Blue filters and fog

Photo Engineer

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Here are the opposites:

C M Y < Secondary colors or subtractive colors
R G B < Primary colors or additive colors

So, a filter that passes red appears red and a filter that cuts red appears cyan. A filter that passes blue appears blue and a filter that cuts blue appears yellow.

The best blue passing filter is a WR98 (or WR99, sorry OTOMH I have forgotten - sorry). The best red passing filter is a WR70.

There are no sharp cutting dense secondary color filters.

PE
 

Ray Rogers

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Q.G.

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Thanks for the primer in colour theory!
I would say that cyan would qualify as (what did i call it?) "blueish".

But anyway: i dug out my Wratten filter thingy too, and found the filters recommended "to give orthochromatic rendering" using panchromatic film.
The book (and recommendation) is from the days that pan film was still available in types B and C (i.e. pre WWII).

Filter K2 (a.k.a. no. 8) was to be used to produce orthochromatic results using Type B panfilm in daylight.
X1 (no. 11), when using Type B in artifical (incandescent) light.
X1 too for Type C material in daylight, and finally
X2 (no. 13) for Type C in artificial light.

To be fair, the spectral sensitivity curves of no two emulsions are alike, and there is no exact match to be expected. But close enough is close enough.

And i believe it would be fair too to to say that none of these are a nice, single frequency, or even pure frequency band blue.
 

Photo Engineer

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Cyan is "cyanish" and is not related to blue in any way being subtractive, since blue is additive.

In any event, a blue filter, seen in block form, reaches from 400 - 500 nm and passes only blue light, not blue and green. Therefore it is not an "ortho" friendly filter. A green filter reaches from 500 - 600 nm and passes only green light. A red filter passes only red light with a reach from 600 - 700 nm. A blue and green filter superposed passes nothing since they block light that each passes. A cyan filter passes Green and Blue and therefore passes as an Ortho filter.

Look at those spectra closely.

PE
 

jp498

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IR cuts through haze/fog well due to the longer wavelength. blue would be the opposite as you head toward the UV end of the rainbow. The shorter wavelength is more attenuated by haze/fog. Here on the coast of Maine, I'm usually wishing for less fog rather than more.
 

Poohblah

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I have seen some diagrams of exactly that out of a book and they were generalized and misleading.
 

Ray Rogers

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Photo Engineer said:
A blue and green filter superposed passes nothing since they block light that each passes.

PE
Click to expand...

???

A blue filter will actually pass m & c
and green...... will pass both y & c
So, c should get through,
the result being cyan which blocks R
 

Photo Engineer

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Ray;

Thank you. I have Dynamic RAM memory and had missed a refresh cycle.

For totally correct information see page 8 of the Kodak book "Color as Seen and Photographed. I have it right here and still got it wrong. Old age or refresh, IDK.

PE
 

Q.G.

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Ray Rogers said:
???

A blue filter will actually pass m & c
and green...... will pass both y & c
So, c should get through,
the result being cyan which blocks R
Click to expand...

Thinking in opposites?

Yellow and cyan are part of the spectrum. Magenta is not.

Depending on how "blue" a blue filter is, it may pass only wavelengths shorter than a certain cut-off wavelength in the short end of the spectrum.
Magenta, being a mix of the opposite ends of the spectrum, will certainly be lost.

And depending how "green" a green filter is, it will pass only wavelengths lying in a band in the middle of the spectrum.
So whether that includes cyan is questionable. Could be. But could also be not.
 

Q.G.

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Photo Engineer said:
Cyan is "cyanish" and is not related to blue in any way being subtractive, since blue is additive.
Click to expand...

Cyan is a colour found in the region of the spectrum where blue gets 'long' and green is still 'short'.

Blue isn't additive. It's a pure spectral colour (band).

But you could say that cyan, being a mix (it only is in simplified speak, where there is no scale of increasing wavelengths, and blue and green are entirely separate entities) is additive.


You're thinking in tri-colour. Think spectrum!
B&W films aren't tri-colour thingies either.

