Re: Filter/Speed/Meter & DON'T meter using a Nikon Camera

["R. C. Lacovara" <lacovara@xxxxxxxxxxx>]

At 11:55 AM 10/3/00, Rolland Elliott wrote:

>You can still meter though an infrared filter on a Nikon camera, but the 
>meter is not picking up lots IR light, it is picking up traces of other 
>wavelengths, probably in the deep red area of the spectrum. Which exact 
>wavelengths doesn't really matter that much. If it works for you, keep 
>doing it. But don't imply to people that the camera is metering IR light 
>because it isn't.

My experiments with an FE body last night indicated that there was some 
residual sensitivity to IR light from LEDs at 810 and 880 nm, but the light 
sources were pretty bright. Of course, you are entitled to ask me how I 
know an IR source is "bright," but I'll explain more below. In any event, 
the response was no less than three stops below a red LED, more like 4 to 5 
stops. At the moment I'm looking for another FE body to experiment on--my 
girlfriend won't let me take one of hers apart for some reason. I don't 
know why, she has two. She says that she loads one with color, the other 
with B&W, I say she should make up her mind... ;-)

>Given the wide lattitude of IR Black and white film, and the fact that 
>many people bracket their exposures, and the fact that there is a good 
>corrolation between visible light and IR light in outdoor scenes. it does 
>not surprise me that people have had success doing this.

This is almost certainly the explanation for their exposure success. In any 
reasonable sun-lit setting, the IR has to follow, in a general way, the 
level of red light in the scene. We are talking about near-IR, since HIE 
isn't sensitive beyond about 935 nm, I presume. The near-IR wavelengths 
aren't even as "wide" as the visible region.

>Just trying to explain the science behind the art.

This is always fun... ;-) Usually, the poor practitioner is lectured by a 
snobbish academic when a problem occurs... with fine disdain the academic 
explains the problem as a combination of lack of knowledge on the 
practitioner's part coupled with lack of skill. Here, where so many people 
make good exposures using a variety of methods, the theoreticians must 
scratch their heads a bit for an explanation. Nevertheless, a correct 
explanation exists, must explain good exposures and bad, and be 
understandable to people of ordinary intelligence. (OK, well superior 
intelligence, as all IR photographers are, undoubtedly.)

Forgive my levity... this is an interesting topic, and there have been many 
insightful comments made by those of you interested in IR photometry.

>If anyone is attending college or has access to a spectrometer, I'll 
>gladly ship the small IR blocking filter I removed from the Nikon N90s 
>camera I have for testing.  It would be interesting to get a curve of this 
>filter's spectral response. It is 1/4 the size of a penny. For being so 
>small it sure does cause a lot of problems for us IR photographers.

I would be happy to have a look at it. I have some equipment at home, but 
have access to more equipment at the Lunar and Planetary Institute, who do 
a lot of NASA's photographic archiving. One of the guys there is my 
neighbor, who has an interest in IR. If we don't get somewhere with it, we 
can always knock on the door at NASA across the street and try to get 
someone interested enough to scan the filter for us... those guys have 
really nice optical benches, you'd just eat your heart out...

If you'd like to send it, contact me at "lacovara@xxxxxxxxxxx" and we'll 
work out the details.

Now for last night's experiments.

I have a set of 4 diodes, which radiate at 735, 810, 880, and 940 nm. My 
experience with LEDs is that they have a half power bandwidth of about 20 
nm, but I don't know about these diodes. (The diode at 735 nm is actually 
dimly visible, as you might guess.)

I first examined them with a macro attachment to the FE body. The FE 
responds, although feebly, to the diodes at 810 nm and longer. At 940 nm, 
the response is visible by a needle twitch, but not really useful for anything.

My suspicion is that without a blocking filter there would be plenty of IR 
response. Add the IR filter of your choice over the lens, and viola!... but 
first things first...

I then used my girlfriends Gossen LunaPro F. That had more of a measurable 
response to all of the diodes, but the response compared to an ordinary red 
LED was down by 4 to 5 stops, pretty consistently. I suspect the presence 
of a blocking filter, but again, despite my promise to use my smallest ball 
peen hammer, my girlfriend discouraged my desire to take the meter apart.

My Weston Master II, III, and IV showed minor twitches with the diodes. 
Part of this was that the diodes were small and off angle, but I don't 
think the Weston cell likes IR much. There's no IR filter that I can see, 
unless it's in a coating.

I then tried out two Seconic meters. One is my L-396M, the other is my 
Seconic Marine. Neither of them moved a muscle... The Seconics are pretty 
much indifferent to IR. Again, on the L-396M, the cell is visible, has a 
(suspicious) blue-green tint to it, and I wonder if it's not coated to 
reflect IR. I can fool with the L-396, as it's mine and I don't like it 
much, but I can't disassemble my Marine: I would never get it waterproof again.

As I think about this, and consider the replies to this point, I realise 
that unless a meter manufacturer relies on his photosensitive element's 
natural response to reject IR, he absolutely must use some sort of IR 
filter, either an element such as the Nikon, or a coating. I will do more 
homework.

Bob


R. C. Lacovara, Ph. D.				
Electrical Engineer in Computer Science Drag and
Principal Member of Technical Staff

The Charles Stark Draper Laboratory			voice: 281 333 2132
Suite 210
2200 Space Park Drive
Houston, Texas 77058

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