Background
594 Photronic Cell
    Photos
    Description
603 Illumination Meter
614 Foot-Candle Meter
615 Illumination Meter
617 Light Meter
650 Photronic Exposure Meter
703 Sight Light Meter
Weston 715 Master Light Meter
756 Foot Candle Meter
819 Cine Exposure Meter
Weston Photographic Analyzer Model 877
SEI Photonic Spot Meter
Light brightness units
Integrating Sphere (separate web & photometers page )
Reference
Photronic Patents
GE 8DW58Y Exposure Meter
Photovolt  200A Foot Candle Meter
Weston Meter Movement
Coils as Resistors
Integrating Sphere
Colorimeter
RCA 923 VT-252 Photocell
RCA 931 Photomultiplier
Incident Light metering
Related
Links

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Background

Light (Wiki: Electromagnetic Radiation) 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 (Wiki).  Near IR (Wiki) 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.  Note that digital cameras have silicon imaging chips that can see near IR light, but they also have a filter that eliminates near IR light so that the overall response of the imaging chip comes close to human eye response so that color photos will look correct.

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 (Wiki: Luminosity, CIE 1931 color space).  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. 

From a 1959 ad for Model 594-YR.  Offered as one off or as matched pairs.  (PS they did not mention the color response matching the human eye.)

For light intensity of 20 foot-candles and max. external circuit resistance of 200 Ohms.  Produces 4.0 (+/-0.3) uA per ft. candle with a linearity factor of 0.83 (+/-0.08).  Linearity is nearly 1.00 into a 3 Ohm load.
(A op-amp would make an ideal dead short load which could drive a volt meter scaled to display foot candles.)

A more modern way to measure light level is to use the current from a photo diode (Wiki)

Weston 594 Photronic Cell

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.  Called a dry disk type photoelectric cell.

An ad in the Dec 1932 issue of Electrical World says that 14 months prior ( Oct. 1931) Weston started advertizing the Photronic cell (and relay).

See:Transactions of the I.E.S.:  New, Color Corrected Photronic® Cells for Accurate Light Measurements by Marlin E. Fogle (R&D Engineer at Weston Electrical Instrument Corp.).
Dr. H.P. Gage of Corning glass developed the blue-green glass filter specifically for the VISCOR filter to remove the excess red and near IR response. 
The other filter is yellow to remove the excess blue response.
Cites a prior article in the same Transactions of the I.E.S., but in XXVII, No. 9, pg 828-835 (1932).
Fig 2 shows the 614 and 756 Illumination Meters.

Photos

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 operational 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.

603 Illumination Meter

This may be the first combined instrument? That's to say a Photonic Cell and a meter so that light could be measured.

Fig 1	Fig 2 This is serial 607	Fig 3 This is serial 607 and comes with a custom calibration. The serial 2410 Model 603 did not have a calibration. see: https://www.westonmeters.info/weston-meter-model-603 The above web page dates the 603 model to 1937, but the calibration certificate here is dated March 15, 1954.
Fig 4 The wire insulation has biodegraded which is probably why the meter does not read light levels.	Fig 5 Inside	Fig 6 Meter Movement While installing new probe wires I checked the photocells and the output was near nothing, so probably a defective probe. Also the meter movement is missing the return spring. so. . . this appears to be a parts mule, i.e. a meter whose good parts were used to restore another meter and is now a collection of bad parts.

Weston 614 Foot-Candle Meter (Reflected Light Meter?)

The 614 and 615 light meters are very similar and I think they both use either the 594 cell or a version of it based on the wording in their description.
"Corrected to Visual Response" on meter face.

The photocell in both the 614 and 615 meters can be rotated up about 90 degrees so that you can hand hold the meter in your right hand using the strap while walking around a room and measure the light intensity falling on a horizontal surface (the photo cell surface should be parallel to the surface being measured.

The 614 does not need a battery.  It only contains the photocell and some wire wound resistors for the range switch.

mentioned in patent:  2050224 Projector for color motion pictures - filed Sep 5, 1933 - so in use then.

Still photographers typically use reflected light metering (Wiki: light meter), i.e. from the camera position looking at the subject.

When the pancake probe of a Bicron 50 radioactive Survey meter is placed on top of the photocell it reads 20,000 counts per minute.
But when the Bakelite (Wiki) lid is closed the reading drops to background count.

