For taking pictures outside, one could use timetables telling us , depending on location, season and time, how to expose our film. Later people invented the exposure meter called the actino meter and also the extinction meter more or less based on comparison of density but these meters are outside of the scope of this website.

Finally in the late twenties and early thirties a way was found to create photo electric sensors sensitive for visible light. When exposed to light the sensor generated a voltage, enough to drive a galvanometer telling us the amount of light measured.

The Photoelectric exposure meter is a product representing almost  100 years of scientific research. Already a lot is written about the photoelectric exposure meter but history here: actually three discoveries lie at its foundation. In 1839 a French scientist , Edmond Becqurel discovered that light increased the output of voltaic cells when it fell upon platinum coated electrodes. Thirty-four years later Willoughby Smith, a telegrapher then working in the Azores, noticed that the selenium in his line-testing instrument changed in resistance to current when light fell upon this element. In 1887 Heinrich Hertz, a German scientist found that electrically charged particles or  electrons were shot  off certain substances when exposed to ultra-violet rays. Since then a lot of scientist are researching on these phenomena’s .

The basis for the photoelectric light meter was the selenium photoelectric sensor. It was long known the element selenium was sensitive for light but there was no proper method to manufacture useful sensors. Weston (USA) did a lot of research on these sensors and created a sensor for a photo electric light meter. However an additional power source was required. After improvements a cell was fabricated that functioned without the need of additional power (battery). When exposed to light, the cell generated a power. Weston, General Electric in the USA. Electrocell and Suddeutsche Apparaten Fabrik both in Germany manufactured photo electric selenium cells to be used in exposure meters.

The type that requires an external power source mostly needs a battery to drive the measurement electronics based on pe. a LDR, a CdS or a Blue photocell (photodiode). The light measured by the cell is processed by the internal electronics and fed to a read-out system. A lot of meters produced in the 60′, 70′ and the 80′ use a little Mercuric Oxide 1.35 Volt battery not longer available and is now banned because of toxicity and environmental fears. There are some alternatives to replace this cell but a 1.5 Silver Oxide cell is not an option.

The light measuring technique with a photoelectric light meter is actually very simple and does not need complex equipment. As an example we take a look at the Gossen Bisix 2 exposure meter for incident and reflected light. This meter consist of a number of parts.
– The reading scale with values
– The sensor
– The correction unit
– A meter element
– A ‘calculator’ disc
– A nulling system to set the meter at the zero position

There is a difference in measuring light as it is coming directly from the light source (incident light) or is reflected by the subject (reflected light). Most meters cannot measure both types of light without making a change to your measurement equipment. For measuring reflected light the meter can be used without anything in front of the cell but when measuring the light source directly a kind of filter has to be placed in front of the cell. By Gossen meters, this filter is a part of the meter and can be placed or removed from the photocell. Meters that don’t have this mechanism are not made for measuring incident light but only for measuring reflected light. There is also a kind of light that is called ambient light. This is the light that is the light that is available at that moment without any intervention of the photographer in the form of a extra light source.

The visible light we use as a photographer is a very small part of a spectrum of electromagnetic energy. The electromagnetic energy radiates with a certain wavelength and intensity. Very much and detailed information about light and the physics of light can be found on Wikipedia.

The function of a selenium cell is actually very simple, when light falls on the selenium cell it generates some power. Because of this power, a current runs trough the circuit which activates the meter.
I did some measurement on a small meter and found the following values: The voltage generated by the photo-electric cell is 0 mV when dark and some 250 ~ 400 mV when light shines on the cell. The actual voltage depends on the type of cell. The internal resistance of the moving coil meter is about 1 Kohm so the current that runs trough the system is aprox. 250 mV / 1000 ohm = aprox 25 uA.

The sensor package consists of several parts, in this case (from left to right) the frame around the honeycomb lenses, the honeycomb lenses, a light filter determining the measuring angle, a metal contact frame with wire, the sensor, another metal contact plate with wire. The sensor contains a metal contact layer to make contact with the electrodes. The contact frame with the wire makes contact with the front (sensitive part) of the sensor. The metal plate behind the sensor makes contact with the backside of the sensor and also puts pressure on the package due to his spring shape. There are a number of websites on the internet with detailed information on how the selenium cell is made and how it actually works.

The Lunasix was the first meter build with this CdS cell somewhere in the late 50’s or early 60’s. A fairly large number of Gossen meters are build with this kind of sensor.

At the picture on the left, we can see that the meter consist of 2 parts, left part the calculator disk, part on the right the meter with the scale. Even in the current situation were the meter is taken apart, it is still functional and can be used for measuring the light values.

The Bix 3 is typical a match needle meter with a small needle, the actual meter needle and the wide needle, the match needle with a slot in this case. the slot must be placed directly above the meter needle by turning the big wheel. as soon as they match we can read the aperture / exposure time pare and select one depending if we want a high or low speed or a large or small aperture.

This ‘Null’ metering system is obviously quicker and easier. Interestingly, it is also unbeatable accurate. Indeed, it’s the method used with many on the world’s most precise laboratory instruments. Other meters may give you mid tone readings, but unless you know the full range of brightness in the scene you may lose critical highlight and shadow detail.

A +/-3 EV scale makes it easy to compare other areas with the key area on which you’ve ‘nulled’. It’s deal for scene brightness measurements and zone-system applications. And it works the same regardless of light level – no calculations.