Archived posting to the Leica Users Group, 2006/11/15

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Subject: [Leica] Some IR filtration experiments
From: davison_m at msn.com (MARK DAVISON)
Date: Wed Nov 15 10:45:21 2006

I have been conducting some experiments to understand the issue of IR 
contamination and filtration in digital cameras.  I have posted some results 
at

http://gallery.leica-users.org/v/MarkEDavison/M8infrared/

to show some comparative examples of IR filtration.  I have included a Leica 
M8 shot where the filtration was done by applying a modified Phase One 
profile which is supposed to correct blacks under tungsten light. My 
conclusion is that the software filtration works surprising well on the 
blacks that are IR contaminated, but hardly affects the other contaminated 
colors at all, but you should look and come to your own conclusion.

(Note: this is a cross post.  There is an on-going discussion of software 
profile methods for IR filtration at
http://www.leica-camera-user.com/digital-forum/9178-magenta-work-around-capture-one-workflow.html

or

http://tinyurl.com/y6cjx2

which gives more detail on the origins of the modified profile I used on the 
Leica M8 image.)

Here's a description of the shots:

The scene was shot with incandescent illumination from ordinary lightbulbs. 
The camera white balances were set to 2800 K  except for the Epson R-D1, 
which was set to incandescent. (The Epson R-D1 does not allow you to set 
white balance in Kelvin.)

The first example is the D200, which is very insensitive to IR. The colors 
in the first D200 photograph are a very accurate rendition of the way the 
scene appears to my eye. Take special note of the maroon and green pile 
blankets, the black Leica M lens, and the black pile jacket at the bottom of 
the photograph. The second photograph shows the D200 with IR cut filtration 
(via a Tiffen standard hot mirror filter). There is hardly any visible 
change in the colors. The third photograph is with the D200 and the IR pass 
filter (a Hoya R72), taken at the same exposure as the first two 
photographs. There is no visble IR at all at this exposure.

The photographs continue in sequence for 3 more cameras: the Leica M8, the 
Epson R-D1 and the Nikon D2h. For each camera I show an image with no 
filtration, with IR cut, and IR pass, all at the same exposure. Note how 
much IR is recorded by the M8--it is the most IR sensitive of all the 
cameras. Note also how the IR contamination has completely bleached the 
green out of the green pile blanket, how the maroon blanket has shifted 
color, how there is a purple sheen on the barrel of the Leica lens, and how 
the black pile jacket has turned dark purple. The shot with IR cut 
filtration knocks down the purple sheen on the lens barrel, improves color 
saturation and contrast overall, but doesn't quite return the green pile 
blanket to the correct color. Note also that there was a glowing IR 
reflection from the "black" pile jacket on the bottom of the apple which is 
taken out by the IR filtration.

Similar comments apply to the Nikon D2h, but the infrared sensitivity is 
weaker and the corrections with the IR cut filter look better to my eye.

The Leica M8 shot which has been filtered by application of the profile 
Jamie Roberts supplied does have better blacks in the anodized aluminum 
objects, but the green of the pile blanket at the top has not been restored, 
and in general the colors of the pile fabrics look faded.  More subtly, the 
IR reflection on the bottom of the apple has not been removed.

My point is that IR contamination doesn't just affect synthetic black 
objects and dark anodized aluminum--it contaminates practially all synthetic 
pile fabrics that I can find in my house. So you can't just hunt down dark 
purple things and change their color. (By the way, if you shoot social 
events and students in classrooms in Seattle in the winter, you are going to 
encounter a lot of pile jackets and incandescent light, so this is not some 
obscure rare combination, at least for my use.)

The Tiffen hot mirror filter which I used in these experiments is obviously 
too weak to restore all the colors (especially for the green pile fabric), 
so I have a better UV/IR cut filter on order--a Heliopan 8152.

Some philosophical notes:

I have been using these other cameras for some time now, and I always had 
more trouble getting indoor shots from the R-D1 and D2h to look "right".  
There was some indescernible purpleness about these photographs that 
reminded me of faded advertising posters. In comparison the D200 photographs 
looked rich and vibrant.  Now I understand the source of the problem. I'll 
be using the IR cut filters on the other cameras when the situation 
warrants.  I have also noticed that foliage never looks right in the IR 
sensitive cameras--it's always a funny spring green.  I will wager anything 
that this comes from high IR reflections in plant leaves, even under 
daylight.

One huge difference between a film and digital camera is that the spectral 
sensitivity functions of the digital camera R, G, and B channels are fixed 
forever.  You can't change the spectral sensitivities by simply loading a 
different brand or type of film. Since this is the case, I think it would 
make life easier for serious photographers if digital camera manufacturers 
would measure the spectral sensitivity functions of their cameras and 
publish them, just as Kodak does with their films.  With such a graph you 
can tell at a glance if the camera has significant IR or UV sensitivity, and 
you can pick your lens filters accordingly.






Mark Davison



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