Sensitizing film to be panchromatic

[BetterSense]

Of course a long time ago film was only blue- and uv-sensitive. Then as technology marched on, the manufacturers managed to make it sensitive to longer wavelengths.

My first thought as to the reason the film only responded to blue is that shorter wavelengths are higher-energy. So it's just that the film is relatively more sensitive to the blue photons. But if that was the case, it wouldn't really be possible to obtain panchromatic film except by making the film LESS sensitive to blue so the other wavelengths can catch up.

So please explain to me how panchromatic film 'works' and why early films were blue-sensitive. Feel free to get technical.

 

[Neanderman]

So please explain to me how panchromatic film 'works' and why early films were blue-sensitive. Feel free to get technical.

It has to do with color absorbtion. The silver halides are yellow in color, which means they are absorbing the blue light while reflecting back all the other colors. Sensitization to the other colors is done with dyes that absorb the other two colors.

I'm sure PE can provide much more detail.

Ed

 

[AgX]

There are different theories about how the energy transfer from the sensitizing dyes to the halide crystal takes place.

 

[Kirk Keyes]

Sensitizing dyes are added to films to extend the range of color sensitivity from UV-blue on up into the IR, depending on the dye/dyes used.

Film that has been sensitized for panchromatic response is relatively less blue sensitive that an emulsion that is not sensitized. The sensitizing dyes cause the film to be less UV and blue sensitive and move that sensitivity to another color.

James Clerk Maxwell made the first color photograph in 1861 using red, green, and blue filters. I believe he had to use much longer exposures to the red filter then for the green and blue. He did not add any sensitizing dyes, as they were not discovered until about the 1890s, so he had to use the sensitivity that is intrinsic in the silver halide.

Look at a graph of the sensitivity of silver halides to light of different wavelengths. The graph drops precipitously as the wavelength gets longer (i.e. it goes from blue to red). The sensitivity does not really just stop, but it gets extremely unsensitive.

It would be interesting to try Maxwell's experiment and shoot something on ortho with a deep red filter and see how much more time it would take to get an image. There's all sorts of issues with this, like reciprocity, and leakage of the filter, as no filter is 100 pure in color.

 

[Photo Engineer]

Actually, early experimenters used chlorophyll as a red sensitizing dye. IDK when this was discovered offhand. I had all of the data here on that and posted much of it in another thread. Maxwell's experiment worked, but due to an accidental error on his part. Again, I cannot recall the details.

As for silver halide, it is actually UV colored, but of course we cannot see that so pure AgCl, which is mainy UV sensitive, appears white to us. AgBr is yellow and AgI is orange. Each of these has longer and longer blue sensitivity. Adding a magenta colored dye which sensitizes the silver halide will then extend sensitivity into the green and make the emulsion appear red - magenta to us.

Remember that a highly sensitized emulsion appears visually to have the opposite color of the light it is sensitive to, so a blue sensitive emulsion is yellow and a green sensitive emulsion is yellow-magenta or red. A red sensitive emulsion is greenish or yellow + cyan.

Many of the sensitizing dyes belong to a class called cyanines, merocyanines and etc. The longer the unsaturated chain, the longer the wavelength that a dye sensitizes to. See the full page chart of spectra and dye structures in Mees and James for example. This will give a detailed description of what is going on.

PE