Re: Infrared Lightmeter

[Patrick Magee <pmagee@xxxxxxxxxxxx>]

On Fri, 13 Aug 1999, Leonard Flanagan wrote:

> Hello Pete.  I am not an electronics whiz, so I can tell you what he did 
> only in general terms.  I'll try to get more details for you.  I have 
> occassionally thought about putting together a kit form of this thing, if 
> there would be enough interest.
> 
> Basically what we did was get several data sheets on the little IR sensors 
> that are used to pick up the signal from TV remote controls units, and 
> compared those sensitivity curves with the curve for Kodak HSI.  Found one 
> that matched pretty well.  He mounted it in the end of a project box 
> (plastic -- a mistake -- should be metal -- the prototype is now coated with 
> foil to prevent IR light leakage from above)and connected it to some type 
> simple mysterious electronics that convert the restulting information from 
> the senor to current running to a little meter on the top, which is numbered 
> from 1-6.  Covered the sensor with a piece of IR filter plexiglass material 
> (I am not sure if that was necessary).  On - off switch on the back.
It _is_ necessary unless the sensor has a built in filter. 

> Candidly, it's not perfect, and I haven't been really careful about figuring 
> out what is 18% gray or anything.  The main problem is that the meter 
> responds in typical daylight scenes only over about 1/3 of its range, and so 
> you have to judge carefully to get a reading.  But still, it is definitely 
> much better than guessing. Given the cost of the film, I like having it and 
> use it every time I shoot HSI.  If I can persuade him to make a second 
> generation version, perhaps he can come up with something more sensitive to 
> variations in typical daylight scenes, and of course mounted in metal.  I'll 
> let you know more soon.

A variation of this is on my "must be done by next spring" list. My
version will be microprocessor based with a LCD display, user calibration,
serial port interface for inputting data for film curves, changeable
filters, and a few other goodies for flexibility. I believe I can use
the film curve data and the sensor curve data to improve the accuracy
of the meter. I'm hoping to have it sensitive enough to read directly off
the ground glass of a view camera.

What kind of interest is there in something like this?

Pat

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