Welcome to the
"Technical Section"
of
Doktor Jons
Guide to Closed Circuit TV (CCTV)

All you need to know about lenses,
for all types of video surveillance.

FILTERS for CCTV lenses

Apart from the miniature ‘barrel’ type lenses mostly used on Printed Circuit Board cameras, and specialist optics like pinhole lenses, the majority of interchangeable ‘C’ (and to a lesser degree ‘CS’ ) mount optics on the market, will accept a wide range of front mounted filters.
Having said that, Doktor Jon would be surprised if any more than one in twenty thousand cameras ever gets anywhere near a filter.

Perhaps the main reason why filtration is rarely if ever used on CCTV cameras, is a glaringly obvious one - hardly anyone knows what filters are available, and how best to use them.

And yet the basics are in fact very straightforward.
Realistically, some cameras will not significantly benefit from the use of additional filters, but in certain key applications, the results can actually be quite dramatic.

There are essentially three main groups of filters that can be used in CCTV work, although in addition, a handful of specialist filters could be considered for ‘mission critical’ applications.
With very few exceptions, all filters are generally screwed onto the front of the optic, although the thread size does vary, depending on the overall diameter of the lens.

BLOCK FILTERS

Despite the name, these most definitely do not come in cube shapes.
The principle of a block filter is simply to pass certain frequencies of light, and normally hold back a specific section of the light (electromagnetic) spectrum.

It may come as something of a surprise, but without the humble block filter, most colour cameras would be producing some quite extraordinary images. If you remove the lens from a colour camera and look straight at the imager, you’ll hopefully see a blue coloured filter inside the camera. This block filter is specifically designed to cut out almost all Infra Red light from reaching the imager, as this would dramatically effect the overall colour accuracy of the picture.

Continued>

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