Halftoning

[mdarnton]

Probably the easiest way to do what you want to do is to make your halftone on the computer from a scanned neg, as smithdoor suggests, then print out an enlarged neg on inkjet transparency film and use that to contact print on your photoresist material. A window screen is much too coarse for this--halftone screens were on the order of 150 (low newspaper resolution) to twice that or more for books.

I don't know what you're expecting, but probably getting the high end of that will be a losing battle, and you'll be lucky to make the low end work right. I messed with halftone offset printing in the 70s, and it was real twitchy. I took a class, and that helped. There was a lot to know, and I don't know where you'd get that info these days. Your best bet is to find someone who's actually doing it, today.

[Nathan Potter]

I assume you are considering a UV sensitive photoresist similar to that once made by Shipley Corp. or other suppliers. Not quite infinite contrast but very high contrast and typically spin coated on plates to thicknesses in the range of 0.5 to 10 µm. You need to make sure the resist is compatible with the etchant to be used. Sometimes it is wise to use an adhesion promoter under the resist to avoid premature lifting of the resist mask.

Yes, you need a photomechanical dot original as a contact exposure mask for the photoresist. It must be in contact mode in order to resolve the dot pattern completely and accurately. For fine dot patterns vacuum contact is almost a must.

I used to use Kodalith Ortho film to generate the dot mask by contact exposing the continuous tone original to the Kodalith through a standard dot mask. You can buy dot masks at different dot sizes and pitches depending on the quality of the final image you desire. There are current equivalents to the Kodalith Ortho photomechanical film available from Freestyle Photo.

It may be possible to generate a dot master of the original continuous tone image using only Photoshop then output that dot image to a clear film like Pictorico as long as you don't use a very fine dot pitch. I'm not adept at such PS practice - others here may be.

I may be able to give you more guidance on the photoresist intricacies if I knew more about the material to be etched, the type of resist (novolak resin?) etc.

Nate Potter, Austin TX, Holliston MA.

[BetterSense]

The resist is standard fare photoresist (SPR3622 or similar) and I plan to experiment with materials.

I have Freestyle Arista Ortho Litho film, but I have been told that when using it for this purpose that one is supposed to use a special developer. Is that true or will Dektol work?

Contact printing through a dot screen sounds easy enough but where to get the dot screens?

[Andrew O'Neill]

I was formally trained as a printmaker, back in the 80's. Lithography and intaglio. All my halftones were made by a blueprinting company, downtown Victoria. It's just an art store now. Your best bet would be to just make them in PS, and print out on Pictorico or something similar.
Here is an example of halftone image I whipped up in a matter of minutes. If I were doing intaglio today, I'd contact print it onto a plate that had been treated with ammonium dichromate sensitized gelatine, and expose under a UV source. Areas that receive UV will harden. Areas that do not will wash away in a warm water bath. Slide the plate in the acid bath and etch.

[Nathan Potter]

OK, a bit more information. I'm not familiar with SPR3622 but if std. fare then it is likely a positive acting novolak resin with azo sensitizer.

Arista Ortho Litho film should work but only with a high contrast litho developer. Aristo should be able to supply such.

Try Edmund Scientific for dot screens. You'll obviously need a screen to match the film size of the continuous tone original.

I'm not at home now so don't have access to my old notebooks but I've done what you want to do some years ago so I'll try to summarize.

1. Grab your continuous tone original B&W negative.
2. Place the unexposed litho emulsion side up on the bottom. Place the dot mask on top of the litho film
emulsion side down. Place the continuous tone film emulsion side down on top of the dot mask.
3. Expose the B&W original thru the dot mask and onto the litho film. The exposure time is critical - you'll need
to experiment with exposure and development times.
4. Develop the lith film in lith developer for a time necessary to yield a dot pattern where the size of the dots is
approximately linear with the density of the B&W original. (The low density areas of the original over expose
the lith film thru the dots causing the dots in silver on the lith film to expand such that in this reversal
process highlight areas of the original create larger dots of silver representing highlight areas in the litho
film).
5. Now since I believe you have a positive acting photoresist and you now have a negative acting lith you will
need to produce a copy of the negative lith film by contact printing onto another piece of lith film. For this use
the lith developer again but with exposure and development times that will not change the dimentions of the
dot pattern.
6. Using the second generation lith film with the integral dot pattern you next contact print that positive image
onto the positive acting photoresist pattern. Note some caveats in this part of the process. The litho film dot
density must be high enough to avoid any significant print thru the opaque areas. Dust is a major headache
with photomechanical film processes so try to be dust free as much as possible depending on your tolerance
for such defects. If you use a fine dot pattern then vacuum clamping of original to photoresist is adviseable.
7. Expose the photoresist thru the lith dot mask typically with UV light depending on the sensitizing region of the
resist (typically 365 nm mercury line for azo sensitized novolak resin). Develop the resist image (typically a
solution of sodium hydroxide or equivalent). Again you'll need to work out the exposure and development
times (not trivial).

It can be common with some metals to use an adhesion promoter (HMDS, Hexamethyldisilazane) to enable better adhesion of resist to metal, especially with copper, aluminum. Not so with titanium and chromium.

Hope this is a bit of help.

Nate Potter, Austin TX., Holliston MA.

[marfa boomboom tx]

Search for aquatint screen...

[invisibleflash]

OP, that stuff is all dead. Digital killed it.

Back in the 70's we used glass or film halftone screens with the process camera. The glass screens were above the camera and rolled down. (I let one fly by accident and the screen got cracked. ) Film screens had to be used with a vacuum back or it took on a moire.

http://graphicdesign.spokanefalls.ed...copycamera.gif

I'd keep an eyey out on ebay if any screens come up. If you want sharp dots go with litho film and litho developer.

[smithdoor]

I have made halftones back to 70's Today if I need one I just use photoshop there are others I use before I had photoshop they all work great.
Note Kodak made a lot of contact screens for halftone work
I also made my process camera

Dave

OP, that stuff is all dead. Digital killed it.

Back in the 70's we used glass or film halftone screens with the process camera. The glass screens were above the camera and rolled down. (I let one fly by accident and the screen got cracked. ) Film screens had to be used with a vacuum back or it took on a moire.

http://graphicdesign.spokanefalls.ed...copycamera.gif

I'd keep an eyey out on ebay if any screens come up. If you want sharp dots go with litho film and litho developer.

[Gary Beasley]

If indeed you can find all the parts to produce a halftone don't confuse a tone screen with a true halftone screen. Tone screens are used to produce a tint in a percentage of area covered, as in a 10% screen makes a light tone and a 50% screen makes a dark tone. You can tell the difference by examining the screen under a loupe, the halftone screen has fuzzy edged dots and the tone screen will have sharp edged dots [not talking of the classic glass screens but the plastic one that take a vacuum]. You can use a tone screen on a photograph but you will merely get posterised images in that tint range instead of a true halftone representation of the image.

[marfa boomboom tx]

If indeed you can find all the parts to produce a halftone don't confuse a tone screen with a true halftone screen. Tone screens are used to produce a tint in a percentage of area covered, as in a 10% screen makes a light tone and a 50% screen makes a dark tone. You can tell the difference by examining the screen under a loupe, the halftone screen has fuzzy edged dots and the tone screen will have sharp edged dots [not talking of the classic glass screens but the plastic one that take a vacuum]. You can use a tone screen on a photograph but you will merely get posterised images in that tint range instead of a true halftone representation of the image.

search: kodak magenta half-tone screen --- that's what this was…