Two vs three filters for multigrade printing

[Duggbug]

In the awesome book Way Beyond Monochrome, the discussion about multigrade paper says that you can use a combination of a low-numbered and a high-numbered filter to duplicate any of the intermediate numbered filters (page 305). But the graphs, including also the graphs from Ilford, show that there are three emulsions in the paper, with varying contrast depending on the green or blue color of the light.

It seems to me that if it were possible to get the full range of contrasts using only two filters, that would suggest they would only need two emulsions. Maybe it would be A and C on the chart on page 305, for example.

So my question is, what is the purpose of the third emulsion, and can it be manipulated independently from the other two?

 

[koraks]

It seems to me that if it were possible to get the full range of contrasts using only two filters, that would suggest they would only need two emulsions.

That's no the case.

what is the purpose of the third emulsion

I expect it's to avoid lumpy response curves. The 'handoff' between two emulsions can be a little coarse, involving an intermediate plateau, so a third, intermediate emulsion can be added to obtain a longer linear slope across as wide a range of grades as possible.

can it be manipulated independently from the other two?

It will also be responsive to blue & green light, so no.

 

[bernard_L]

A question I’ve been asking myself. And related:

• Is the third (intermediate) emulsion sensitive to an intermediate wavelength, like 500nm?
• Or, does it have a balanced sensitivity to blue and green?

The answer being hidden in Ilford trade secrets. Ilford multigrade filters work; split grade practitioners seem to be satisfied. So this will remain a rhetoric question.

 

[Milpool]

They give a high level overview here: https://www.ilfordphoto.com/wp/wp-content/uploads/2017/03/Contrast-control-for-Ilford-Multigrade.pdf

Basically, the three emulsion components have the same sensitivity to “blue” and increasing sensitivities into the “green” part of the spectrum.

A question I’ve been asking myself. And related:

• Is the third (intermediate) emulsion sensitive to an intermediate wavelength, like 500nm?
• Or, does it have a balanced sensitivity to blue and green?

The answer being hidden in Ilford trade secrets. Ilford multigrade filters work; split grade practitioners seem to be satisfied. So this will remain a rhetoric question.

 

[MattKing]

The three emulsion components all exhibit the same contrast.
They differ in their sensitivity ("speed") and spectral sensitivity - essentially sensitivity to green, green-blue, or blue light.
One builds print contrast by adjusting the relative proportions of image density laid down by the respective slow, moderate, and higher speed emulsions, and you make that happen by adjusting the relative amounts of green and blue light.

 

[Kilgallb]

They give a high level overview here: https://www.ilfordphoto.com/wp/wp-content/uploads/2017/03/Contrast-control-for-Ilford-Multigrade.pdf

Basically, the three emulsion components have the same sensitivity to “blue” and increasing sensitivities into the “green” part of the spectrum.

Some time ago I got this reply from Ilford. I was asking about what wavelengths for LEDs to do split grade printing.


The maximum green sensitivity of our Multigrade materials is at approximately 530nm, and you should aim for an LED source emitting within about 5nm either side of this.

The blue sensitivity of the material is actually quite broad, and thus the LED wavelength is less critical, provided it is not so long that it is starting to hit the green emulsion.

As a starting point, I would suggest you try an LED emitting somewhere within about 10nm either side of 450nm.

 

[Milpool]

You can get a rough sense of that from the (relative) spectral sensitivity charts for the papers. Sometimes people find they don’t get the absolute maximum range depending on the LEDs but that shouldn’t matter unless for example you regularly need to print at the maximum contrast the paper is capable of.

Some time ago I got this reply from Ilford. I was asking about what wavelengths for LEDs to do split grade printing.


The maximum green sensitivity of our Multigrade materials is at approximately 530nm, and you should aim for an LED source emitting within about 5nm either side of this.

The blue sensitivity of the material is actually quite broad, and thus the LED wavelength is less critical, provided it is not so long that it is starting to hit the green emulsion.

As a starting point, I would suggest you try an LED emitting somewhere within about 10nm either side of 450nm.