Photoelectric Exposure Meter

Photographers judged exposure by eye, by printed tables, or with extinction meters that asked them to read the dimmest visible number. Edward Weston (the instrument maker, not the photographer) used a photovoltaic cell that generates its own current in light and fed it straight into a sensitive meter marked in exposure terms. No battery, no judgement. **US 2,016,469**, "Exposure meter", Weston, assigned to Weston Electrical Instrument, filed 31. December 1931, granted 8. October 1935: an instrument "for the direct determination of the correct exposure conditions", improving on earlier current-generating cell meters whose output was too small and whose construction was impractical. **You build the same thing** from a small solar cell and a multimeter, calibrate it against a lux meter, and turn its readings into shutter speeds.

مبتدئ

90 minutes

التعليمات

Cell and meter

Clip the solar cell's two leads to the multimeter set to DC milliamps (or microamps). This short-circuits the cell through the meter - the current is what you read.

المواد لهذه الخطوة:

Solar Cell Mini - 350mV1 قطعة

الأدوات المطلوبة:

جهاز قياس متعدد

Alligator to Alligator Cables

A hood

Glue a black card tube 30 mm long around the cell so it sees only light from in front, as Weston's meter did.

المواد لهذه الخطوة:

مقوّى أسود1 ورقة

الأدوات المطلوبة:

سكين حِرَفية

مسدس شمع لاصق

Calibrate

Hold the cell and the lux meter side by side, facing the same way, in four very different lights: a dim room, a lamp, a window, open shade outdoors. Note both readings each time.

الأدوات المطلوبة:

مقياس اللُكس

Current to exposure

جارٍ تحميل دفتر Jupyter…

Use it

Stand at the subject, point the hood back towards the camera, read the current, and set the camera manually from the notebook's table. Compare with the camera's own meter.

الأدوات المطلوبة:

جهاز قياس متعدد

History & Context

**What the patent specifies:** a light-sensitive current-generating cell connected to a measuring instrument reading directly in exposure terms. **Modern build (derived):** a silicon solar cell and a multimeter. Weston's selenium-cell meters, sold from 1932, made metering routine; cameras later built the cell in. Its limits: selenium cells lose sensitivity in dim light and age, and any meter averages the scene - a snowfield or a black cat still fools it.