Murray, I'm thinking you're trying to come up with a way to calibrate your meter. There are probably many ways. If you already have access to a known accurate meter, it's real easy. If not, then you could employ the zone system.
Meter a gray card in direct sunlight and using the sunny f16 rule assign an exposure value. Meter a black card and assign that an exposure value 4 or 5 stops less, depending on the blackness of the card (zone 1 or 0). Meter a white card under the same light and assign that a value 4 or 5 stops greater (zone 9 or 10). Graph the points and draw a calibration curve between the points. The real question I guess is what to measure. With my real simple meters, I was measuring resistance of a CdS cell. I seem to recall it changed resistance at something like 3 1/2 decades per stop. It made for a straight logarithmic curve. Easy. VERY sensitive. With the phototransistor, I don't know what you'd measure. Maybe the voltage across a resistor in series with the transistor? Gene ----- Original Message ----- From: "Uptown Gallery" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Friday, January 17, 2003 5:52 PM Subject: [Cameramakers] Speaking of fluorescent cold light heads , help with luminance question also > Getting uniform lighting over a large area with a homebrew fluorescent > source may take some effort...I have an idea to follow some day. > > I am working on my homebrew spotmeter idea. > > Silicon photodiodes have typical sensitivity rating in mV per uW/cm^2. > > I see that as meaning voltage as a function of optical power density. > > The number of dimensional units involving illumination and luminance and all > the physical variations and unit systems are confusing. > > I've charted luminances vs. EV numbers, shutter speeds and apertures. Now I > need to relate luminance and the above-described photodiode sensitivity. I > have the sensor area for a couple different size sensors if that is needed. > I'm having a hard time figuring out the dimensional analysis (what physical > parameters are missing or needed). > > Any help appreciated. > > Murray > > > --- > Outgoing mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.391 / Virus Database: 222 - Release Date: 9/19/02 > > _______________________________________________ > Cameramakers mailing list > [EMAIL PROTECTED] > http://rmp.opusis.com/mailman/listinfo/cameramakers > _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
