HSL-Array for RNP Digital Negatives

[mkochsch]

The Third, and hopefully final (but you never know), Colour to Density Stepwedge. This one is based on the HSL model which is very similar to the HSB model. HSL is often shown as a "double-cone" where Luminance is the value that runs vertically from 0 to 50 (dark side) and 50 to 100 (light side). The only drawback to this model is that PhotoShop doesn't have a palette option to work in HSL natively. The HSB colour space is so close though that the Luminance and Brightness values can be easily interchanged. The red writing along the edge show the HSL Luminance values with the equivalent HSB values in black ink.
Why is this needed? I noticed on the last array, the HSB-Array, that there were usable colours which started inside the single cone toward the centre. To keep the stepwedge two dimensional it made sense to switch to the HSL double cone model which displays these values on the surface rather the inside. I then proceeded to "skin" the double cone.
(On a side note, if you want to mine even more colours from the HSB array, that is go inside the cone, you just need to starting turning down the saturation variable. It a little hard to imagine but you start creating a cone within the cone.)
I've also made a smaller version of the HSL-Array so it doesn't eat up so much OHP material. The squares are just elongated "compressed" somewhat.
Original tiffs can be downloaded at my website

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[mkochsch]

Don't know what happen there but I can't get the graphics to come up. Here there are.

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[smieglitz]

So lemme see if I have this correct. I printed the array onto Pictorico OHP using an Epson R2400. Then I exposed a sheet of Cranes' Kid Finish white with either cyanotype or VDB emulsion at a standard exposure of 200 units on the NuArc plate burner I use. 200 units appears to give maximum density at step 1 of a Stouffer 21-step wedge for both emulsions on this paper. Here's what I got:

I measured the Hue of the chosen white square (adjacent to a light gray and indicated by arrows on each example) using the info palette and came up with C=62 M=00 Y=100 K=00 for the cyanotype, and C=65 M=00 Y=100 K=00 for the VDB so again very close. I copy/pasted the column (H=120) for each to illustrate the color to be used in the next step.

When I go for the next step and use H=120 and Saturation =100, the CMYK values are slightly different (78,0,100,0). Should I use the Hue + Saturation numbers or the measured square CMYK values?

Here's what the screened negative layer looks like usinh H/S:

This sound right so far?

Joe

 

[mkochsch]

Yep looks good. Your initial HSL Arrays look very similar to my own cyanotypes coming from my UV-BL lights and my R300 printer.
Your colour choice looks fine too. Once you print the first 101 Step you can tweak the choice a little bit if need be.
If you like working in CMYK that should be fine. I prefer using HSB/L first and then using RGB afterward. There are small differences between the colour models but it shouldn't be a problem.
~m

 

[smieglitz]

Well, I tested some more today and then realized I made an error in not recording the Brightness value of the hue I used for the negative. Based on the HSB square I chose, the values should have been H= 120, S=100, and B=95 (which seems light to me but I'll try it). I'll make another negative tomorrow and see if anything changes.

Anyways, here's what I came up with today after applying the color to the test wedge and reprinting a VDB. Then I scanned that print and analyzed it in ChartThrob to get the curve.

There appears to be a little bit of exposure through the densest green area causing slight fog. Perhaps using the B=95 value will correct that. But, if it doesn't, I'm wondering if using a darker value of the hue might be in order? Or perhaps using a Hue/Saturation adjustment layer and the colorize option rather than a filled screen layer?

After looking at the curve awhile I decided to compare it to a couple curves I had for the 2200 printer and VDB. One was from Sandy King and the other was my adaptation. The curve I did with the automated program today looks very similar to the curves from 3 years ago if I rotate it 180. I assume this is because the 2400 curve I'm working on was applied to the positive image while the other was applied to the negative image IIRC.

Oh well. More to think about...

Joe

 

[mkochsch]

Beautiful. Oh you're there my friend...or very close. Me, I might sample the 3% maybe 4% square as my blocking colour. It's always hard to tell just using a web posting but I think H120 S96 B95 green would fix up the toe a bit and get it starting right in the corner -- then recalc my curve. BTW, the orientation of the curve dialog all depends on if you use bytes or per cent when you're curving. I prefer per cent but many like bytes because it's a tiny bit more accurate.
~m

 

[smieglitz]

Testing three, four. Almost there. For the last test I used a darker value of the hue because I was getting slight fog with the brighter color. I think I'll be able to tweak the curve slightly tomorrow to get a bit darker maximum and separate the lightest highlight values slightly.

Using the HSL array and Chart Throb made this a lot easier than it was to do manually a few years ago. Thanks for putting the files and links online.

Joe

 

[smieglitz]

Tweaked the curve today and think I'm there.

Here's my final curve for VDB on Cranes Kid Finish White out of the Epson 2400 on OHP:

Joe

 

[mkochsch]

The highlight is pretty good.
You need a tiny bit less "negative" density at 10-40% (move the curve toward the median) and a tiny bit more "negative" density at 85-95% (nudge the curve a little bit away from the median).
I would also put down a control handle right on the 50% (128 byte) line. It's the vertical line right in the middle of the curve dialog.
~m

 

[smieglitz]

Thanks for the suggestions. I'll give them a try Monday when I'm back at it.