Twister fries – More RA4 curves, now with a dichroic head

The other day I set up a Durst M305 color enlarger that I temporarily have with me – just to verify it works. Since I had it out anyway, I figured I could make some step wedges with it. I’ve been doing this with LED light sources recently as well. I couldn’t resist the temptation to see what one might get from a dichroic head. It’s still a bit of the gold standard for darkroom color printing, after all.

The recent LED experiment I’m referring to is the one where I tested the 550nm LEDs, most notably. I made some test strips using a Stouffer T3110 step wedge and the individual color channels on my LED heads, and concluded that LED wavelengths do matter. Well, some of the time at least. And to an extent.

One illustration from that blog I’d like to repeat here by means of a starting point for the present writing. Here it is:

These are a* (horizontal axis) and b* (vertical) from Lab* measurements of the color patches for as pure as possible cyan, magenta and yellow. The round dots are the combination of 480nm blue, 550nm green and 680nm red. The crossmarks underneath are from the test wedges made with 450nm blue, 525nm green and 650nm red. While cyan and yellow seem to be pretty much the same, for all intents and purposes, magenta is a different story. Overexposure with the 550nm LEDs just produces more density until magenta dmax is reached. But the 525nm LEDs quickly start to activate the yellow-forming layer as well, resulting in a color shift (crossover) towards red.

Alright, so on that basis, I know that LED colors matter. I already knew/suspected this, but earlier tests, some years ago, were more informal and subjective.

Fast forward to the present – I had this little Durst M305 sitting here, with a good dichroic color head on it, too, and I needed to verify that it works before it (hopefully) goes off to a community darkroom nearby. So I used it to make these here:

Talk about a spoiler! The end result is right there! But what to look for?

First, what did I do? With the LED enlarger, I can simply use any of the LED channels and print a pure (depending on the color) step wedge. A dichroic enlarger like the M305 is a bit of a different story. It uses a subtractive filter system, which means I cannot directly manipulate the exposure of a single layer in the paper. Well, sort of – but the question is how well it works.

To get a feeling for what dichroic filters do, I grabbed some transmission curves of cyan, magenta and yellow dichroic filters from an Edmund Optics datasheet. I don’t know if they are an exact match with the kind of filters Durst used on the M305, but the pattern must at least be very similar. I put these curves together in a single plot, showing the amount of light each filter transmits / allows to pass through on the vertical axis (100% at the top of the graph) and the wavelength of the light on the horizontal axis.

Edmund Optics dichroic filters cyan, magenta and yellow, spectral transmission. Source

In words: the yellow filter allows all light to pass up to about 500nm – so all blue light – while green, orange and red are blocked. The magenta filter passes all light up to about 480nm (which is an ‘ice blue’ or cyan-like blue) as well as all light from about 600nm upwards, which corresponds with yellow, orange and red. The cyan filter passes all light up to about 580nm (yellow) as well as some light in the near-infrared spectrum around 780nm.

Since I couldn’t directly expose the paper to the primary colors red, green and blue with the dichroic enlarger, I used a simple workaround using its dichroic filters. To expose the cyan-forming/red-sensitive layer, I dialed up the filter knobs for yellow and magenta all the way up as far as they would go, meaning I blocked out all blue and green light, leaving only the red light. I did the same for magenta-forming/green-sensitive (dial in full yellow and cyan) and for yellow-forming/blue-sensitive (dial in full magenta and cyan).

I’m working on the assumption that in this dichroic head, at full filter values, the filters are in fact entirely blocking the light path. I’ve not taken this particular head apart, but other filter mechanisms I’ve worked on did do this, and I hope/assume that Durst has the same approach.

Then it was a simple case of contact printing the step wedge onto strips of paper and running them through the color processor, resulting in the step wedges shown in the photograph at the top of this page.

Now for the big question: what are the results and in particular, how do they compare to LED exposure? For the comparison with LED, I’ll rely on the ‘newer’ set of 480nm blue, 550nm green and 680nm red, although the blue and red LEDs performed very similarly anyway.

Here are the h/d plots that show the measured densities of the step wedges. I’ve plotted cyan density for the cyan curve, etc. The solid lines are the dichroic head, the dotted lines are LED exposure. The horizontal lines are the dmax I obtained from Fuji for their DPII paper (the paper I did my tests on).

I think Fuji have a different way of defining/measure yellow density than what my i1Pro gives, but I also note that the cyan and magenta dmax end up pretty close to what they apparently should be.

The main thing I see is that the plots are virtually identical. This isn’t particularly surprising, but it’s good to know that dichroic and LED exposure creates virtually the same response in the paper.

Well, sort of. Because there are differences. After all, just look at the strips; here are the dichroic wedges one more time:

And the LED-exposed strips I compared them to:

Note how the cyan strip exposed with the dichroic head has a green border, while the LED-exposed cyan strip is…cyan. The magenta strip exposed with the dichroic head shifts towards red – just like 525nm green LEDs did, but different from the 550nm LEDs which produced a pure magenta without crossing over upon overexposure. Yellow seems to be pretty much the same.

Plotting all this in a* vs b* from Lab* values, it looks like this – again with solid lines denoting dichroic and dotted lines for LED exposure:

The differences and similarities are really clear. What the above means, is that as long as each color layer is exposed up to the level it needs to produce its maximum density, the hues remain quite pure (especially cyan and magenta) and are also practically the same between LED and dichroic exposure.

What’s different is when more exposure is given than needed to produce dmax. With the LEDs I’ve started using recently, this only results in the yellow channel from crossing towards magenta and ultimately cyan upon (massive) overexposure. The 525nm LEDs also produced such an effect, but the 550nm LEDs retain a pure magenta. However, with the dichroic head, all channels cross over heavily (and abruptly) beyond the exposure level needed to reach dmax.

How relevant this seemingly ‘bad’ crossover when using a dichroic head is, remains to be seen. It’s subject matter for a follow-up on this blog, and in fact I’ve already been playing around with some illustrations. Stay tuned!

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