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)
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.
Box on eBay:
Matched Cell AT
25 FC & 200 Ohms
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 - filedSep 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
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
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:
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.
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 -
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.
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
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 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 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 -
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.
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.
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.)
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 -
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 -
from Wiki Foot-Candle, Exposure Value,
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.
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(1 sec f/1)
|Indoors Window Light
|Maglite 2 AA Flashlight
|Full Moon, clear sky
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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
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.
Design probably from the late 1920s, using "GE" film
speeds. They switched to ASA film ratings in 1946 (Wiki).
Note: this meter says on back "For film or plates use exposure index American Standard speed number". Sold by the Army Exchange Service.
|The front just pulls off - snaps back
136/256, 428/668, 428/621, 428/686, 428/931, 257/431, 257/E21.75, 257/42, 428/657, 204/192.26, 338/15
cadmium (sulfide?) gold sensor
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 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 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
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.
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 -
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:
RE10944E Electric Conductor, E. Weston, July 17, 1888 - Copper alloy to nullify resistance change with temperature.
- The resistance changes with terperature
- The coil has inductance which may or may not be a problem
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
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.
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