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Re: Infrared Exposure Trick...ery?

Ray,
The theory is the same as it is for regular light. IR is light too, just
invisible. Most meters use silicon sensors. Silicon sensors are
primarily sensitive to IR light. Their peak sensitivity is in the 800nm
to 950nm area, perfect for Kodak IR film. So, stick a #87 filter in
front of the meter and measure. I would not assume all meters are equal
though. Since they are designed primarily for visible light photography,
some may have filters over the sensor to make the sensor have the same
sensitivity as film. (If I were building a meter, that is what I would
do. You don't want to have your measurements influenced by light the
film can't record.) If that is the case, the meter is no longer IR
sensitive and there is nothing you can do with it.

I have a Canon A-1 and meter through the lens with a #87 filter. I use
ASA 8000 or ASA 10000. I have exposures of  1/60 sec @ F/8 in full sun
light off of rocks or buildings or trees. It varies quite a bit of
course, but just to give you an idea. I'm not making any 3 second
exposures, in fact I hardly ever need a tripod when shooting in
sunlight. I develop in HC-110 at 70F for 6 minutes. I find the film to
be sensitive to bromide drag and sometimes get dark streaks coming from
the edges of the film aligned with the sprocket holes. So be careful
with that. Now I roll agitate the tank instead of the normal inversion
method.

Is it really measuring IR? I've metered a fluorescent light bulb with
the #87 filter over the camera and got almost nothing, while even a low
wattage incandescent bulb gives me quite a high reading. The difference?
They are both "white" light, not exactly the same color temp, 60 watt
incandescent bulb versus a 3500 K fluorescent. The 60 watt is much lower
color temp because it has very little blue in it. The peak output of the
60 watt bulb is in the IR though, and the fluorescent bulb has very
little IR. The transmission of the #87 is 0.8% at 740 nm. The equivalent
density for this would be about 2.10 D You can just barely see the
bulb's filament through the filter. You might try taking a TV remote
control and shining it directly into your meter to see if it reads it. I
haven't tried this, so I don't know if the IR LED in the remote control
is powerful enough.

IR is not for taking pictures in the dark, unless you've got an IR light
source like a flash with an IR filter over it. It doesn't record heat.
Virtually everything that it sees is reflecting IR radiation, not
emitting it. Kodak has information on how hot an object would have to be
for it to emit enough near IR radiation for the film to see it. There
are also other sources for this info available from some web sites.

What else can I say, just try it. It works for me.

Dave




Ray Rogers wrote:

> Greetings!
>
> I was reading the posts and I am well,
>
> I am a bit confused, could you perhaps expand upon the
> theory of using the IR filter and a regular light
> meter for measuring IR exposure?
>
> What hangs me up is a faded memory of information that
> said light meters are NOT sensitive to IR...not that
> they are only very weakly sensitive, but that they are
> not sensitive at all....was this faulty information?
>
> What kind of reading do you get when you use the high
> EI and IR filter over the meter?
>
> Does it give you both F stop and shutter speed? or do
> you ignore one and use a constant?
>
> Can you demonstrate that it really is measuring IR and
> not visual light which has seeped in? For example, can
> you measure visually dark, high IR emitting or
> reflecting objects? In other words, can you measure IR
> with this method in the dark?
>
> Thanks!
>
> Ray Rogers
>
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