Weston 614 Foot Candle Meter	Weston 614 Foot-Candle Meter with photocell rotated up	Inside
Radioactivity measured using Bicron 50 Surveyor & PGM pancake probe. The radioactivity is coming from the photocell and not other parts of the 614. When a sheet of copy paper is placed between the photocell and the probe there is a slight drop in the count.	Without Paper  CPM = 20,500	With Paper  CPM= 17,500 (varies from 17,000 to 20,000)
When Aluminum foil folded twice (4 layers) is interposed the Counts Per Minute drops slightly more than for paper.	Without Aluminum foil CPM = 20,500	With 4 layers of Aluminum foil  CPM= 13,500
When the Bakelite lid (3.9mm thick) is closed no radiation is sensed. One theory is that they use thorium for refraction reduction and uranium for color filtering?	With Bakelite cover closed CPM = 0

Weston 615 Illumination Meter (Incident Light Meter?)

The 615 meter uses a common 9 Volt battery and contains an op amp and associated precision metal film resistors.
The 615 has a white cap on the photocell.
"Corrected to Visual & Cosine Response" on meter face.

Movie photographers typically use incident light metering, i.e. the light meter is at the subject location and measures the light falling on the subject. The Sekonic movie light meter (Wiki) has a white hemisphere on a swivel mount and it is the brand I used with a Bolex H16 16mm camera (Wiki).

Since the Model 614 and 615 have sequential model numbers and one is for still photography and the other is for movie photography, they may be the first light meters intended to measure light for the purpose of exposing film.  But the table inside both models is for light for various tasks like reading, etc.

The Geiger counter reading is only 4,000 counts per minute for this meter, much less than for the 614 above.  Probably because the white diffuser blocks the radiation.
With the Bakelite lid closed only background radiation is measured.

Weston 615 Foot Candle Meter Under the yellow paper is says: Weston Schlumberger Made in U.S.A.

Weston 615 Foot-Candle meter with photocell up and back off

Weston 615 Foot-Candle meter with back off showing 9V battery and zero and another adjustment pots.

Weston 617

Uses two the 594 cells (maybe to get enough power to drive an analog meter movement).
Ollinger's - Light Meter Collection - 617 Type 1 -

Federal Reporter, 2nd Series, Vol 133. pp. 778-781; WESTON ELECTRICAL INSTRUMENT CORPORATION et al. v. DEJUR-AMSCO CORPORATION - info on use of honeycomb to control look angle.

Scott's Photographica Collection - Weston Electrical Instrument Corp. Model 617 Exposure Meter -

Weston 7

One of the earliest photoelectric exposure meters and marked the introduction of the Weston film speed system (Wiki).
It wasn't till 1949 that the Weston exposure meters switched to the then new ASA system. 
Note:  The difference between the Weston film speed and the official ASA film speed was a difference of 1/3 stop.
I expect at that time it amounted to no difference and may have been done so that Weston could not claim to be the total basis for the ASA system.

Weston 650 Photronic Exposure Meter

In the late 1920s film speeds (Wiki) were not yet standardized between manufacturers, so each brand had their own system.

	Patents shown on front: 1579849  see below 1779574 see below 1982406 see below
	Place for Emulsion Speeds since films were not standardized.
	Weston Film Ratings Booklet

D97827 Photoelectric Exposure Meter, Edward F. Weston (Weston Electrical Instrument Corporation), Dec 10, 1935, Application September 12, 1935, Serial No. 58,536

Weston 703 Type -3 Sight Light Foot-Candles Meter 

About 1926 based on patents on meter face.  Note not intended for photographic exposure, but rather Illumination (Wiki) and Photometry (Wiki), Foot-candle (Wiki).

1579849 see below
1635595 see below
1661214 see below
2003919 see below

Back Cover Removed

Weston 715 Master Light Meter

This is very similar to a light meter I used decades ago.  I remember the hinged light baffle that also changed the meter scale.
Note in the photo where the baffle is opened the meter is reading the daylight in the room.

