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.)
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 cell
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.
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.
|Indoors Window Light
|Maglite 2 AA Flashlight
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
An Integrating sphere (Wiki) was used by Edison when testing his light bulbs. It collects all the light from a source and so removes any directional aspect so gives you a measure of the total light output. I've watched eBay for a used one that's small, but they are still thousands of dollars.2018 May discovered a YouTube on making your own.
Internet Lumens vs Actual Lumens, and the 100 watt LED test -
DIY Lumen Measuring Device. Integrating Shpere, and Lumen Tube.
How to measure Flashlight Lumens and Candela using ANSI/NEMA FL1 standards
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LM631A Amprobe (Meterman) LM631A Digital Light Meter
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page created 22 May 2008.