Difference between revisions of "Selenium meter"

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|image_text= Typical selenium meter window<br>-here on a [[Gossen]] Sixon
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|image_text= Typical selenium meter window,<br/>here on a [[Sixon|Gossen Sixon]]
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|image_by=AWCam
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A '''selenium meter''' is a light measuring instrument. The most common usage of such [[light meter]]s is measuring the [[exposure value]] for photography. The electric parts of such a meter are an electromagnetic measuring instrument which is connected to anode and cathode of a [[selenium]] photo cell that produces more or less electric power when exposed to more or less light. The optical part of such a meter is a window in front of the photo cell's light sensitive side. The window's surface is usually structured like a honeycomb made of convex lenses. This type of window helps to bundle the light coming from the direction to where the photo cell is directed. The mechanical part of a selenium meter is an analog calculator which accepts exposure value and film speed as input parameters for showing the possible aperture/shutter speed combinations for correct exposure.
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A '''selenium meter''' is a photoelectric [[light meter]], used to determine exposure settings. In the middle four decades of the 20th century, light meters based on [[selenium]] electrical cells were the best available to photographers, and were included in many camera designs. By the mid-1960s, physically smaller [[CdS | CdS cells]] had replaced selenium in most uses.
  
===match-needle meters===
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A [[selenium]] photocell produces more or less electric power when exposed to more or less light. Its output of electrical current is connected an indicator, typically a pivoting electromagnetic coil with a lightweight needle attached. The optical part of such a meter is a lens array in front of the photocell's light sensitive side. This is usually structured like a honeycomb made of convex lenses, which narrows the angle of view to which the meter is sensitive. The remaining part of the light meter is an exposure calculator (often in concentric dials), which takes the light reading and film speed as input parameters, then shows the possible aperture/shutter speed combinations for correct exposure.
The simplest type of match-needle selenium meter shows a clockhand on the meter's scale. This can be moved by turning one slice of the analog calculator. When the clockhand matches the instrument's needle the EV-value is set right on the calculator.
 
  
Another type is common especially where a selenium meter is built into a camera. Then the scale of the instrument shows just one mark. When the needle matches that mark the user can see appropriate aperture/shutter-speed combinations on the analog calculator.
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Dr. Bruno Lange in Germany developed the first practical selenium photoelectric cell. But the [[Rhamstine]]'s Electrophot DH of 1931 is believed to be the first selenium light meter manufactured for photographic purposes.<ref>"J. Thos. Rhamstine and the First Electric Exposure Meter" by Richard W. Holzman in Photogram Newsletter, April-May 2008 published by [https://sites.google.com/site/miphsmain/the-photogram The Michigan Photographic Historical Society]</ref>
  
====coupled meters====
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As with other light-meter technologies, selenium meters may be entirely separate from the camera, or be made an integral part of one. Large selenium cells, with their "fly's eye" covering, greatly altered the appearance of many mid-century cameras.  
More sophisticated cameras have the match-needle instrument coupled directly to aperture and shutter speed setting rings on the lens tube instead to a separate analog calculator. This is the most convenient way of match-needle instrument usage, especially when the meter's scale is mirrored into the viewfinder.
 
  
In the beginnings of exposure automation (ca. 1960) the instruments were even used for setting exposure settings directly, mainly for automatic aperture setting. A simple method for this was called "trap-needle"; pressing the shutter release mechanically gripped the meter needle, then moved an aperture control up to hit the needle, setting the aperture to a value controlled by the meter.
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Built-in selenium meters are often coupled to a camera's shutter speed control, with a read-out indicating the appropriate aperture (or, vice versa). More sophisticated camera meters may be coupled to both—or even offer exposure control that is fully automatic. (However this was uncommon during the era when selenium cells prevailed.)
This kind of attempt to automate exposure setting was common in an era were precision mechanical engineers didn't know any limits of complication so that even further steps were gone to to use selenium photo cells to control all exposure settings, or to control them more exactly with help of electro-mechanical aids. [[Heinz Waaske]]'s SLR construction ''Edixa Electronica'' was such a costly attempt. The construction problem was the low electric power delivered by the photo cell. It was a precision mechanical wonder that a handful of cameras were made which had automatic exposure control driven by a selenium meter, without need of further electronic or electromechanical support, for example the [[Optima]].
 