Weston 756 Foot Candle Meter

Weston 756 Illumination Meter with VISCOR filtered Photronic cells.  Note the paddle probably has two sensors connected in series in order to get enough voltage to drive a passive (no batteries) current meter.  The alternative at this time would have been a vacuum tube amplifier and many pounds of batteries to power it.  The serial number of the paddle and meter are the same.  The Engineering department that had this meter made a number of f/stop overlays, maybe to make it easier to use as a light meter for photography?  The wire connected to the paddle has degraded and needs to be replaced.  This is the model used when making the movie "Gone With The Wind" (Ref IMDB, WestonMeter.org).

Range switch: Off, 600, 120, 60, 12, 6
One the 6 Candlepower range one tick is 0.1 CP  See Light Brightness Units below for some common values.  Note daylight is over 15,000 CP, way off scale for this meter, hence the need for the f/stop masks.

Date stamp on back of meter: Dec. 10, 1964.
S/n on meter and target: 2770.
at the bottom of meter face:
Use only with target bearing same number as meter
New Candle       Model 756 No. 2770   Viscor Filter

Fig 1	Fig 2	Fig 3 masks needed for use outside, since the Sun pegs the meter on the highest range.
Fig 4 Replaced degraded cord.  Works fine.	Fig 5 Instructions in lid of box.

Harvard Collection: Weston Foot Candle Meter -

Weston 819 Cine Exposure Meter

Physically looks like the 650 still camera exposure meter, but the circular calculator and meter face are specific to movie cameras.
Dates:  Instruction book 1937
Weston Film Speed booklet: 1938
Kodak K135 Kodachrome Film sheet: 2-1957
Patent numbers: identical to the Weston 650 above
A letter to the first owner explains how to use it for still cameras.

Weston Photographic Analyzer Model 877

This is a multi function instrument where one of those is measuring the density of a negative.  This one way of determining the film speed.  In the Ansel Adams Zone System (Wiki) where (AFAICR) first the enlarger exposure for unexposed film is determined to get solid black.  Then an 18% gray card is exposed at various ASA values and after a standardized development process those are printed using the standardized enlarger process.  Part of that is printing not only the exposed part of the film but also the space between frames that was not exposed and comparing those.  For more on the Zone System see the SEI Photonic Spot Light Meter below.

Fig 1 Original box	Fig 2 Back	Fig 3 Front with model 594 Photonic cell unsnapped from head.
Fig 4	Fig 5	Fig 6

Patents:

	1818003 Process and apparatus to be used in printing photographs from negatives, Joseph W Myers, (not assigned), Application: 1927-12-19, Pub: 1931-08-11,
	2235590 Apparatus for determining proper exposure in making photographic prints, Jr Harvey P Rockwell, Weston Electrical Inst. Corp., 1941-03-18, - This is the model 877
	2306641 Photographic exposure calculator, Jr Harvey P Rockwell, Weston Electric Instrument Corp, 1942-12-29 - Relates Negative density, illumination, Exposure time, paper speed. - built into the meter scale. (patent No. appears on meter scale.)

877 manual at Camera Manuals.org.

SEI Photonic Spot Meter

I used one of these to go along with the Ansel Adams (Wiki) Zone System (Wiki).  This was the subject of a camera class at De Anza College.  I used the Calumet 4x5 view camera for that class. The book for the class was The Zone System Manual by Minor White (Amazon). See: Ollinger's Light Meter Collection - SEI Photometer - Related Patents 165764 Improvement in photometers, O. Schuette, July 20, 1875 - variable density filter 464059 Photometer and actinometer (Wiki), L.H. Barker, Dec 1, 1891 - 552371 Method of and apparatus for measuring illumination, E.J. Houston & A.E. Kennelly, Dec 31, 1895 -

1315972 Photometer, P.A.M. Lacroix & J. Richard, Sept. 16, 1919 - French This has the look and feel of the SEI, but it's much older and uses a pocket flashlight battery.

Light Brightness Units

Lumens (Wiki) are a measure of the total visible light emitted by a source and it typically measured using an integrating sphere (Wiki).  For example the Lumen output of a LED would be the same after it's lens was filed off as it was with a narrow beam.  But the Foot-Candle reading with a narrow beam will be much higher than for the same LED after filing off the lens.  Some LEDs have their light output specified in MCD (milli candelas Wiki) and they get high numbers by using a very narrow beam lens.

