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Non-photo blue (wikipedia.org)
138 points by aoldoni on Nov 21, 2015 | hide | past | web | favorite | 41 comments

A discussion on "why" this is happening:


The reason older photocopiers don't copy blue is very simply that the drum was activated by reflected light from the scanner. The chemical process that creates the "semiconductor" on the drum surface is sensitive to a fair proportion of the light spectrum, but frequently lost sensitivity t the blue portion of the range. Organic photoconductors had a better response to blues but still cut off a portion of the spectrum. Newer Photocopiers (Digital) operate differently and are dependent on the frequency range of the CCD in the scanner, they are usually pretty good at the blues, and frequently have settings that enable Text enhancement, enabling a setting of a contrast point, anything darker than that point is registered as black anything lighter as white.



Back in the late 70's, early 80's, Dungeons and Dragons modules (self contained adventures) used to print all their dungeon maps in light blue because photocopy machines would not pick them up. This was effectively a poor mans protection against piracy.

I had a manual for an assembler (Laser Genius on the C64) that used the opposite approach: the text was black but the pages were a deep red, which would theoretically photocopy to black.

In practice the original was about as much a pain to read as the photocopy.

EDIT: I remember the letters in the copy having a sort of lighter colour halo around them, which made it readable. I figured was due to the blackness of the letters pulling the limited amount of toner towards them...

DRM seems to have a long history of targeting their paying customers with abuse.

And Hewson Consultants did the same in their "interactive video adventure" Avalon circa 1984. The game -for the Sinclair Spectrum- asked for a four digit code printed in non-copy blue that came in the box. Quite a few games would do the same later, like Larry's, monkey island or Elvira, Mistress of the dark, with different approaches, but Avalon's was pure blue over white paper.

Any art or writing instrument store will have gobs or pens and pencils (or lead inserts for mechanical pencils) in this shade of blue if you're interested. As the article states, it's good for sketches that you'll ink or pencil over later, since they will disappear if you scan or photocopy them in high-contrast B&W.

It sort of reminds me of the old graphics method of using RGB(255,0,255) for sections of a sprite that should be transparent, since an alpha channel wasn't an option, because it's such a hot pink and such a precise color it would never naturally appear in artwork.

Somewhat related:


Also I fondly remember Hubertus Bigend's IKB suit…

Apparently this is Blue Man Group blue too.

And my engineering notebooks from the EurekaLabNotebook company use this color for the grid which is a bit disconcerting if you make a copy of the page and the grid disappears and you have only your writing and diagrams.

Interesting, do you have a picture? Most engineering paper has a grid printed only the back so it doesn't show when scanned, but I haven't seen it in blue.

These are the folks I get them from, http://www.eurekalabbook.com/BlueQuadEnlarge.html and a picture of the Blue printed one.

As long as we're talking about shades of blue:


When the article refers to 'graphics-arts cameras', what do they mean?

Is there a spectral notch filter applied to the CCD, or is the article a troll?

"Scanning in black-and-white makes it possible for the non-photo blue still to serve its original purpose, as notes and rough sketching lines can be placed throughout the image being scanned and remain undetected by the scan head."

Any black-and-white scanner should have a spectrally-flat response, picking up blue just as black and white photographs see the sky as darker than white.

It's entirely possible that older lithographic film didn't have much response in the blue, but there's really no way that a modern imaging system won't pick it up.

What am I missing?

Edit: Experiment is the arbiter of truth: I took a picture of the screen with my digital SLR. As expected, every color swatch in the article is blue. Desaturated the RAW image. Looks grey.

The "graphics arts cameras" they are referring to are pre-digital. They looked like this: http://www.forgottenartsupplies.com/?what=artifacts&image_id...

The goal was to compose a layout into a single image.

You created a layout by literally cutting and pasting things onto a board. Then you placed that board in the area at the bottom and took a picture of it that was transferred to film loaded in the top.

You're right that the film was special; but it's the other way around from how you were thinking. The film was not sensitive to red light. To this film, red is "black" and cyan or blue is "white".

Why this was useful:

- You could open the box of film (it came in sheets) in a room that was darkened except for a red bulb, without exposing it.

- You could use overlays of transparent red material (rubylith) to mask things precisely. Even though you could see through to the layer below, the camera would see it as all black.

- And, as the article mentioned you can add notes to the layout with blue pencil and it would be invisible to the transfer. We always called this "non-repro blue" though, as in, the camera wouldn't reproduce it.

And just to add a couple things to your answer:

-- Litho film was also very high contrast so everything pretty much came out black or white. (Photos weren't actually reproduced as greyscale but rather as a set of larger or smaller black dots using a halftone screen. This still applies when things are printed.)

-- Because litho film was sensitive to blue, the non-repro blue writing on the white paper would, like the white itself, be an exposed part of the image. This results in a black area of the negative where silver halide has been turned into metallic silver. This black area would then become white again when the negative was used to create a printing plate.

