Selenium Photo-Electric Cells

Megatron closed down operations at the end of February 2010. OTC provides these Megatron pages for information only.

Construction

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.

Selenium Photo-Electric Cell The diagram shows an idealised 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.


Uses

Megatron photo-electric cells are chiefly used for lightmeters, 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, colour 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 despatched.
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 lightmeters, 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.



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:

  ľA/lux/sqcm
Type B 0.07
Type M 0.03
Type MFII 0.028


Photocell Characteristics Current-illuminance Diagrams
graph-Type B graph-Type M&MF


Spectral Response and Cosine Correction

Two important characteristics of photocells, which can give rise to errors, are their Spectral Response and Cosine Correction. Further information can be found under Choosing a Lightmeter.

The Effects of Temperature

The temperature coefficient is dependant on load resistance and illuminance - higher load resistance and higher illuminance both making the coefficient increasingly negative. The temperature coefficient also depends on the wavelength of the light. At low illuminance and with a low load, the coefficient is negative and fairly uniform between 400nm and 600nm; between 600nm and 700nm it changes sign and is increasingly positive with longer wavelengths. This pattern is maintained with lighter load resistance, but shifted towards the negative.

Our Type B and M cells have a temperature coefficient of about -0.1% per degree C, with a low load resistance and white light.



Details and Dimensions of Standard Mounts
Cell 67 dia not potted
Mount cosine correcting
black 'Perspex' with transparent 'Perspex' dome
Cable 2m side
65 dia potted
Cell 67 dia potted
Mount black reinforced synthetic resin
Cable
or 6BA
Terminals
2m side
55 dia potted
Cell 55 dia potted
Mount black reinforced
bynthetic resin
with black anodised aluminium back
6BA
Terminals
Cell 45 dia not potted
Mount cosine correcting
black 'Perspex' with transparent 'Perspex' dome
Cable 2m side
45 dia potted
Cell 45 dia potted with glass face
Mount black reinforced synthetic resin
Cable
or 6BA
Terminals
2m side
p_dim-b.gif
Cell 25mm potted
Mount cosine correcting, black acetal with opal 'Perspex' diffuser
Cable 2m side
25mm potted
Cell 25mm potted
Mount cosine correcting, weatherproof, black acetal resin with opal 'Perspex' diffuser
Cable 2m side
16 dia potted
Cell 16 dia potted
Mount cosine correcting black reinforced synthetic resin with opal 'Perspex' diffuser,
thick mild steel base option (to act as weight)
Cable 2m side or rear
16 dia potted
Cell 16 dia potted
with glass face
Mount spun aluminium sealed with epoxy resin compound
Cable 25mm rear
50 x 37 potted
Cell 50 x 37 potted
with glass face
Mount black reinforced synthetic resin
4BA
Terminals

 

Price List

megatron - for measuring light
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