On the other hand some flashlights advertize high lumen numbers, meaning they contain a bright LED, but do not have the optics to focus all that light into a beam to provide a high Candela value on the subject being illuminated.  The Xenide AEX25 1500 Lumen HID flashlight uses a High Intensity Discharge Xenon Short Arc lamp which allows designing an optical system that gets very close to all the light on the target, but an LED is far from a point source, i.e. it has a very directional light output pattern making it difficult to get all the light where you want it.

from Wiki Foot-Candle, Exposure Value,

	Foot Candle	Lux (lumen/m2)	Exposure Value (EV)	594
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
	1	10.764
EV reference (Wiki) (1 sec f/1)	0.23	2.5	0
Indoors Window Light				0.02 ma
Maglite 2 AA Flashlight				0.35 ma
Full Moon, clear sky	0.023	0.25
Quarter Moon, clear sky	0.0009	0.01
Moonless, clear sky	0.0002	0.002

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

^{The meaning of Candlepower (Wiki) has changed over time.}
1860 - defined by the Metropolitan Gas Act 1860 (Wiki)
1909 - defined as 9/10 of the output of a Hefner Lamp (Wiki)
1921 - CIE defined in terms of a carbon filament Incandescent lamp (Wiki)
1937 - defined based on freezing point of liquid Platinum as 58.9 candles per square centimeter.
1948 - SI Candela (Wiki) replaced candlepower.  1 CP = 0.981 candela, but now they are considered the same. <- This is the meaning printed on the lower part of the meter face on the Weston 756 (dated: Dec 10, 1964)

Reference

John's Collection of Weston Meters - Publications & Further Reading -

Patents

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 applications speed of response was very important.  But for measuring light for photography or to determine the light needed for various industrial applications a light meter with a response matching the human eye was required.

1579849 Pointer for electrical measuring instruments, Edward F Weston (Weston Electrical Instr Corp), Apr 6, 1926, 116/330 - helical spring screwed onto pointer for balance
1635595 Electrical measuring instrument, E.F. Weston, et al (Weston Electrical Instrument Corporation), Jul 12, 1927, 324/151.00R, 324/156, 324/155 - meter movement
1661214 Zero corrector, Carpenter Grant (Weston Electrical Instr Corp), Mar 6, 1928, 116/291, 324/154.00R -
1779574 Direct-reading photometer, Samuel Wein (Radiovision Corp)Oct 28, 1930, 356/218, 429/111, 250/214.0SG, 315/55 - photocell + meter
1982406 Exposure meter, Ferdinand Tonnies Hans (), Nov 27, 1934, 396/293, 356/225, 250/214.1 -meter points to camera setting (speed or f/stop)
2003919 Meter, Canfield Earl L, Jun 4, 1935, 356/227 - Weston 703  (This patent and the ones above are related to the 703 meter)

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 - cylindrical with variable iris
2041816 Light sensitive cell, Carpenter Arthur, Ross Ernest (United Res Corp), May 26, 1936, 338/19, 369/120, 338/199, 136/259 - variable resistance
                Citations:
                1940245  Light sensitive cell, Arthur W Carpenter (United Res Corp), Dec 19, 1933, 428/433, 136/264, 338/15 -
                                variable resistance  - mixtures of platinum, osmium and iridium, either with or without palladium
                1942958   Light-sensitive cell, Arthur W Carpenter (United Res Corp), Jan 9, 1934, 338/15, 136/256, 338/292, 338/309 - variable resistance       
                1948766 Light sensitive cell (mounting methods)
2042665               
2203209 Exposure meter, Weston Edward F (Weston Electrical Instr Corp), Jun 4, 1940, 356/228, 235/64.7, 235/86 - knob moves belt with exposure values
2046665 Exposure meter, Weston Edward F (Weston Electrical Instr Corp), May 19, 1933, 356/225 - cylindrical
2058562 Light intensity self-adjusting camera, Albert Einstein, Gustav Bucky (), Oct 27, 1936, 396/213 - mentions "Weston Photronic"

