Light measurement technique

Update: 07.29.2012


The photoelectric light meter for photography purposes and ambient light measurement.

Ever since the beginning of photography there was the question; "how do I get my photographic correctly exposed?" It does not matter if it is material in the camera or in the darkroom.

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

The sensor or measuring element is actually an photo voltaic element which generate power when light falls on the element. it is an selenium element and used to produce first Gossen light meter in 1933. So it is a old system but still used for powerless light or exposure meters.
When light falls on the photo element the voltage is fed to the meter trough a correction unit to correct the power coming from the selenium element to the meter to give the correct read-out. The correction unit has to be calibrated to give the correct reading by a given quantity of light. To convert the meter from reading incident light to reflected light or vice versa a diffuser has to be set in front of the photocell. In this case this is a blind that can be shifted.

The meter element is mounted on a bracket that can me moved a little bit so with this construction the meter can be nulled i.e. the starting position can be set at the right zero value point with no light falling on the meter.
Measuring the incident light or the light reflected from the subject consist of pointing the meter in the right position with or without the diffuser and reading the measured light value from the reading scale. After reading the measured value, the value hast to be transferred to the 'calculator' disc by turning the scale until the red arrow points to the correct value. 

But first the right film speed has to be set on the scale by turning the little knob in the center of the meter at the right DIN / ASA value equal to the film used. After transferring the light value on the scale, the 'calculator' disc is set and the exposure time can be read from the  scale by selecting the required exposure time / shutter speed combination. 

Light

We use a light meter or an exposure meter to measure the amount of light. Actually both type of meters are basically the same instrument
except that the exposure meter has a extra device for calculating an exposure time and aperture combination for a given film speed. Light is measured in different values. In case of photometric measurement you want to measure the amount of light that is current in form of light intensity; lux, foot candle or candela/m2 Lx, fc, cd/m2, lxs, fcs and more. The light meter can be used for photographic purposes and for photometric purposes. In case of Photographic measurement you want to measure the amount of light that is available or reflected from the subject and set the camera accordingly to get a proper exposure. Setting he camera can be done by changing the aperture or the exposure time or both depending on how you want the exposure to be.

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.


Type of sensors

As far as known Gossen used 3 types of photo elements in their meters, at first they used the selenium photo element since the Ombrux, the first Gossen meter brought on the market in 1933. The advantage from the selenium exposure meters is that they don't need extra power like a battery. A disadvantage is that they don't live forever. Mostly the contacts wear out so these are insulated from the element and this stops the meter from functioning. (for the selenium cell see pictures further in this article)
After the selenium cell, the electronic world came with the CdS cell, this is a light dependent resistor or cadmium sulfide cell of which the resistance decreases when the light shining on the element increases.
The Lunasix was the first meter build with this CdS cell somewhere in the late 50's or early 60's. A fairly number of Gossen meters are build with kind of sensor.
The SBC cell was the next step for sensor elements of Gossen to build into their meters. This new cell the Silicon Blue Cell (SBC) is actually a fast photodiode and was integrated in the system exposure meter the Profisix. The exposure meters equipped with this type of photo sensor are recognizable by the SBC shield on the meter. Newer meters make use of more than 1 SBC as photo receiver. The Mastersix for instance has 2 of these diodes as sensor. The newest have even color corrected SBC sensors inside.

Selenium cell

Selenium Photo-Electric Cell
 

 

 

 

 

Selenium photo-electric cells
Selenium photo-electric cells convert the energy from the light falling on them directly into electrical energy. The advantage of selenium photo-voltaic cells over other cells is that their response is very close to that of the human eye; this makes them particularly suitable for use in light measuring instruments. Their efficiency as energy converters of the total spectrum is not as high as some other photocells, and so they are not used as solar cells. The diagram shows an idealized barrier-layer selenium photocell in section. The steel support plate 'A' provides the rear (positive) contact, and carries a layer of metallic selenium 'B', which is a few hundredths of a millimeter in thickness. 'C' is a thin transparent electrically-conductive layer, applied by cathodic sputtering; it is reinforced along its edge by a sprayed on negative contact ring 'D' and protected from damage by lacquering. The rear support of our photocells is protected from corrosion by a metallic spray coating 'E'; this also improves the electrical contact.

