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 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.

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.

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.

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.

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 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.