2073790 Photographic exposure meter, Goodwin Jr William Nelson (Weston Electrical Instr Corp), Mar 16, 1937, 356/222, 235/64.7 - Model 617 Type 1
2088070 Photoelectric tube, Spencer Percy L (Raytheon Production Corp), Jul 27, 1937 - sensitive in red and near IR
                313/538, 252/181.4, 427/74, 445/11, 445/12, 427/124, 204/164, 445/13, 427/126.5
2105255 Exposure meter, Green Newton B, Joseph Mihalyi (Eastman Kodak Co), Jan 11, 1938, 235/88.00R, D16/238, 235/64.7, 356/228 -w/exposure calculating disk
2123670 Electrical measuring instrument, Weston Edward F (Weston Electrical Instr Corp), Jul 12, 1938, 356/226, 235/64.7 - Light Meter (filed: Aug 16, 1935)
                Compact square with light sensitive rectangle in meter face.
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? Ans: so that the included angle of light would match a normal 46 deg. camera lens. 
            See:  Ollinger's Light Meter page & patent 1982406
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,
2433566 Process for manufacturing photoelectric cells of the dry disk type, Anthony H Lamb (Weston Electrical Instr Corp), Dec 30, 1947, 136/256, 136/259, 257/42, 257/431, 257/658
                 May be for Photronic cells              
2606215 Encased and hermetically sealed photocell, Anthony H Lamb (Weston Electrical Instr Corp), Aug 5, 1952,
                136/259, 174/50.52, 228/124.6, 174/50.6, 174/50.54, 313/523, 174/564, 174/528
2629039 Selenium Cell and Process for Making Same, George H. Shoemaker (Weston Electrical Instrument Co), Feb 17 1953,
             338/15 ; 136/264; 257/42; 257/E21.07; 29/620; 427/123; 427/372.2; 427/383.7; 438/102; 438/486 -
            Simpler and better process to convert amorphous Selenium to crystalline form.  In glass tube.
            Calls:
                866462 Selenium Cell, W.J. Hammer, Sep 17 1907, 338/19 ; 114/21.2; 136/259; 250/214.1; 338/237 - cell acts as variable resistance
                2186085 Method of Making Selenium Rectifier Films, Samuel Weta (B-L Electric Mfg Co), Jan 9, 1940,
                2196830 Photoelectric Cell, Clarence W. Hewlett (GE), Apr 9 1940, 136/255 ; 338/15 -
                2342278 Manufacturing Selenium Cells, H. Herrmann (Germany), Feb 22 1944,
                2364642 [typo in patent?]
                2413013 Method of Making Selenium Rectifiers, A. Von (Federal Telephone & Radio Corp), Dec 24, 1946,
                2433401 Selenium Paste and a Method of Making it, Otto Saslaw (Intl Tel & Radio Corp), Dec 30, 1947, 438/102 ; 106/287.35; 252/62.3S; 257/E21.071 -
                2479301Selenium Rectifier, Wayne E. Blackburn (Westinghouse), Aug 16, 1949, 257/42 ; 252/62.3S; 257/E21.071; 257/E21.073; 257/E21.074 - blocking layer

2855567 Instrument adjuster element, Arthur D Clark, Anthon H Lamb (Daystrom Inc), Oct 7, 1958, 324/154.00R, 116/291 - zero adjuster

Photovolt  200A Foot Candle Meter

The Photovolt Corp was founded in 1939 according to their current web page.
Foot candle meter using a sensor very similar to the Weston 594 Photonic.  Introduced in 1948 (Ollinger's Light Meter Collection).
Wiki has information on Exposure Value and how it relates to the light and camera settings.  EV=5 : 7.43 fc (ISO 100).

The Model 200A has ranges: Off, 250, 50, 10 & 2.5.
The Model 200    has ranges: Off, 500, 100, 25 & 5.
The 200M model seems to have a nose on the sensor, maybe to fit a microscope?  The range switch is OFF, Lo or Hi. Meter shows 0 to 100.

Fig 1	Fig 2  Black button above meter is zero adjustment knob. The two pieces of wood inside the lid are for the exposure calculator.	Fig 3  just under 10 fc. @ 200 ASA Nikon D300S says: f/5.3 & 1/3 sec
Fig 4	Fig 5 Four adjustment post, one for each range. Don't know what grunge is on metal parts? let me know.

