View Full Version : UV diffusion vs point source

8-Apr-2017, 07:40
When making platinum/palladium/kallitype prints the sensitizer gets absorbed into the paper fibers. Would extraneous light from a diffusion source 'sneak' into the paper fibers on either side of fine black lines to diminish their acuteness?

Is it something you can't really put a finger on, but when prints made from the same negative with both light sources are compared side by side, there is a quality of greater sharpness to the point source print?

I live in a relatively isolated area and have never seen hand coated prints on cotton rag papers.

8-Apr-2017, 08:08
If good contact is made between the film and paper, no, one will not see a difference between the sharpness of the two types of light sources. There will be other printing characteristics that will differ (printing speed, contrast).

However, I carbon print and I do see a difference with the very thick emulsion I use.

Emmanuel BIGLER
8-Apr-2017, 08:31
Hello from France

The reason for a loss of sharpness when using a non-collimated beam for contact printing is explained by the simple diagram posted below.

My experience with UV photolithography on thin photoresists is that when we use a large diffuse UV source like the one used for printed circuit boards, it is hard to achieve a resolution in contact imaging below about 50 - 100 microns.
With a mask aligner, same UV light (mercury arc lamp, with wavelength emitted at 365, 400 and 435 nm) but highly collimated (equivalent to a point source located far away) we can easily achieve 5 microns of lateral resolution.
But the object used for contact printing with a mask aligner is a chromium mask, the absorbent layer is about 0.1 micron think! Photoresist thickness can be between 0.5 micron and up to 100 microns for certain kinds of very thick photoresist layers used to fabricate micro electro mechanical systems.
Resolution is actually limited by the source size, and various residual gaps between the rigid mask and the rigid substrate, since both are never perfectly plane. The size of the blurred spot is simply proportional to the residual gap and to the source width.

For a photographic contact print with a vacuum press, actually I have no idea of what the residual gaps are, since film and prints are not rigid and are hard pressed against each other. However even if there is no air gap, the minimum effective gap is something like the the sum of the thickness of the absorbing layer in the film and the actual thickness of the sensitive layer in the print.
And there is certainly an effect of light diffusion inside film and print, reducing lateral resolution of in the image, my feeling is that a large diffuse source will probably generate more scattered light than a point source, but I have no quantitative argument for this.

But this blurring effect due to the use of a large diffuse source might not be visible when the final resolution of the whole process in contact imaging is between 50 and 100 microns, the naked eye's resolution limit on a print examined at 10" of distance is about 80 microns.


8-Apr-2017, 09:11
You both bring to mind another issue that may come into play, and that's the 'rough' surface of the paper itself, some with more texture than others. Where a fine black line crosses over a 'valley' in the papers surface, there would be some extraneous light sneaking underneath. Might not be perceptible to the human eye as Emmanuel suggests, but then again...

8-Apr-2017, 09:39
Nice diagram!

Since I print late into the night/morning, I have been known to accidently print a negative upside down (both platinum and carbon printing). The prints look like mush...I increased the size of that gap you mentioned by the thickness of the film (0.007" for most films I believe).

I use BL tubes, a single 750W self-ballasted mercury vapor bulb, and a NuArc (Mercury vapor) for platinum and see no significant (noticable) sharpness difference. However, I see a difference between the BL tubes and the other light sources in the way I carbon print. The mercury vapor light sources are not true point sources. The 750W bulb has a built-in reflector and even the NuArc has a lot of UV bouncing around, however both are much more collimated (if I am using the word properly) than a bank of BL (or BLB) tubes.

With the merc vapor lamps I get more sharpness in my carbon prints than the eye can see at normal viewing distance or with a loupe for that matter. It is not hard to do and I am not compromising other characteristics to achieve that sharpness. But why, one may ask, achieve a level of sharpness that the eye can not see? Because even if it can not be seen, it can be felt. In addition, my carbon prints have a pronouced raised relief...the blacks are a significantly thicker layer of gelatin than the highlights. When a black and a white are next to each other, the difference in thickness creates an additional increase in accutance -- or a feeling of sharpness. A diffuse light source would dull all those borders between whites and blacks.

I only print from camera negatives. I believe that some carbon workers using inkjet printed negatives and use sharpening tools in PhotoShop that can counter the loss of sharpness of banks of BL tubes. And some carbon printers do not wish to have the raised relief, thus use thinner tissues (and thus a reduced 'gap').

8-Apr-2017, 10:19
But why, one may ask, achieve a level of sharpness that the eye can not see? Because even if it can not be seen, it can be felt.

Love that!

8-Apr-2017, 10:45
From Sandy King on the matter;

HID printing units can be adapted from commonly available commercial units. Lamps of 1000 watts put out a lot of light and give very fast printing times, from 2-4 times as fast as fluorescent tubes. There have also been claims that the prints made with such units are sharper and of greater contrast than those made with fluorescent tubes. In my testing of printing units I found that HID lamps print with more contrast than fluorescent tubes. However, the differences were not great and with most processes there are controls that would allow us to equalize contrast. As for sharpness, when making small prints where it was possible to maintain very good contact between the exposing negative and printing paper during printing I could see no difference in sharpness, regardless of whether the exposure was made in a contact printing frame or with a vacuum easel. On the other hand, when making large prints in a contact-printing frame I did find a small difference in sharpness between prints made with the metal halide light and those made with fluorescent tubes, with the advantage to the metal halide unit. My theory is that contacting printing frames in large size are not capable of maintaining tight contact between the negative and printing paper. This lack of close contact leads to a greater scattering of the light (and loss of sharpness) with a diffuse printing source — where many of the lights rays pass through the negative at very low angles — than with a semi-collimated unit — where the rays pass through the negative at relatively high angles.


Greg Davis
8-Apr-2017, 12:36
Theoretically you could make a print on a sunny day and another on an overcast day and compare.

Mark Sawyer
8-Apr-2017, 13:22
When making platinum/palladium/kallitype prints the sensitizer gets absorbed into the paper fibers. Would extraneous light from a diffusion source 'sneak' into the paper fibers on either side of fine black lines to diminish their acuteness?

Yes, light from a diffused source would bounce around into nearby paper fibers a little after entering the paper, but so would light from a point source. Once light hits the paper, it doesn't matter whether it came from one point or many, it will bounce around a little. But compared to the light hitting the emulsion directly through the negative, it's pretty inconsequential. As Vaughn said, if you have good contact between the negative and the paper, it won't matter.

8-Apr-2017, 14:12
Good points.

As stated earlier I've never held a hand coated platinum or platinum toned kallitype print in my hand, so I've sent a request for paper samples to get a sense of weight/texture.

I don't foresee using heavily textured paper, but if there were 'valleys' of any sort then there would be an effect on fine dark lines with a diffuse source, wouldn't there? Maybe not enough to see easily by the naked eye, but as Vaughn suggests, a point source print of the same negative may 'feel' crisper by comparison?

Jim Jones
8-Apr-2017, 19:00
Years ago when duplicating halftone 135 dpi negatives onto diazo film, we used large metal halide lamps that may have subtended an angle of 30 degrees from a good vacuum table. Intimate contact was essential to record 2% or 98% dots. Draw-down took several minutes on 30x40 inch sheets of film.