  
A compromise for expensive [[rangefinder camera|rangefinder]] or [[SLR]] cameras was to offer a selenium meter as optional device that could be coupled to the cameras's shutter speed controls. The scale of these instruments show not an exposure value but an aperture value to be set "uncoupled". An example is given by the [[Minolta SR-1]].
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Selenium cells may generate less current after many years of exposure to light, heat, and moisture; and thus old selenium meters are often inaccurate (or dead) today. However examples do surface of aged selenium meters still in perfect working order.
  
 
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||[http://www.flickr.com/photos/amberflykezzie/3239063926/in/pool-camerawiki http://farm4.static.flickr.com/3090/3239063926_6ed206d668_m.jpg]
 
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||modern [[Gossen]]<br>match-needle<br>selenium meter
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||modern [[Gossen]] Bix 3<br>match-needle<br>selenium meter<br/><small>by {{image author|Uwe Kulick}}</small> {{with permission}}
||[[Paxette electromatic]] with<br>meter controlled aperture<br>and light sufficiency signal
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||[[Paxette electromatic]] with<br>meter controlled aperture<br>and light sufficiency signal<br/><small>by {{image author|Paul Analog}}</small> {{with permission}}
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||[[Weston]] Master V<br/><small>by {{image author|Howard Somerville}}</small> {{with permission}}
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||[[Samoca]] miniature meter to be<br/>mounted on camera's accessory shoe<br/><small>by {{image author|Steve Chatfield}}</small> {{non-commercial}}
 
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==Notes==
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<references/>
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{{glossary}}
 
{{glossary}}
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[[Category: Camera parts]]
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[[Category: meters|*]]

Revision as of 16:39, 1 February 2012

A selenium meter is a photoelectric light meter, used to determine exposure settings. In the middle four decades of the 20th century, light meters based on selenium electrical cells were the best available to photographers, and were included in many camera designs. By the mid-1960s, physically smaller CdS cells had replaced selenium in most uses.

A selenium photocell produces more or less electric power when exposed to more or less light. Its output of electrical current is connected an indicator, typically a pivoting electromagnetic coil with a lightweight needle attached. The optical part of such a meter is a lens array in front of the photocell's light sensitive side. This is usually structured like a honeycomb made of convex lenses, which narrows the angle of view to which the meter is sensitive. The remaining part of the light meter is an exposure calculator (often in concentric dials), which takes the light reading and film speed as input parameters, then shows the possible aperture/shutter speed combinations for correct exposure.

Dr. Bruno Lange in Germany developed the first practical selenium photoelectric cell. But the Rhamstine's Electrophot DH of 1931 is believed to be the first selenium light meter manufactured for photographic purposes.[1]

As with other light-meter technologies, selenium meters may be entirely separate from the camera, or be made an integral part of one. Large selenium cells, with their "fly's eye" covering, greatly altered the appearance of many mid-century cameras.

Built-in selenium meters are often coupled to a camera's shutter speed control, with a read-out indicating the appropriate aperture (or, vice versa). More sophisticated camera meters may be coupled to both—or even offer exposure control that is fully automatic. (However this was uncommon during the era when selenium cells prevailed.)

Selenium cells may generate less current after many years of exposure to light, heat, and moisture; and thus old selenium meters are often inaccurate (or dead) today. However examples do surface of aged selenium meters still in perfect working order.

Notes

  1. "J. Thos. Rhamstine and the First Electric Exposure Meter" by Richard W. Holzman in Photogram Newsletter, April-May 2008 published by The Michigan Photographic Historical Society


Glossary Terms