Ilford B&W films - spectral sensitivity querie

[martian bachelor]

I've been looking at spectral sensitivity curves of various films in

manufacturers' publications and am wondering if anyone knows what

Ilford means exactly when they say "SPECTRAL SENSITIVITY - Wedge

spectrogram to tungsten light (2850K)" in, e.g., their data for <A

HREF="http://www.ilford.com/html/us_english/pdf/FP4Plus.pdf">FP4+</A>

or <A

HREF="http://www.ilford.com/html/us_english/pdf/Pan_F_Plus.pdf">Pan-

F+</A>.<P>

 

I thought film sensitivity measurements factored out the composition

of the light being used to affect the measurement, since what one is

looking for is a property of the film which should be independent of

how it's determined. So why does Ilford mention it? If I want just

the film's spectral sensitivity do I need to factor <I>out</I> 2850K

tungsten light from the data they provide? (-in which case they

should IMO label it a "response" and not a "sensitivity"...)<BR>

Opinions?<P>

[patrick_gainer]

I don't understand your question. The spectrum in the general sense includes the entire spectrum, of which the spectrum of a 2800 K lamp is part. The Wedge spectrograph shows the relative sensitivity of the film to each element of the total spectrum when it is exposed to a uniform spectral distribution of light (as near as is experimentally possible). It should at least include the spectrum of sunlight.

 

Are you expecting to do the mathematics to use the wedge spectrogram to find the effective film speed when it is exposed to a partial spectrum, such as a 2800 K lamp? Many if not all modern light meters have a spectral response close enough to that of panchromatic film so that readings need not be corrected when incandescent lamps are used. If you are using orthochromatic film, readings will have to be adjusted because the film does not see all of the spectrum that the meter sees. In such cases it is possible to filter the light entering the meter's sensor to match the response of the film you are using unless the part of the spectrum you want to use goes beyond that to which the meter responds.

 

I am really trying to help, not ridicule. If I seem pedantic it's because I don't know how much of what I have said you already know.

[martian bachelor]

I suppose another way of saying it is that I'm looking for what the film's response to an equal-energy illuminant spectrum would be.

 

When I look at the curves that I linked to above I see quite a lot of red sensitivity (or response). I'm wondering if that is native to the film or is an artifact of the 2850K tungsten light used.

[jan_brittenson]

At some point you'll have to factor in the illuminant to predict how a particular scene will appear. Converting from one temp to another (e.g. 2850K to 5500K) is quite easy to do mentally by just looking at the curve and imagine it tilting (e.g. towards the red). Keep in mind the information in the data sheets is for photographers, not scientists. It doesn't need to be accurate to the Nth digit to be immensely useful.

[kirk_keyes]

WHat they mean by spectral sensitivity at 2850K is exactly that - the graph shows the repsonse to 2850K light. The plotted line shows relative density (note the y-axis only says "sensitivity" so we don't really know what units they are showing) at a given wavelength.

 

But it all depends on how one wants to prepare the chart. If Ilford had wanted to factor out the 2850K light source, they could have done that. But they just chose not to.

 

The data they present shows the response of the film when exposed to that specified light source. The spectral sensitivity of those films would be different for any other light source, i.e. at 5500K, they would have much more response in the blue and much less in the red.

 

"I thought film sensitivity measurements factored out the composition of the light being used to affect the measurement, since what one is looking for is a property of the film which should be independent of how it's determined."

 

Wouldn't that be nice...

 

So if you are trying to generate a curve based on equal energy, yes, you do need to factor out the energy of the 2850K light.

 

Are you trying to make predictions for other light sources?

[kirk_keyes]

PS - I bought a spherical difraction grating mirror recently so I can build myself a recording spectrograph. The mirror and brass mounting wieghs about 20 lbs - I think it came out of a emission spectrometer from the 50s or 60s.

 

Seems like a fun project!

