Cyanotype UV lightbox - sharpness problems

mgatrn · Jun 17, 2019

I started cyanotypes using the sun, which gave me nice sharp resolution (making engineering drawing blueprints).

So then I recently bought a big 8 BL tube UV lightbox, thinking that it would be the ultimate...

… how wrong I was.

The paper is fully flat in contact with the glass...

… and I even built a little box, in an attempt to channel the light more perpendicularly through the negative...

… but no cigar.

My prints are all blurry towards the edges (sharpish at the centre, although still not as good as the sun).

My gut says that the problem is with:

too diffuse light (radiating at oblique angles and spreading the shadow vs the crisp shadow produced by the sun)
the wrong UV tubes (dunno what they are, some 20W Chinese BL tubes that came with the unit off eBay)
possibly the glass (too thick at 5mm)

The glass is 115mm above the tubes with the current setup.

Anyone else had this problem with their UV lightbox and solved it?

Should I change the tubes, get a face tanning sun lamp, or try a more directional light source (if such exists)?

Nick

koraks · Jun 17, 2019

Your "chanelling" box will not result in collimated light. You could try a small diameter honeycomb, although you'll lose some light and it will still not be completely collimated. However, I find that for contact prints collimated light is not a necessity. The blurriness you experience likely has other causes than the nature of the light itself. What kind of negatives do you use, and is the emulsion side on the negatives in direct contact with the cyanotype emulsion? How does your pressure/contact frame work; does it really guarantee good contact between negative and print?
FYI I also use an array of BLB tubes at even a smaller distance to the contact frame and get tack sharp prints with all alt processes.

mgatrn · Jun 17, 2019

I'm using plain transparency acetate negatives...which work perfectly in a photo frame out in the sun (sharp and crisp lines).

Absolutely positive negative/paper is flat on glass, I weigh it down with a heavy book.

What distance is your frame from the BLB tubes?

Trying BLB tubes is probably gonna be my next attempt at making this box work.

ced · Jun 17, 2019

Maybe the paper after coating & drying is too buckled that causes contact issues.
Is the neg/pos/paper as someone already asked emulsion to emulsion?
Allow enough time for the pressure to get the air out.

koraks · Jun 17, 2019

mgatrn said:
Absolutely positive negative/paper is flat on glass, I weigh it down with a heavy book.

That doesn't sound good to be honest. I never got good contact between film and negative this way when I started out. With the sun (=highly collimated) you may get away with it, but not with tubes.

> What distance is your frame from the BLB tubes?
Something like 10cm/4". Less than in your setup I think.

Trying BLB tubes is probably gonna be my next attempt at making this box work.

What kind of tubes do you currently use? Anyway, if exposure times are acceptable, the tubes evidently are sufficient.

mgatrn · Jun 17, 2019

i think it might be contact issues now that I think about it.

I use a spongy packing layer in a photo frame when I use the sun. But I haven't been doing this in the lightbox...

... next thing I'll try.

Mine is 11.4cm from the tubes.

mgatrn · Jun 17, 2019

How do get full contact for your prints?

koraks said:
That doesn't sound good to be honest. I never got good contact between film and negative this way when I started out. With the sun (=highly collimated) you may get away with it, but not with tubes.

> What distance is your frame from the BLB tubes?
Something like 10cm/4". Less than in your setup I think.

What kind of tubes do you currently use? Anyway, if exposure times are acceptable, the tubes evidently are sufficient.

koraks · Jun 17, 2019

mgatrn said:
i think it might be contact issues now that I think about it.

I use a spongy packing layer in a photo frame when I use the sun. But I haven't been doing this in the lightbox...

... next thing I'll try.

I think that may make a difference, yes. I've had the same issue when I just started out and I found that a hard/solid backing did not allow for sufficiently good contact between the negative and the print.

My contact frame (a bit of a messy DIY-job) basically consists of what could be described as a heavy-duty picture frame with 5mm hardened glass. The back is a split-back type consisting of a wooden back panel lined with a wood/paper-fiber liner (ca. 5mm; the green stuff that is used underneath laminate flooring). Pressure is exerted on the backside through 8 screws with wingnuts. This setup works quite well in practice even at fairly large sizes of 30x45cm (I constructed it to fit A3-size prints with room to spare) and heavily curling carbon tissue. You can probably get away with the old spring-type contact frames especially with papers/print materials that don't curl much and smaller print sizes.

In any case, some form of pressure combined with a compressible backing material is the most reliable approach in my experience if you don't want to resort to a vacuum frame.

mgatrn · Jun 17, 2019

This is great.
I tried my contact frame in the lightbox and it solved the sharpness issue.
Not quite as good as sunlight, so I'll probably need to dial up the pressure with a frame like yours.

The other issue though now is depth of exposure.
In midday sun, sunny day, exposure time is 2.5 mins...and that produces incredible deep and smooth Prussian blue (esp. after hydrogen peroxide bath)

With the box prints though, I'm seeing graininess in the blues...as if the light isn't penetrating deep enough into the paper.
Is this a bulb wattage issue or just exposure time issue?

I just upped my box exposure to 8mins and saw an improvement.