3293540 Temperature compensated circuit arrangements, Bela A Silard, Schulkind Herbert, Photovolt Corp, 1966-12-20

Weston Meter Movement

	D19896 Magnet, E. Weston, June 10, 1890 - Horseshoe type but with wide area where meter scale can fit.
	D20318 Magnet, E. Weston,Nov 18, 1890 - has cylindrical field between the poles with narrow gap - for instruments.
	292714 Electrical Indicator, E. Weston, U.S. Electric Light Co, 1884-01-29, -Fig 1 bell, Fig 2 meter.
	D28392 Design for an index needle or hand, Edward Weston, March 22, 1898 - tubular for meter movement - balanced about pivot axis
	392387 Electrical measuring apparatus, E. Weston, Nov 6, 1888 - uses the above designs

427022 Electrical measuring Instrument, E. Weston, Apr 29, 1890 - Fig 9 shows jewel for pivot.
446489 Electrical measuring Instrument, E. Weston, Feb 17, 1891 -

611722 Electrical measuring instrument, E. Weston, Oct 4, 1898 -

678706 Electrical measuring instrument, Edward Weston, 1901-07-16 -
678707 Art of manufacturing movable coils for use in connection with electrical measuring instruments, Edward Weston, 1901-07-16 - works with index needle above
678708 Apparatus for constructing movable coils of electrical measuring instruments, Edward Weston, 1901-07-16 - works with index needle above
730061 Reflecting-galvanometer, Edward Weston, 1903-06-02 -

Coils as Resistors

In many early test instruments where a resistor is needed it is made by winding copper wire into a helix or coil.  Since the resistance of cooper wire varies with its diameter (AWG) and the Ohms per foot is directly related it's possible to predict the length of the wire needed as well as the cross sectional area required.  The power rating also follows.  But there are problems such as:

• The resistance changes with terperature
• The coil has inductance which may or may not be a problem

RE10944E Electric Conductor, E. Weston, July 17, 1888 - Copper alloy to nullify resistance change with temperature.
RE10945E Electric Conductor, E. Weston, July 17, 1888 - Copper alloy to nullify resistance change with temperature.
497482 Shunt for electric light and power stations, E. Weston, May 16, 1893 - stacked parallel plates
906498 Shunt or electrical resistance, Edward Weston, 1908-12-08 - for measuring high currents

Integrating Sphere (separate web page)

Used to measure total light output, like from an Edison lamp.  Lumens are a measure to total light output and ignore any optics used to shape the light.
The Hipco flashlight uses s couple of lenses to get a very sharp beam.

Colorimeter

1495763 Colorimeter, Colorimeter Thomas D Simpson, Woodman A Clayton, Sinclair Refining Co, 1924-05-27, 356/70; 356/415; 434/98; 434/367; 434/388 - measure the color of lubricating oil according to an A.S.T.M. standard.

The Datacolor Syyder 4 measures the color of my LCD computer monitor.
The Photelometer determines the amount of oxygen in blood by it's color.

RCA 923 VT-252 Photocell (Wiki: Phototube, Photoelectric Effect, Photocathode, Einstein Nobel Prize)

In the early 1900s there were a number of applications for sensing light.  Mechanical television (Wiki) and talking pictures (Wiki) were big things.

(also see: Weston 594 Photronic Cell, Metascope\patents)

RCA 923 Phototube Frank.pocnet: 923Datasheet.pdf
Peak response 800 +/- 100 nm i.e. near IR

A photocell is used in the Western Union Desk Fax.

RCA 931 Photomultiplier

The Photomultiplier Handbook (Photomultiplier_Handbook.pdf) is the bible for these tubes and includes a lot of information about their development.

1. Introduction
2. Design
3. Electron Optics
4. Characteristics
5. Applications

Historical Applications:
Developed for an optical sound pickup used for motion pictures.

Early applications: Astronomy and Spectroscopy, Radar Jammer (very wide band noise), Scintillation counting (PDR-56), headlight dimmers for cars, gamma camera (Wiki) for medicine,

The Secondary Emission Multiplier - A new electronic device, V.K. Zworykin, G.A. Morton, and L. Malter, RCA, March 1936, IRE Vol 24, No. 3. (Zworykin_Morton_Malter.pdf)

The only Nobel Prize Einstein won was for the Photoelectric Effect (Wiki).  E=(mV²)/2.  This is very similar to his equation E= mC² (Wiki).

While the modern term for these is photomultiplier (Wiki) the patent (2231697) calls them Electron multipliers.

