Reading Number Plates

[Rosko]

Hi there - - I'm intending fitting a camera to view cars and lorries coming into a yard and it would be a bonus if on occasion we could read the number plate. The distance involved is 80 metres - - has anyone any advice on what size lens I should use? - - and any other relevant points.

Thanks

[Service Engineer]

16mm + I reckon, depends on exactly how much else you want to see on the same camera.?

Have a look here: http://www.skywaysecurity.com/cctv_lenses.cfm#visual

[billythebellbox]

Hi there - - I'm intending fitting a camera to view cars and lorries coming into a yard and it would be a bonus if on occasion we could read the number plate. The distance involved is 80 metres - - has anyone any advice on what size lens I should use? - - and any other relevant points.

Thanks

a large zoom 8-80 or 7.5 to 120

you need to do a clac really, based on format, focal lenght type on plate is 80mm(ish)

[ilkie]

a large zoom 8-80 or 7.5 to 120

you need to do a clac really, based on format, focal lenght type on plate is 80mm(ish)

As a start, assume a screen height of 1.5m at a distance of 80m......

1/4" CCD = 128mm

1/3" CCD = 192mm

1/2" CCD = 256mm

And as the others will say, there are other factors you need to consider:- eg lighting, shutter speed, resolution, camera height etc

Ilkie

[Service Engineer]

I origionaly suggested 16mm+.

a large zoom 8-80 or 7.5 to 120

1/4" CCD = 128mm

1/3" CCD = 192mm

1/2" CCD = 256mm

I'm curious as to why my 16mm is so far out from suggestions after.

I'm no cctv engineer so am willing to stand corrected.

[djandysp]

I origionaly suggested 16mm+.

I'm curious as to why my 16mm is so far out from suggestions after.

I'm no cctv engineer so am willing to stand corrected.

16mm would probably do as youd get an image of the vehicle at the same time but if moving too quick then youd miss the plate, id go more for 8-80mm or 50 mm and have it fixed just for plates or even get an anpr system. The vista ones arent bad in quality or price, Derwent are brilliant but leave a big hole in the wallet

[ilkie]

I origionaly suggested 16mm+.

I'm curious as to why my 16mm is so far out from suggestions after.

I'm no cctv engineer so am willing to stand corrected.

A 16mm lens would provide the following scene size

1/4" CCD 16m wide x 12m high

1/3" CCD 24m wide x 18m high

1/2" CCD 32m wide x 24m high

If we look at the Home Office guidelines (yes I know I keep on about them)...........

say 120% of a 1.6m high target (Rotakin) with 16mm lens with a 1/4" CCD at 8.89m or 4.44m with a 1/2".

Incidently with a 3.6mm lens and a 1/4" CCD the same figure for recognition is just 2m.

Hope this helps

Ilkie

[arfur mo]

this is not as simple task as it might seem, and its not all about the lens - although it is a very important factor.

i find getting good clear number plate shots is often a vexed task especially at that sort distance, even if you get satisfactory images things like rain mist, fog and stray light sources can come into play and virtually knock out it's usefulness.

when i've tackled the problem i've had the vehicles either slowed down with humps or better stopped at barriers also funneled into a strict approach so the lens could be as narrow as possible. ideally reduce the distance so other influences have less effect.

number plates are not all at the same height so two high quality camera's with fast shutter settings to reduce blur are generally used.

once the camera's are set up it's then the question is what are you going to use to record it on? the best camera in the world will suffer from a poor recording unit much as the other way around.

tbh it can tax the best brain so your far better advised to ask a pro cctv installer in to quote, that way at least all the problems are theirs to get it to work as you want and i'd suggest can be cheaper in the long run.

regs

alan

[Doktor Jon]

I origionaly suggested 16mm+.

I'm curious as to why my 16mm is so far out from suggestions after.

I'm no cctv engineer so am willing to stand corrected.

As has already been mentioned, there are lots of factors / variables that need to be either accounted for, or else minimised in order to use CCTV cameras with a high degree of success, when capturing vehicle number plates.

Personally I generally tend to refer to vehicle plates in terms of their width, expressed as a percentage of the screen width.