Photo Engineer said:
In any event, a blue filter, seen in block form, reaches from 400 - 500 nm and passes only blue light, not blue and green. Therefore it is not an "ortho" friendly filter. [...]
Click to expand...

It still depends on how "blue" a blue filter is, how "green" a green filter is, etc.
 

Photo Engineer

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I am thinking spectrum. Contained in that spectrum are the Additive and Subtractive colors which behave differently when viewed by transmission or reflection.

Since we are talking about film here, and not a spectrum, a film is "tri color", even a B&W film.

A sharp cutting Wratten blue filter does not pass green light. And, a blue sensitized film does not see green light. If you add something to the blue sensitive film to see green then it is ortho. How much green it sees depends on the amount of additive present. If you are giving this film a WR blue filtered exposure however, you will not see any change in sensitivity. If you give it a WR green exposure you will see an increase in sensitivity.

Since magenta is a combination of Blue and Red at opposite ends of the spectrum, it falls off the norm and is not easily represented on CIE charts or by other methods of describing color other than by being anti-green. Of course we see it. Just as we see yellow which is a combination of Red and Green.

PE
 

Q.G.

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I'm sorry, but it doesn't make much sense to talk about additive and subtractive colours when discussing what filter filters out what part of the spectrum.
We have seen that it opens easy-to-fall-into trapdoors to do so anyway, in the post that suggested that magenta is passed on by a blue filter.

If you look at spectral sensitivity graphs of B&W emulsions (and you know all this), that are produced by exposing said emulsions to white light turned into spectra by a wedge, the resulting densities (after processing of course) do not show a tri-colour behaviour. Not that the resulting curves are flat. But it's not a discrete, tri-colour affair.
So it's not correct to even suggest that B&W films are tri-colour.

More to the point, however, is that if you compare such spectral sensitivity photos, it's clear to see that panchromatic films react to a part of the spectrum that orthochromatic films do not register (ortho's sensitivity stops just short of 600 nm, while pan emulsions go on to about 680 nm).

All we have to do to get an orthochromatic response out of a panchromatic film is to block that 'extra' part of the spectrum. Is that possible, and do such filters exist? Why, yes of course!

Now we can argue about what colour that filter would be for ages, i'm sure.
But it's not more difficult than that.


Yellow, by the way, is not "a combination of Red and Green"! It's a pure spectral colour.
You're still caught in the wrong frame of reference!
 
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Photo Engineer

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If your eye is stimulated by a pure red and a pure green light, you see yellow. If your eye is stimulated by pure red and pure blue, you see magenta, and etc... These combinations are used in the additive process to produce full color in TV. So, yellow is classed as a mix of two primary colors, of which there are 3, R, G and B. You see shadings of 2 of the secondary or subtractive colors where the primaries exist, namely cyan and yellow. As such, the human eye is a 3 color system as is all photography including B&W.

B&W films can be sensitized by one Panchromatic dye or by 3 Single spectrum dyes. Depending on the purity of the filter, any one region or one small portion of a region can be cut from the image.

I use a spectrosensitometer with and without filters to test each emulsion and can easily demonstrate these to you in the lab right here at home. A good source is the Kodak book "Color as seen and Photographed" and a good text is "Principles of Color Technology" by Billmeyer and Saltzman. Another good text is "An Introduction to Color" by Ralph Evans and an additional text is "Principles of Color Photography" by Evans, Hanson and Brewer.

These were used in the college level course given at EK. I have copies of all 4 referenced materials right here. I also have several on color TV and they all seem to agree on definitions of primary and secondary colors and additive and subtractive colors namely R/G/B and C/M/Y.

I have tried to reproduce their comments here as closely as possible.

PE
 

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I've thought of another illustration of this while looking through the books above.

If you have subtractive pigments and subtractive light images, they behave identically and their absence is required to produce a white while the presence of all 3 are required to produce a black. This is why a subtractive system works well with film. In fact, it works better than an additive system for these very reasons.

An additive system behaves differently. The three additive colors give a white when combined in a lighted format, but yield a black when in pigment form on a reflective support. Therefore they work in a different fashion, depending on medium whereas subtractive colors behave the same regardless of medium.