Sensitivity
The average current into a low load resistance is given in the table. The active area of the cell comes to about 3mm from the edge - the diameter of the active area is thus about 6mm less than that of the cell in
A/lux/sqcm: Type B 0.07, Type M 0.03 and Type MFII 0.028

Megatron photo-electric cells are chiefly used for light meters, exposure meters, and other light measuring devices such as lighting controls for buildings. They are also used for a wide range of other instruments such as colorimeters, color temperature meters, smoke and turbidity measuring equipment as well as a number of devices such as flash actuators, smoke detectors and alarms, and aligning mechanisms for paper.

Performance
Each individual photocell is thoroughly aged, checked and re-checked being dispatched.
Cells have a more linear performance: when the load resistance is low, the cell is small, at low illumination intensities. Megatron manufacture two different types of cell, the Type M cell is also available with a special filter:

Type B Cells - general purpose, high stability and sensitivity, used for light meters, exposure meters, etc.
Type M Cells - are specially manufactures for photometry, as their spectral sensitivity, without the use of filters, closely matches the CIE standard observer,
Type MF II - uses a filter giving even better performance.
http://www.megatron.co.uk/selenium/index.html


 

 

 

The first Gossen electric light meters for photographic exposure and cine applications were made with the selenium photo-electric cell. These meters were equipped with a button for high and low light intensity reading.

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.

graph-Type B

This is an curve from a Megatron cell type B
 


CdS, Cadmium sulfide Cell

After the selenium cell, the electronic world came with the CdS cell, this is a light dependent resistor or cadmium sulfide cell of which the resistance of the cell decreases when light shining on the element increases. This change in resistance influences the current running trough the measuring circuit. A disadvantage is the need for an external power source like a battery while the selenium cell produces the power by it self due to light shining on the element. In the period when the CDS meters were build, a popular battery was the mercury cell with 1.35 Volts of power. However these cells are band because they contain a lot of mercury which is very bad for the environment. The cell that replaces the mercury cell is the zinc-air cell but this one has a lot of disadvantages like the very high price and short life time. Another disadvantage is the memory property of the CdS cell after the last measurement of bright light. This only takes a few minutes to disappear but the photographer must take this in consideration when doing the next measurement.

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.
Giant CDS Cell Assortment

SBC, silicon blue cell

The silicon blue cell is the next step in the improvement of light measuring sensors. The photodiode is very fast, stable and can be color corrected to match the correct measurement of light. In some cases 2 photodiodes are used (Multisix, Lunasix F) to get a better result in light measurement. Sometimes a combination of a photo diode and photo transistor are used. The sensor is sensitive for

Most of the modern exposure meters are now equipped with the SBC sensor. Meters equipped with this kind of sensor can be recognized by the little 'sbc' shield on the meter.

The SPD cell or Silicon Photo Diode and the SBC Silicon Blue cell are both sensitive for infrared light and needs to be corrected with a filter. However the GPD or GAP Gallium Arsenide Photodiode is not sensitive for IR light.

The sharps silicon blue cell manufactured by photain controls is claimed to be the world's first photo-electric diode possessing high sensitivity over the entire visible light spectrum. It is more reliable than the selenium or cadmium-sulphide photo-cells and has superior time response. No bias power is required, it has a lower noise level than the other two type and it is non-directional.
 


Type of meters

There are differences in type of meters actually in the way how to work with them. The oldest meters consist of 2 parts, a meter measuring a voltage (or current) and pointing to a value on a scale. and a kind of calculator disk to calculate the exposure by transferring the measured value to the calculator disk in some way. there is no electrical or mechanical connection between the 2 parts of the meter.

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.

Another type of meter is the math needle meter. Here we have also a meter for measuring the light and a calculator disk for calculating the exposure vales. But instead of reading a value from the scale, there is another needle in the meter that can be placed directly above the light meter needle so the match in position. The movement of the match needle is done by for instance turning the calculator disk. As soon as the 2 needles match, we can read the exposure values in aperture and time from the calculator disk.

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.

In order to get the correct exposure, you not only have to know the average reading , but you must also know the total range of brightness in the scene. Some Gossen meters operate on the zero metering principle (The null system). You simply align the needle on zero and the meter's readout automatically shows you all applicable aperture / shutter speed combinations in 1/3 stop increments. No more reading a numerical value, setting it on the dial and then reading out the exposure values.

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.

Copyright E. van der Aa 2010, All rights reserved.