Weston Model 594 Photronic Cell

Background

Light is part of the electromagnetic spectrum and broadly speaking covers the wavelength range of 40 nm to 1,000,000 nm.  This is from short Ultraviolet through long infrared (IR).  Only the narrow band of about 400 to 750 nm (blue to red) is what's called visible light.  Near IR light (about 700 through 1200 nm) can be seen by using a digital camera that has the IR blocking filter removed.  This is sometimes called X-Ray since you can see through some clothes.  When measuring the light output from something that makes light there are two ways.  One is to measure the total power output at all light wavelengths and the other is to measure only the visible light output.  If the light is to be used by people seeing something with their eyes then the visible light output is the important thing.  If a sensor is going to use the output then the light that the sensor can see is the important thing.

There are a number of ways that light can be converted into an electrical signal allowing it's brightness to be measured.  One of the key problems they face is to have a response that matches that of the human eye.  If the spectral response of the sensor does not match that of the eye then the reading may be too high or too low.  When modern Silicon sensors are used to measure light something needs to be done to cut the IR response of the Silicon that's much more sensitive than the eye.  The  filtering that's used on Silicon sensors is a reasonable approximation, but not as good as the Weston 594 Photonic Cell with the Visible Correction (VISCOR) filter..

I think the Weston 594 Photonic Cell is the basis of a number of specifications for visible light and they are still being made today by Huygen (the company is named after Christiana Huygens [Wiki]).  Which is triply fitting since the son of Edward Weston 1850-1936 (309 patents mostly relating to electric meters) named his son Edward Faraday Weston 1878-1971 in honor of Michael Faraday [Wiki].  The patents signed Edward Weston are the fathers and Edward F. Weston the son.  Although the "Weston" light meter was/is a standard of the photographic industry and popularized by Ansel Adams and Edward Weston the photographer used them it was made by Weston Electrical Instrument Co. and mostly due to Edward F. Weston.  That's three different people called Edward Weston.

Description

Prior art Photoelectric Cells did not have a stable light to output conversion factor, they degraded over time so could no be used to make a light measuring instrument.

This is a Selenium based photo cell where each cell only generates a small voltage, but with strong current and so a number of these can be connected in series to get a useful current.  This version (called YY or Yellow Yellow (see the color dots in photo above) has an output of 46 to 58 micro amps at 20 foot candles with a 200 Ohm load.  The (output at 200 fc) / (output at 20 fc) >= 8.6.  The 200 Ohm load was used back in the vacuum tube days, but today a virtual short input amplifier will improve the linearity compared to what you would get with the 200 Ohm load.

There are different filters available for the 594 but I don't yet know which is on this one.

Related Photoelectric Sensors

In the 1920s there was a lot of patent activity related to adding sound to moving pictures typically done using vacuum tube technology.  There was also a lot of work on transmitting images electrically, later called television, again typically using vacuum tube technology.   In both these applicaions speed of repsonse was very important.

Light brightness units

from Wiki Foot-Candle, Exposure Value,

	Foot Candle	Lux (lumen/m2)	Exposure Value (EV)	594
	1	10.764
Sunny Day	15220	163,840	16	163 ma
open Shadow	950	10240	12	0.55 ma
Indoor Art Gallery	60	640	8
Christmas Tree Lights	4	40	4
?	0.23	2.5	0
Indoors Window Light				0.02 ma
Maglite 2 AA Flashlight				0.35 ma

594 Notes:
Sunny Day 13 Mar 2009 39 deg North into ma range of Fluke 87 DMM.
A photographic light meter that reads in EV can be used to measure Lux.  See Table 3 in the Wiki EV article

L [lx] = 2.5 x 2^EV

Reference

Patents

2000642 Photoelectric Device, Anthony H. Lamb (Weston Electrical Instrument Co), May 7, 1935, 136/251 ; 136/244; 250/214.1; 257/42
2016469 Exposure Meter, Edward F. Weston (Weston Electrical Instrument Co), Oct 8, 1935, 356/225 ; 250/237R; 356/228
2123470 Control Device, Anthony H. Lamb (Weston Electrical Instrument Co), Jul 12 1938, 361/173 ; 250/206; 250/214AL; 250/229; 250/554; 315/159; 361/157 -
2244264 Photoelectric Cell, Louis J. Seitz Jr.  (Weston Electrical Instrument Co), Jun 3, 1941, 136/256 - This is an improvement on the Photronic Cell.
2320185 Photoelectric Cell, Anthony H. Lamb (Weston Electrical Instrument Co), May 25 1943, 136/256 ; 257/E31.13 - ridged surface so sensitivity is dependent on the direction of the incoming light.  Why?
2403863 Photoelectric Cell, Anthony H. Lamb (Weston Electrical Instrument Co), Jul 9, 1946, 136/256 -
2425250 Encased Electrical Device, Anthony H. Lamb (Weston Electrical Instrument Co), Aug 1947,
2629039 Selenium Cell and Process for Making Same, George H. Shoemaker (Weston Electrical Instrument Co), Feb 17 1953,

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