This was the major departure taken when Dufay was abandoned and both Agfa and Kodak came out with subtractive systems. They appeared brighter and had better color, and could be duplicated as prints.

So thus, you cannot combine a Green and Red pigment and get a Yellow, but you can combine Green and Red light and get a Yellow. This behavior is typical of systems based on primary colors.

Now, films can be sensitized in any way one wishes, but the best results are obtained by sensitizing to primary colors and if it is a color material, then the dyes are secondary (subtractive) dyes for the above reasons. Since this sensitization extends into B&W, one can then see that a B&W material is essentially a tricolor material without the imaging dyes. Use of R/G/B filters removes respectively the portion of the spectrum related to the filter and in the amount based on the density of that filter. Using C/M/Y filters create holes between the primary sensitizations proportional to their respective densities.

So, for example, for a color material, a dim Yellow or Amber light can serve as a safelight taking advantage of a "hole" in the sensitivity of color paper, and a Red safelight can serve in the presence of an orthochromatic material.

However, a bright Red safelight can fog an ortho film. Speed is also a controlling factor in the "removal" of sensitivity.

So, there are arguments within arguments on this topic.

PE
 

Ray Rogers

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Where is a Munsell Man when you need him?

Q.G. said:
Depending on how "blue" a blue filter is, it may pass only wavelengths shorter than a certain cut-off wavelength in the short end of the spectrum.
Magenta, being a mix of the opposite ends of the spectrum, will certainly be lost.
Click to expand...

I suspect a blue which passes no magenta would technically
not be blue any longer but rather... "cyan".

and this goes back to your earlier use of the term "blueish" to describe "cyanish"....

In everyday English, yes, many people do call cyan "blue" but they are just floating on an acceptable slack in accuracy for easy-going
Kirk-like (as opposed to Spock-like) communication.

In a world where the difference is understood and meaningful,
it is incorrect to mix the terms.

You said Magenta is not a part of the spectrum.
Is this true? I mean REALLY true.

I THINK it might be more accurate to say that magenta,
unlike some other colors,
cannot be represented by a single wavelength
nor a single very narrow band of contiguous wavelengths...

Never the less, I wonder how closely this matters, as gold and silver might also be considered not part of the spectrum (?) yet as a photographer, one might need to deal with imaging both magenta and gold....
 
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Sirius Glass

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"cannot be represented by a single wavelength nor a single very narrow band of contiguous wavelengths..." defines a color as not being in the spectrum.

Violet is in the spectrum while Purple falls in the "Line of Purples" in the color gamut and hence Purple is not in the spectrum.

[Reference:
http://en.wikipedia.org/wiki/Color_space
http://en.wikipedia.org/wiki/CIE_1931_color_space
==> see The CIE xy chromaticity diagram and the CIE xyY color space]
​
These terms for characteristics of color has specific technical definitions. If one does not use these definitions, all discussions become incomprehensible.

Steve
 

Ray Rogers

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Sirius Glass said:
"cannot be represented by a single wavelength nor a single very narrow band of contiguous wavelengths..." defines a color as not being in the spectrum.

Steve
Click to expand...

Except that logic leads to the paradoxical situation where removal of what IS there, (ca. 500-530nm ?) creates the presence of something which is still not there, since nothing was "added".

I would prefer to conceptualze it as being there but hidden.

About definitions I think you nailed it.
 

Photo Engineer

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CIE chart

This is the CIE chart used by EK and others for color rendition.

As you can see, it stretches from about 400 - 700 nm with Blue and its shades being considered from 400 - 500 nm, Green from 500 - 600 and Red from 600 - 700. This encompasses yellow, being a mix of Red and Green and the other colors. Magenta is not on this chart but is represented as points on the lower horizontal(ish) axis of the chart.

Since G = -M, this can be represented directly with no problem in Subtractive systems.

PE
 

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Q.G.

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Ray Rogers said:
I suspect a blue which passes no magenta would technically
not be blue any longer but rather... "cyan".

and this goes back to your earlier use of the term "blueish" to describe "cyanish"....
Click to expand...

Nope. Not at all.

