Technical 18% Kodak Grey card conversion question

[tom_jenner1]

This is kind of technical and I didn't know where to post this. I wish to

convert an 18% Kodak grey card reading (iso, shutter, f stop) to

umol/meter^2/sec-1 equivelent. Any clues how?

 

I wish to use my camera (Nikon D70s) as a three chanel optical spectrum

analyzer (daylight white balance) to measure light levels with the above

conversion for growing plants using LEDs.

 

If you look here at

 

 

you can see what I'm trying to do.

[ellis_vener_photography]

Kodak 18% gray cards are not designed for color accuracy just reflective luminosity so it is the wrong tool for the job you want to do.

[tom_jenner1]

Don't people also use the grey card to set white balance since it's color neutral.

[barry_kenstler1]

Tom,

 

I'm not sure how to go about the conversion, but regarding Kodak's gray card, Kodak's technical data for the card shows a fairly even reflectance across the visible spectrum. I know many people claim it is not color neutral, but I'm one of the dopes who has used it as such. So, what's the real story?

 

Oh, by the way, cool experiment you've got going.

[tom_jenner1]

Yeah, I think I'll contact Kodak for help. It's a rather tricky question because I'm attempting to make photosynthetic active radiation (PAR) measurements. By looking at the specs of PAR meters and sensors made for PAR measurements, I'm thinking my D70s is going to be more accurate.

 

I could try using the "Sunny 16" rule since full sunlight is 2,000 umol/meter^2/sec-1 and put a grey card next to the plant then meter off that but I'd like to confirm this as a sound testing method. Having a 18% calibrated reference will also allow me to measure the amount of light being reflected from the leaf tissue.

[twmeyer]

"<i>full sunlight is 2,000 umol/meter^2/sec-1</i>" <p>At what time of day, at what latitude, at what time of year?<p>"<i>and put a grey card next to the plant then meter off that... Having a 18% calibrated reference will also allow me to measure the amount of light being reflected from the leaf tissue.</i>".<p>Light reflected from the leaf onto an adjacent "grey" card will be green. ish.<p>Empirical is empirical, dude, and you're working in a closed loop. What's your control? <p>Definitely contact Kodak for help. And let us know how <i>that</i> goes... t

[tom_jenner1]

Thanks for the response, Tom.

 

The 2000 umol number I believe is for 30 degrees lat, equinox, noon. I've also heard 2,200 umol so I'm trying to figure this out.

 

I believe it's actually an open loop test. There's no feedback process control going on.

 

Yeah, I've wondered about reflections off the leaf material. My idea was to measure the leaf, move it out of the measuring area then take a grey card measurement.

 

I've been having a hard time finding an answer. "Ask a physicist" didn't know what a 18% grey card was. I think what I need to do is find a photobiologist who also understands cameras.

 

Yes, I need empiricism.

 

Thanks for wishing me luck with Kodak, I'll probably need it!