Now trying 10 mins.

koraks · Jun 17, 2019

I dot recognize graininess as a consequence f exposure; in my experience that is usually related to the chemistry or the original image (I got graininess with digital negatives as the inkjet dots were simply printed by the process - which I didn't like). Perhaps an example/scan would make things a little easier to understand for us.

mgatrn · Jun 17, 2019

ok, I'll take some pics

could you send me a photo of your contact frame by any chance?

koraks · Jun 17, 2019

It's kind of in the 'crude, but effective' category, resulting from lacking woodworking skills and some design flaws/compromises, but here you go:

I opted for 5mm hardened glass so that it could (1) withstand any pressure I could reasonably through at it and (2) it won't throw razor-sharp shards all over the place in case it does break somehow. One of the things that I'm not really happy with is the thin wood strips around the edge holding back the glass plate. These have to resist the pressure of the frame and the construction is just a little too flimsy to do this convincingly. I never got round to upgrading this aspect but I will do so whenever the current construction fails. So far it's holding up though. I wanted to keep these side panels as thin as possible as they would otherwise too much of a shadow, reducing the effectively usable surface.
The red mask is rubylith, which I use to get clean borders around my prints. You can also spot the sheet of clear acetate that I use to separate the print from the negative. This degrades sharpness marginally, but not problematically so.

This is the pressure mechanism; note the two hinging frames with the screws that can be used to locally adjust pressure. This really fits the 'crude, but effective' description - just what it is. Works a treat, but it isn't necessarily elegant. The locks/clips on these hinges are horrible and are candidate for replacement.as you can see, the metal retainer strips bend all the way through, which is unsurprising as all of the pressure is held by these two strips. Major engineering flaw, I confess. Again, it has held up so far.

This shows the green (turned brown due to UV exposure) compressible material. Also note the paper sheet which mostly serves to keep particles from the green stuff from floating all over the place.
Note the tightening screws of which you can see the points; I screwed on rounded bolts which connect with O-rings glued to the pressure back (see previous picture). This prevents the screws from 'eating into' the wood of the back. Works much better than I had hoped.

mgatrn · Jun 17, 2019

that looks impressive
the localized pressure is probably the go

i really need something like that
do you know where you can buy
or could you make me one?
i need A3 like yours

koraks · Jun 18, 2019

Honestly, you don't want me to make you one; you'll curse me forever Then again, if I can do this, so can you. No special tools needed apart from a drill, saw and a screwdriver and all the bits and pieces were from the local hardware store.

mgatrn · Jun 18, 2019

you're probably right, it's just the time/effort is a factor for me right now
one question: how did you do the groove for the glass pane?
did it come pre-routed into the wood?

mgatrn · Jun 18, 2019

it's just window framing right?

koraks · Jun 18, 2019

There is no groove, it's just an L-shaped profile made from two bits of wood screwed to each other at a straight angle with the thinner bit (that faces the light source) overlapping the sturdier bit. The glass rests on that.

The glass is hardened, so it's not window glass exactly but that could be used nonetheless. I'd go for 5mm as thinner glass may give problems with the pressure involved.

mgatrn · Jun 18, 2019

Oh I see, quite simple then.

mgatrn · Jun 18, 2019

Do you think your design, with the localized pressure, would be superior to the standard contact frames with two spring steel strips?

koraks · Jun 18, 2019

I think it depends on the springs used, mainly. If there are something like 4 of them and they are fairly heavy-duty, then I think it is quite similar. However, what I don't like about these steel springs is that when loosening them, they tend to spring back and I don't like the thought of that spring steel inadvertently catching behind my fingernails or something like that. Being as clumsy as I am, that is bound to happen at some point, and if they are heavy-duty springs, it may very well cause injuries. Secondly, I didn't have access to suitable springs of the right size etc. so this design was easier to implement with materials I could obtain at the local hardware store.

Come to think of it, there was another design consideration; this screw-type system makes it very easy to accommodate printing materials of varying thicknesses. I made this frame with the thought in mind that I might at some point use it for solar plates as well, which are much thicker than paper. A spring-loaded system would have exerted much more pressure with a thicker printing material, potentially causing damage to the frame. In addition to this, the screw system compensates for variations in manufacturing tolerances (which are huge due to my ineptitude at wood working).

mgatrn · Jun 18, 2019

So I bought some BLB tubes and am finding it easier to get sharper detail.

Maybe it has a smaller wavelength, and this helps detail.

It isn't quite as good as the sun, but it's close, and I reckon a contact frame could close the gap.

Right now I'm playing with moving the tubes up closer to the paper, effectively increasing power (lacks power).

Emulsion saturation is becoming a factor too. So testing half n half coats of 1x and 2x applications.

I'm finding there are more variables to control with a lightbox than the sun.

mgatrn · Jun 18, 2019

This print was done in the summer midday sun, 2.5 min exposure.

Deep blues and crisp bright whites.

So far unable to replicate in lightbox

mgatrn · Jun 18, 2019

Light box 7mins exposure (left) vs winter sun midday 2.5 mins exposure (right)

Photogjimi · Apr 5, 2021

mgatrn said:
I started cyanotypes using the sun, which gave me nice sharp resolution (making engineering drawing blueprints).

So then I recently bought a big 8 BL tube UV lightbox, thinking that it would be the ultimate...

… how wrong I was.

The paper is fully flat in contact with the glass...

… and I even built a little box, in an attempt to channel the light more perpendicularly through the negative...

… but no cigar.

My prints are all blurry towards the edges (sharpish at the centre, although still not as good as the sun).

My gut says that the problem is with:

too diffuse light (radiating at oblique angles and spreading the shadow vs the crisp shadow produced by the sun)

the wrong UV tubes (dunno what they are, some 20W Chinese BL tubes that came with the unit off eBay)

possibly the glass (too thick at 5mm)

The glass is 115mm above the tubes with the current setup.

Anyone else had this problem with their UV lightbox and solved it?

Should I change the tubes, get a face tanning sun lamp, or try a more directional light source (if such exists)?

Nick

I know Im late to the party on this one... But what is the wavelength of the LED? It should be 365nm. anything higher doesn't "set" the print properly.

Cyanotype UV lightbox - sharpness problems

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