What we have to get clear is that blue, cyan, yellow, red, green are all pure spectral colours You can assign a single wavelength to them, or a band of wavelengths you will want to call "blue" etc.

But magenta is not. Magenta does not exist except as a mix of blue and red.

A blue (!) filter will block red (and, as PE points out, every other colour too - Now that is a matter of blue or blueish.). So lacking the red component needed to create magenta, there really is no possibility on earth how the light coming through a blue filter will contain magenta.


We really have to stop thinking in terms of tri-colour!
It may be appropriate when discussing how CRTs and such produce colours, but it is completely out of place, wrong, here.


Ray Rogers said:
You said Magenta is not a part of the spectrum.
Is this true? I mean REALLY true.
Click to expand...

Yes. It is REALLY true.
You can split the entire EM radiation into a spectrum, and nowhere will you encounter magenta.

Ray Rogers said:
I THINK it might be more accurate to say that magenta,
unlike some other colors,
cannot be represented by a single wavelength
nor a single very narrow band of contiguous wavelengths...
Click to expand...

It being REALLY true that magenta is not part of the spectrum, that's obviously not more accurate. There's nothing more accurate.

But it's true: magenta can only exist as a mix of spectral colours.

It cannot, by the way, be represented by a single band of wavelengths as broad that it contains even the entire EM spectrum.
 

Q.G.

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Still that tri-colour thing...!

Yellow is in between red and green. It is not a mix!

You can take away every other bit of the spectrum, including red and green, and yellow will remain unchanged.


We're getting nowhere with this unless we stop believing that all colours in the world exist as a mix of three Real Colours.

B&W film's sensitivity is not to only three Real Colours, but each line, each wavelength, each distinct colour in the spectrum it responds to will produce an image.

Yellow, for instance, has a single wavelength or band of wavelenghts, which will leave it's mark on film. It does not consist of Red and Green, and the effect yellow ligth has on film is not the combined result of red and green.

So please, please: let's forget about this tri-colour thing!
It's a Red herring!
 

Photo Engineer

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A magenta can be represented by a dye or pigment with an absorption peak centered at about 550 nm. A yellow can be represented by a dye or pigment with an absorption peak centered at about 420 nm, and a cyan dye or pigment would have an absorption peak at about 690 nm.

The half band width of these dyes or pigments represent the "purity" or "hue" of the dye.

OTOH, the additive colors represent only dyes or phosphors, not pigments and are represented by transmission peaks at about those same wavelengths.

So, with subtractive systems you think of density absorbing light, but with additive systems you think of transmission of light.

So, while magenta can only exist as a mix of colors, it can exist as a single dye. Yellow also is a mix of colors but exists as a single dye and etc. Look closely at a TV screen and you will see no yellow dots, but you will see R/G/B and black. Yet you see yellow. BTW, some current screens use a yellow emitter to jazz up yellows. George Takai has been the spokesperson for this new type of 4 emitter screen type.

PE
 

Q.G.

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It was.

You're now considering the impression the totality of what is there leaves, vs the constituent parts.

It's not a difficult thing. Just think about "white" light.
Where in the spectrum is that?
Same with magenta.

It's not that it's not there.
It is, but as the sum total of certain bits of the spectrum.
But it is not there as a single constituent in the spectrum.

And that's the important bit to remember when returning to the blue filter and magenta thingy.
A blue filter (pure blue) transmits one part needed to form magenta. But it does not transmit the other bit needed.
So even with the best will in the world, there cannot be magenta in light that came through a blue filter.
 

Q.G.

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O.K. I give up.
I thought we were discussing how a filter (blue, maybe) could be used to approximate, or even equal, the result an orthochromatic film produces.

But you appear to be hell bent on discussing printing techniques and computer screens.
That's some topic drift!

One last try though: what happens, PE, when i put on a Sodium safe light in the dark room while processing panchromatic film?
And why?
 

Photo Engineer

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Actually, I'm trying to show that a magenta does exist. And that R + G = Y.

But a wide blue filter with low absorption could pass green light, but I would not call it a blue filter. As you noted above, we might agree on another color.

PE
 
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