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
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
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:
Megatron manufacture two different
types of cell, the Type M cell is also available with a special filter:
- when the load resistance
- the cell is small
- at low illumination intensities.
Type B Cells - general
purpose, high stability and sensitivity, used for lightmeters, exposure
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.
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: