Blog posts tagged with 'PHOTOCELLS'


Hah, you all thought you had escaped the photocell tech-o-fest but today, we are going to talk about them further and in particular 'gas amplification,' an interesting phenomenon shown by gas filled photocells.

Although the manufacturing methodology is the same as for a standard thermionic valve, in the case of the gas filled photocell, a precise aliquot of a noble (inert) gas was introduced after a vacuuum had been pulled from the envelope.  The low vapour pressure of the injected gas means that it's molecules are distributed within the envelope and indeed the electrode cage.  In operation, a portion of cathode electron emission will collide with the inert gas molecules resulting in ionisation. The secondary emission electrons so produced would then be attracted to the anode and conversely, the gaseous cations would be attracted to the cathode where they 'belt it' so hard that even more cathode emission occurs.   The effect of this cascade reaction is that anode current would be greater than with an equivalent vacuum cell by up to a factor of 10X!  And this my friends, is 'gas amplification.'

Gas filled photocells are very sensitive to low light levels and can discriminate very small changes in light as may be seen with sound-on-film equipment.  Mullard produced a whole range of this type of photocell and so help you recognise them should you come across examples, I present below a photograph of the range, aren't they sweet: -



Photo-electric cells may be distingusished as belonging to one of three different types, these are: -

The barrier layer photo-electric cell; in which a voltage is produced when light falls upon the junction of two different materials. - a good example of this type is the Selenium cell fitted to the famed Weston Master (Megatron) light meters.

The photo-conductive cell; in which the resistance of the cell varies in proportion to the amount of light falling on them - a good example of this type is the Cadmium Sulphide (CdS)  cell which was the mainstay of most SLR cameras having through the lens (TTL) light metering from the early 1960s to the mid 1980s.

The photo-emissive cell; in which electrons (and hence a current) are liberated from the cathode when irradiated by light.

The first two types of photo-electric cell are mentioned for completeness and interest only and all further discussion will centre around the latter type - the photo-emissive cell which just like a typical radio valve, has an anode and a cathode.  The cathode of a photocell consists of a flat metal plate coated with a photo-emissive material.  The anode typically takes the form of a metal rod or loop situated in front of the emissive cathode.




The operation of any valve type depends on the current flow caused by drawing free electrons from an emissive cathode to the anode by a positive potential being applied to the anode.  By applying a potential to grids interposed between the anode and cathode of a valve, the flow of electrons may be controlled - vary the grid voltage and a corresponding variation in anode current results which may be greater than the voltage applied to the grid and hence this is how a valve ampifies a signal.

However, rather than using grid potential to produce an electrical variation, we sometimes need to produce electrical variations which correspond to variations in light and this is where the photocell comes in.    To understand the photocell we have to do a little chemistry, as we know, some elements are very stable, however others are less so.  In Caesium, the energy from light can upset a Caesium atom's stability and force it to give up electrons.   However, 'dope' Caesium with Silver and Antimony to form an amalgam and an element of stability is introduced whereby only specific quantities of electrons are produced in reaction to a specific level of light and it's colour and it is on this basic premise that a photocell works  Right, that's enough chemistry for today so for anyone still awake, I shall just let you know that in my next blog entry we will look at types of photo-electric devices in a little more detail


An acquaintance came to see me, all self important and gloating - he had a Baird Televisor lamp and it was picked up for a fraction of it's true value, obtained for a song and Stukey Bill had sat in front of it for ages, yeah, right.    Well done said I with a brittle smile and a demeanor tinged with a soupcon of envy, however, this soon evaporated just like his dreams of a healthy profit when he produced his prize with a flourish and I lampooned his hopes by showing him a photo from contemporary Mullard documentation showing a similar specimen of his 'Televisor lamp' : -  

Pretty good huh, but alas not Televisor material.   What my hapless pal had stumbled upon was a photocell designed for sound-on-film projectors, although there were no markings I pronounced it a Mullard emission photocell type 20CV - unfortunately, slightly less desirable than a Televisor lamp!!!

These devices crop up from time to time and the technology behind the is interesting and not often described so, in  my next blog entry, I shall describe the theory, operation and usage of these valves and indeed other photo-electric phenomena - I'll bet you can hardly wait!!!!