This was used in the PDR-56, maybe in the MD1 Star Tracker.  Probably the first photomultiplier and AFAIK still being made.  Used in all kinds of scientific instruments.

Frank.pocnet: 931A.pdf
Base: Small-shell Submagnal 11-pin.
S-4 Spectral Response, Cs3Sb, 950 Lumens/Watt, QE: 12%, See Photomultiplier Handbook page 16.

Fig 1	Fig 2 The light input grill is about 15/16" x 5/16".	Fig 3
Fig 4 Circular-cage multiplier structure pg 28 Photomultiplier Handbook

Photocell & Photomultiplier Patents

	1450265 Hot-cathode tube, Slepian Joseph, Westinghouse (CBS), App:1919-04-18, Pub: 1923-04-03, - secondary emission, Hot-Cathode,
	1831314 Photoelectric tube, Archie J Mcmaster, Charles E Parson, GM Labs, 1931-11-10, - sensitive to visible light from a filament type lamp.  copper photocathode plated with a mixture of caesium chloride and calcium. However, in most cases it is found desirable to admit an inert gas such as argon, helium, neon, krypton, Xenon, or even a less inert gas such as nitrogen, before the bulb is sealed off.
	1858210 Electronic tube, Archie J Mcmaster, Charles E Parson, GM Labs, 1932-05-10, - Caesium salt on Copper photocathode
	1869342 Photo-electric tube, Archie J Mcmaster, Charles E Parson, GM Labs, 1932-07-26, 313/523; 252/181.7; 427/77; 427/106; 427/108; 427/123; 428/929; 428/935; 428/937; 445/11; 445/12; 313/346R -
	1903569 Electron tube, Kenneth W Jarvis, Russell M Blair, 1933-04-11, - early attempt at electron multiplier.
	1909115 Optical system, Archie J Mcmaster, - single photocathode, for reading the sound track on motion pictures
	1909701 Photoelectric tube, Archie J Mcmaster, single photocathode, not assigned, 1933-05-16, -
	1925054 Photoelectric tube,  Archie J Mcmaster, GM Labs, 1933-08-29, - single photocathode, , precursor to the GM Autronic Eye or GuideMatic?
	1980702 Phototube, Salzberg Bernard, RCA, 1934-11-13, -First generation 2-starge, photomultiplier
	1984312 Photocell circuits, Archie J Mcmaster, GM Labs,1934-12-11, - two tube transformer coupled amplifier for TV application. Note this was the time of mechanical (flying spot) TV (Wiki).
	2028153 Radiation detector, Gunn Ross (Wiki), 1936-01-21, - photocathode, selenium cell or thermopile as electron sources.
	2035907 Photoelectric system, -photocell &  relay driven counting
	2073599 Electric discharge device, Malter Louis, RCA, 1937-03-09, - photo multiplier
	2077634 Photoelectric tube,  Archie J Mcmaster, GM Labs, 1937-04-20, -  photocell If we wish to obtain a tube which is highly sensitive to light in the red and yellow regions of the spectrum we use a caesium salt such as caesium chloride, caesium carbonate, caesium nitrate, caesium trinitride, caesium silicate, or caesium azide. If we wish to obtain a tube responsive to shorter wave lengths we use potassium chloride, potassium carbonate, potassium nitrate, potassium trinitride, potassium silicate, or potassium azide. For still shorter Wave lengths salts of the alkaline earth metals are used such as barium, strontium, and magnesium carbonates. All of these metals are high in the electromotive series. The method of processing the silver plated copper plate in the tube will first be described in connection with the caesium salts.
	2077810 Phototube, Salzberg Bernard, RCA, 1937-04-20, - secondary emission amplification,
	2078304 Electric discharge device, Vladimir K Zwerykin, Jr Ernest A Massa, RCA, 1937-04-27, - similar to 2073599, but longer
	2082210 beam breaker - protection of hands of punch press operator
	2147825 Electron multiplier device, Vladimir K Zworykin, Malter Louis, RCA, 1939-02-21, - uses magnetic filed to control electrons - multistage, not practical
	2231697 Electron multiplier, Vladimir K Zworykin, Jr Richard L Snyder, RCA, 1941-02-11, - probably the 931 Photomultiplier
	2231698 Electron multiplier, Vladimir K Zworykin, Jan A Rajchman, RCA, 1941-02-11, - Photomultiplier
	2241670 Electric target range, Archie J Mcmaster, Rayolite Rifle Range Co, 1941-05-13, - a flash of light from pistol is detected by phototube.  A precursor to modern electronic games.
	2308469 Exposure meter, Archie J Mcmaster, GM Labs, 1943-01-12, - does not use phototube Cites: 1779574 Direct-reading photometer, Wein Samuel, Radiovision, 1930-10-28, - 1982406 Exposure meter, Tonnies Hans Ferdinand, 1934-11-27, - 2064987 Photometer, Reinhold Ernst, Joseph M Bing, Photo Marketing, 1936-12-22, -
	2908840 Photo-emissive device, Robert H Anderson, RCA, 1959-10-13, - end input photomultiplier

1869379 Emulsions and materials, methods and processes for the preparation thereof, Richard W Lewis, 1932-08-02, 516/50; 106/502; 106/487; 106/277 - there's an error for this patent number.  The .pdf version has the text part of patent 1869342 above, but not the figures.

1944194 Radiant energy detector, Sackville William, 1934-01-23, 338/17; 136/254; 338/325; 250/214.1 - this patent is for a resistance device, not a photocell.

Defines three groups:
I. Non wavelength selective radiometers such as the thermopile.
II. Includes resistance cells such as the thalofide cell (mainly sensitive to red and near IR), which have the property of de Creasing in electrical resistance when exposed to 5 radiant energy of short wave lengths, the char acter of the phenomenon depending entirely upon the wave length of the radiant energy stimulus.
III. Includes substances called photo-electric cells which when charged to a negative potential in an evacuated chamber, lose their charge when exposed to light of short wave length.

2096862 Gaseous discharge device, Percy L Spencer, Raytheon, 1937-10-26, 313/537; 313/541; 315/149; 313/538; 313/564 - vacuum tube used in a manner similar to a photoelectric tube

2448517 Photocell, Robert J Cashman, Northwestern U, 1948-09-07, 338/18; 252/519.4; 137/251.1; 313/523; 338/19; 338/237; 338/293; 423/624; 427/74; 427/109; 423/512.1; 445/11 - Thallium Sulfide photo-resistive

Defines three types:
I. photo-resistive or photo-conductive: Selenium, resistance is function of light intensity.
II. photo-emissive: Sodium, potassium or cesium are placed in series with an electromotive force, and light falling  upon these substances has the effect of causing electrons to be emitted from the surface of the metal in suc manner that the total current through the circuit is functionally related to the intensity of the light falling upon the cell.
III. photo-voltaic: Silicon solar cells convert light directly to electricity where the current into a short is proportional to the light intensity.

2689313 Photosensitive cell, Pakswer Serge, William O Reed, Rauland Borg Corp, 1954-09-14, 338/19; 313/594; 428/471; 428/642; 428/929; 428/935; 428/936; 428/938 -

Incident Light metering

Most of the meters on this page measure reflected light.  Typically from the subject and sometimes from an 18% gray test card (Wiki).  All the light meters built into cameras are working on reflected light.  But when I was shooting movies with a Bolex H16 movie camera I used an incident light meter.  AFAICR the incident light meter was supposed to give better results when shooting positive film and reflected light meters for negative film.  At that time (1960 - 1980s) most movie film was exposed based on incident light metering.

Note with modern digital cameras, like the Nikon D300s, when set to output RAW files and with the extended 14 bits per color channel, the dynamic range from a single exposure is so high that you only need to get the exposure setting sort of close.  Before I discovered the 14 bit setting I was looking into High Dynamic Range image stacking.  But this is not needed if you can get 14 bit RAW images.

I've been looking for one of these for over a week on eBay.  So far no seller has answered by question about what the meter reads in direct sunlight.  Also most of the plastic hemispheres seem to look yellow rather than white.  Maybe these have a limited lifetime?

2214283 Exposure meter, Donald W Norwood, 1940-09-10, 356/221; 250/228; 250/214.1 -

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Links

Weston The Early Years - Other useful Sites for the Collector -

James Ollinger's -The Eclectic Photography Resource - Exposure Meter Collection -

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page created 22 May 2008.