[martian bachelor]

Wow, a recording spectrograph would be cool to have. I've thought about a simple one using a cheap transmission grating, but have too many other demands on the time to even work up a design. Any idea how you'll do calibration?<P>

 

> ...the graph shows the response to 2850K light.<P>

 

Then they should say "response" and not "sensitivity" IMO.<P>

 

> ...note the y-axis only says "sensitivity" so we don't<BR>

> really know what units they are showing.<BR>

 

I hope it's not really density, because then how the film was developed would also factor in...<P>

 

> Are you trying to make predictions for other light sources?<P>

 

Yes, I hardly ever shoot under 2850K tungsten. So I'll make the easy conversion to D65 in spite of the hassle factor.<P>

 

I think I basically just wanted confirmation of what I already suspected. The height of those curves in the red just didn't seem right - though one never knows for sure (ie, the Delta 100 curve is about flat with that light source).<P>

[helenbach]

Compare the Ilford curves - which they label as wedge spectrograms, possibly hinting at being 'uncorrected' along with the shape - with Kodak curves. Kodak give more information, and the curves are given as if they are for equal intensity across the spectrum as far as I know (I'd like to be more certain of that). The blue end of the spectrum shows the difference.

 

Best,

Helen

[kirk_keyes]

Yeah - the recording spectrograph does sound fun! (I know, I'm a geek!) The focal length of the mirror is 998 millimeters so it's not very small, but the entrance slit and the film are on the same side of the box with the mirror on the opposite side which helps with size. It seemed like a good design to get a large enough spectrogram to be useful and the spectrum is linear in wavelength. No extra optics are required other than the the mirror.

 

Do a search on "Rowland Circle" for more info on this design. I actually found instructions in an old Amateur Scientist article from a 1948 issue of Scientific American (you can get all the articles on CD now - and if you look around it's probably not very expensive).

 

As far as calibration, what I envision, is placing the spectrograph under the lightsource and making exposures onto film or paper. For calibrating wavelenght, I figure a laser pointer or better a red and blue LED could be used to make wavelength markers on the film.

 

For measuring response, I plan on scanning the spectrograms into a computer with a calibrated scanner. I think the spectrograms will be a several inches long by about 1/2 inch. Once scanned, I'll use Photoshop to pull a one pixel wide strip of pixels out of the middle of the spectrogram. Once that's done, then I'll pull the pixel values from the image and import them to a spreadsheet for calculations and graphing. I don't need absoute photometric results, just trying to compare materials.

 

I suspect that there are astrophotography software packages that can already do that sort of data reduction thing, but I have not looked too hard at that yet.

 

So that's where I'm interested in going with it...

 

"Then they should say "response" and not "sensitivity" IMO."

 

Yes, I agree.

 

"I hope it's not really density, because then how the film was developed would also factor in..."

 

Well, yes, sort of. The spectral properties of the film should not be changing based on the developer used - at least I don't think it does! Changing the gradient of the film will change the density of the response, but when you have a graph that doesn't have any real units for the y-axis, it really doesn't matter...

 

I suspect that they did merely measure the density of the tested films and then graphed the results against wavelength. That would be the easiest way to do it.

[martian bachelor]

Ok, after some thought, calculations, and even exposing some real film (ie, an experiment!) I've changed my mind and have now convinced myself that Ilford is plotting the film's actual "native" sensitivity. So the wedge spectrogram language is just there to show off (ie, it's superfluous).

 

What happened was I got to looking at the curve for a 2850K blackbody and noticed that it has more than three times the energy at 480nm as at 630nm. I knew tungsten was warm, but had forgotten just how warm. The practical consequence is that relative filter factors for a light or medium yellow filter versus an orange or red filter will be very different under the two possible scenarios.

 

So I digitized the sensitivity curve for the 100 Delta film assuming it did *not* contain any effect from the 2850K tungsten illuminant. I combined that (mathematically) with a UV/Haze filter and D65 illuminant to give a base response (or "normal" ASA you might think of it as). Then I did the same calculations with Wratten filters #4 (light yellow), #15 (deep yellow), #22 (orange), and #25 (red). The relative areas under the respective net curves give the theoretical filter factors for the four filters. In case you're curious, these came out 1/2, 1, 1 3/4, and 2 2/3 stops (rounding to "nice" numbers), which seemed about right.

 

I then applied these filter factors to four test exposures under daylight (had to wait for a sunny day...), developed all together, and scrutinized. Much to my surprise (and delight) the calculated filter factors were nearly right on - enough so that a casual observer would say all four negatives looked the same. If the 2850K light had been influencing things there would have been, I believe, a quite noticeable trend along the sequence, with the deeper filters having needed progressively more exposure than I gave.

 

This suggests the high red sensitivity of FP4+ is real - assuming its curve was determined by the same method as the 100 Delta curve which I actually tested. This is good to know since I've heard Kodak's highly red-sensitive Tech Pan film is being discontinued and there are occasions where I want that feature in a film.