the tip about eye protection. I wear those orange UV blocker glasses.