I.R. Metering - Step up to the challenge

[danzig@xxxxxxxxxxxxxx]

Hi :)


The title must be confusing some of you. Let me explain it as 
clearly as can be, it quite simply means,

that I intend to make an IR sensitive meter.

It's irritating not being able to use convinient TTL metering for our IR 
imagery, and modifying our camera's like W.J. pretty much 
relegates that camera body to specifically being used for shooting 
IR. Which is not a good idea financially for a young pro like me.

W.J. once said that he had quite simply, placed a thin red gel filter 
*I think* in front of his Canon EOS 1's internal meter.

A VERY smart idea as this will filter out visible light pretty much 
the way it does on the front of your lens, except that since this is 
your camera meter where this is taking place so the meter will only 
be reading the IR light falling on it for measurements for it's 
metering recommendations.

( Tho I admit I do have several technical reservations which W.J. 
might care to explain, first of which, on the suitability of the meter 
for measuring I.R. as it was designed & optimized for measuring 
available light spectrum & also the fact that I suppose, that the 
measuring device of the meter is not in a convinient place & I doubt 
very much you could easily change that piece of gel filter. Why is 
this a problem you ask ? Well simply because there are several 
kinds of IR filter, like the light red Hoya 25A and Hoya's opaque IR 
filters. So if you use a piece of opaque filter on the meter, you'd be 
getting the correct reading for an opaque filter. If you say used a 
25A filter, you could still use it, but you'd have to know & 
remember how compensate for the extra light, & vice versa if you 
used a 25A on the meter. Since this IR metering system is meant 
to be simple, I do intend it to be able to display different readings 
for the different kinds of filters , on this subject refer to next para. )

Going on the basis on an reflected light meter, I suppose it would 
not be so critical to get the meter to be able to display readings for 
various meters, AS OPPOSED to getting a material for the 
metering ' plate ' which would be suitable for trasmission & 
measuring of IR. Also important is making sure that the meter itself 
is optimized for measuring IR wavelengths.

Assuming the material of the measuring device, & the device itself 
is optimized for IR. All we would have to do is to place the filter over 
the metering ' plate ' so as to account for the light loss, or better 
still compensate by reducing by an appropriate number of stops 
the effective ISO of the film *on this IR meter* to account for light 
loss of the particular filter     

I'm basing this last paragraph on my latest piece of equipment, the 
Minolta 4F lightmeter, it's metering ' ball & plate ' & also the 
recommendations I've recieved over reducing the ISO speed on the 
meter to account for filter factor, as this is a better solution that 
placing the filter over the measuring ' ball ' as this method 
introduces a level of inaccuracy to your data.

Well, I suppose this IR meter will only be a reflected & spotmeter 
type. Since as most of you know, the IR effect recorded on the film 
is mainly due to the IR reflective properties of the object being 
photograped.

Which would make incident metering undesirable & probably just 
plain dumb.

Well, it's time for all us electronic engineers to get our mental 
muscles working ;) advice from fellow engineers or anyone else 
versed in light transmission/measurement techniques  would be 
MUCH appeciated *I forgot what they called those guys who's 
speciality is light, I remember seeing one of those guys on this list 
before*      

C'mon guys, this is for all of us ; )


Yours sincerely,
Ket


A young man who has lived 21 long years and yet still hasn't the 
foggiest idea what life will expose him to him tommorow.    
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