Kind of strange for the info box to specify a color used in the pre-digital era using a digital sRGB triplet.

Yeah, I expect that's just someone with a mania for Wiki-standardization. It's not a precise shade; any cyan-ish color would do. In practice non-repro pencils and markers varied from sky blue to a rich turquoise.

The article seems confused - it's implying that there is some magic shade of blue that cameras can't see (even today), which is totally wrong. I think that's why someone found it interesting to post here.

Graphic arts film wasn't at all fussy about the shade of blue (as you note) and so while there were expensive non-repro blue markers and pencils, everyone I knew (at the very end of the era of graphic arts cameras) used blue highlighters, so design studios were full of them.

I've stuck with blue as the only highlighter colour I'll ever use, more than 20 years since the original rationale.

Also, used to freak people out scribbling (non-repro) obscenities on a flat that was going to be sent to photo and turned into a newspaper the next day.

Especially since an sRGB triplet only specifies how to perceptually reproduce the color, and film has a different spectral response from the human eye. The dye in non-photo blue probably should actually spectrally be in the blue range rather than having any dye in the red or green range, since it would likely show up on film otherwise.

I believe they are referring to a technology of the ancients where they made thin films of photosensitive chemicals, exposed them to light, then processed them to make images. The chemicals varied in which wavelengths would activate them.

For instance, red would not activate the paper commonly used for black and white prints, hence the red lights in dark rooms.

It is also possible the cameras illuminated the artwork with a light to which the non photo blue ink was transparent.

The magic word here is "orthochromatic". Orthochromatic photo emulsions (the light-sensitive part of film or photo paper) are only sensitive to short wavelengths of light. The first photo emulsions were all orthochromatic, which makes skin look weird. Later we developed Panchromatic film which is equally sensitive to all colors. It replaced ortho in the camera, but ortho continued to be very useful in the darkroom and in compositing because it allows the red safelight and tricks like non-photo blue.

Not necessarily. Orthochromatic ("correct colour"), or ortho, materials were actually improved-spectrum materials that were sensitive well into the yellow-green. Prior to that, film and paper were really only significantly sensitive to blue/ultraviolet or "actinic" light. Getting to panchromatic ("all colours") was indeed significant, but ortho was advanced technology at the time. (And yes, being able to see what you were doing in the process room was a Good Thing™. Also, rubylith for masking.)

Whoa, didn't know that. Very interesting. Thanks!

It's a real thing: in ancient times when I worked on a yearbook staff, we used non-photo blue markers to mark up the physical pages we sent to the publisher.

I don't know how these pre-digital reproduction systems excluded the blue, nor to I know if this system is still in use in the digital era.

The article does mention diddling with the contrast and brightness as well as desaturating it. However, it doesn't give references to digital-based workflows working like this. I associate non-repro blue grids and pens with doing physical paste-up on a light table. I wouldn't think they'd be part of a typical digital flow although someone in that business would know better than I.

[Edit: As someone wrote, the article just seems confused. Yeah, you can adjust a digital B&W image so that a light blue goes away. You can also adjust it so a light yellow or a light anything goes away. Digital sensors do have different wavelength sensitivities but the use of non-repro blue and rubylith were a function of the specific sensitivities or lack thereof of litho film.]

The article was written first in 2005. Back then, digital photography was not so popular. :)

Apparently you remember a different 2005 to me!

I mean that at that time non-digital photography hadn't completely died out. I recall watching a TV show comparing digital and film photography's quality.

When I was younger I worked at a small newspaper which had only recently made the transition from self-publishing the old-fashioned way to laying out the paper with software and sending it off to a publishing company.

The old printing press easily took up half of our little room, and old supplies still littered the shelves and floor. I loved digging through papers (we had copies dating back to the first half of the 20th century) and learning tidbits about the old way of doing things. The rest of the workers had been there 30+ years and would reminisce about late nights spent pasting strips of paper with text and images onto a board, annotating it with a pen that wouldn't show up, taking and developing the photographs, and running the press.

I don't really have anything to contribute, but it's cool to see this pop up on here and learn a little more about it.

totally off topic, but also a lovely song by the band pinback.

I was actually listening to the song and then searched for the name. :)

came here to say this. going to go off and listen to it now

There are certain variety of blue flowers that, when photographed, always come out as pink. Back in the film days (with the delay to process film), it was often the source of a mind trick. "Could've sworn that these flowers were blue…" :)

It apparently has to do with infrared reflectivity and even has a name: Ageratum effect.


Interesting! I thought it had to do with UV reflectivity. Thanks for the link.

they still make and use non photo blue pencils in animation and comics. now a days though they remove the blue layer with photoshop.

I made it my new HN topcolor--not bad.

How much karma do you need to customise it?


If that's correct, you're close(r).

Ah, this is why some government documents need to be signed using blue ink!

It's more likely that they require blue ink so they can distinguish the original from a photocopy.

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