It's something that over the years I've found is relatively easy to explain to end user clients; plus there is also the advantage that unlike plate height which relates to vertical resolution on cameras (something which is generally constant), width is easier to relate to different cameras that have various (horizontal) resolution figures quoted - so for example, a 400 line resolution camera will need to produce a slightly larger plate on screen, to achieve the same character legibility as a 570 line camera (bit of a generalisation, but I'm sure you get my drift).

Obviously, as arfur mo has correctly indicated, overall plate visibility, dirty housing windows, vehicle speed, angle that it presents to camera, and lighting, quality of camera, quality of lenses ( transmission, record system, display etc. etc. ) are just some of the more obvious factors that need to be considered.

Now if we take the 16mm lens as an example on a very usable format like 1/3", at roughly 35 metres from the camera, the plate will only appear to occupy about 5+% of the screen width, so would probably appear too small to be legible. At 65 metres distance, the percentage would drop to around 2.5%, and at 80 metres, it's not even worth calculating.

The only way it would work with a 16mm lens is if the imager were a decent MegaPixel device, and that's not really practical in many situations.

If a 50mm lens were used (generally my preferred optic for most vehicle plate recognition), at 50 metres it should provide a plate image which occupies about 10% of the screen width. Ideally, if the lens is set up to cover an 'optimised' recognition zone, perhaps 25 - 40 metres distance, if the camera is arranged correctly the plate will increase in size and clarity at a point nearer to the camera, although in general, light levels will also need to be raised for night use, so as to allow a lens to be stopped down, and the 'depth of field' increased accordingly.

In general terms, distant plate recognition (perhaps 70 metres plus) is not ideal as longer telephoto lenses are required ( as correctly stated by Ilkie ), and then there are problems with a very short depth of field, slow optics, more movement often from even a gentle breeze etc. etc.

[arfur mo]

As has already been mentioned, there are lots of factors / variables that need to be either accounted for, or else minimised in order to use CCTV cameras with a high degree of success, when capturing vehicle number plates.

Personally I generally tend to refer to vehicle plates in terms of their width, expressed as a percentage of the screen width.

It's something that over the years I've found is relatively easy to explain to end user clients; plus there is also the advantage that unlike plate height which relates to vertical resolution on cameras (something which is generally constant), width is easier to relate to different cameras that have various (horizontal) resolution figures quoted - so for example, a 400 line resolution camera will need to produce a slightly larger plate on screen, to achieve the same character legibility as a 570 line camera (bit of a generalisation, but I'm sure you get my drift).

Obviously, as arfur mo has correctly indicated, overall plate visibility, dirty housing windows, vehicle speed, angle that it presents to camera, and lighting, quality of camera, quality of lenses ( transmission, record system, display etc. etc. ) are just some of the more obvious factors that need to be considered.

Now if we take the 16mm lens as an example on a very usable format like 1/3", at roughly 35 metres from the camera, the plate will only appear to occupy about 5+% of the screen width, so would probably appear too small to be legible. At 65 metres distance, the percentage would drop to around 2.5%, and at 80 metres, it's not even worth calculating.

The only way it would work with a 16mm lens is if the imager were a decent MegaPixel device, and that's not really practical in many situations.

If a 50mm lens were used (generally my preferred optic for most vehicle plate recognition), at 50 metres it should provide a plate image which occupies about 10% of the screen width. Ideally, if the lens is set up to cover an 'optimised' recognition zone, perhaps 25 - 40 metres distance, if the camera is arranged correctly the plate will increase in size and clarity at a point nearer to the camera, although in general, light levels will also need to be raised for night use, so as to allow a lens to be stopped down, and the 'depth of field' increased accordingly.

In general terms, distant plate recognition (perhaps 70 metres plus) is not ideal as longer telephoto lenses are required ( as correctly stated by Ilkie ), and then there are problems with a very short depth of field, slow optics, more movement often from even a gentle breeze etc. etc.

not just because i was mentioned , but i'd vote the above as one of the clearest, illuminating and concise post's made in this forum

many thanks Doktor Jon

alan

(note to self - i now must try much harder).