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May 23, 2016

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Facilities: CCTV camera settings getting the best picture possible

As part of SHP’s focus on facilities Simon Lambert BSc (Hons), MIET, MASC, RISC, CCTV Consultant, Lambert & Associates looks at how to get the best out of CCTV. 

Do you want your CCTV images to be ‘ho-hum’ or ‘wow’? Installers and owners who are happy with ‘ho-hum’ read no further.

People who are willing to make the effort for ‘wow’ can read in the next few minutes what can make a big difference to their CCTV clarity.

It’s fairly well known that you need enough pixels in your image to achieve the details you need. However, it still frightens me how many installers and owners I meet who actually don’t understand this basic concept.

They genuinely and mistakenly think buying HD cameras and relying on CSI labs will be enough! No it won’t.

Without sufficient pixels in the face you want to identify, magic won’t make them appear. Let’s see some real world captures from an HD camera I tested.

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The left-hand Digikin target is 550 pixels high. Zooming the lens out made the Digikin in the centre become only 285 pixels high (about half as many).

Guess what? There’s only half as much small detail can be resolved. It’s really that simple.

The right-hand image is the small one magnified so you can compare its limitations. Same camera, same day, same zoom lens but, importantly, no Photoshop fakery that we see from so many CCTV advertisers.

Ok, so you have enough ‘pixels on target’ for identification, but what if your target is moving? People often walk. Using one of the UK Home Office CCTV test faces we can see how sufficient detail can still get destroyed (see below right).

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The left-hand image has a camera shutter of 1/30s, which is probably true of every camera that’s fitted and left on factory default camera settings. The right-hand image has the shutter deliberately shortened to 1/125s. See the advantage?

The factory default CCTV camera settings give us motion blur at walking speed which destroys the identification details that all those pixels were meant to give us. You need to set a sufficiently fast shutter to freeze the motion.

Almost no one ever sets this up properly. Be the one who does. Bear in mind that this will naturally reduce night-time sensitivity so make sure there is enough light.

Noise created by cameras struggling in low light can also destroy details that would be kept in good light. So don’t ever believe datasheets saying a camera is good down to 0.1 lux because your own proper testing is the only way to discover the truth.

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The left-hand night image (of the two on the immediate right) has insufficient light so the camera relies on its internal AGC (automatic gain control) to boost the weak signal, which boosts noise too.

The right-hand image has more light so the camera doesn’t need to boost the signal, which keeps noise much lower.

The brightness of these two images is similar. Nothing changed but the light provided. Clearly, noise reduces what detail you thought you would achieve. So you should limit AGC boost to, say, 24dB and compensate for the lower boost with better lighting.

You would be surprised at the visible differences between good quality lenses and how this can affect clarity.

The images below were taken with three examples of the same camera in the same room, each with a different varifocal lens from one famous, high quality manufacturer. The differences in sharpness are surprising. Can you imagine how much clarity could be lost using a cheap lens?

Any lens needs to be focused properly for good clarity. It never ceases to dismay me how many appear out of focus at night.

Incompetent installation and maintenance is to blame. The solution is easy but that needs to go in another article on another day.

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Nonetheless, when many modern cameras have automatic focus set-up tools built into their software let’s think about maximizing clarity over as wide a distance as possible by taking an example.

Let’s say your camera looks along a corridor 50 metres in length. It is fitted with a 9mm lens with an F1.6 aperture on a 1/3-inch camera.

So, to maximize the number of things in focus do we carefully focus on the further object of interest 50m away? No. We should focus it only 4.65m from the camera! That’s called the hyperfocal distance.

By definition, everything further away than the hyperfocal distance is in focus, so pretty much all of this long corridor. In addition, objects as close as 4.25m will be in focus when depth-of-field is shallowest at F1.6 in low light.

It will be even wider when the iris closes in brighter light. To calculate the hyperfocal distance for any camera and lens you can try this online calculator or a smartphone app for out on site. Get the most wide-ranging clarity when focusing any lens. It’s straight physics.

Analogue video is still popular in spite of growing IP and SDI video. With so much Cat6 cabling in the world the use of baluns is still very widespread for carrying analogue video but can destroy clarity if passive baluns are used according to some manufacturer’s advice.

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A quick-and-dirty workshop check with an analogue camera looking at a test card shows the out-of-the-camera clarity, above.

Then using passive baluns over Cat6 cable their manufacturer advises on the packaging that they are good for 400 metres. However, after 216 metres the high frequency content is far too weak (measuring the chroma burst on an oscilloscope) to meet ITU-R PAL video standards.
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The loss of picture detail can be seen above where horizontal resolution drops from around 450TVL to 400TVL. How bad would this loss of clarity be over the full 400m they recommend? I dread to think, and so should you if you go above 100m with them.

If you’re using cheap, fake ‘Cat5’ that is really copper coated aluminium (CCA) then distances much more than 60m will see too much signal loss. Try it on your workshop bench. You’ll see.

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Light levels at night can easily fool people too. We check with our light meter under the street lamp (see images on the right) and measure 12 lux but that is horizontal.

This value is much higher than the lux measured on the vertical human faces which we’re interested in seeing clearly. Measure this instead, facing towards the camera location, for a correct design. Here it is only 3 lux.

Not only that, but make sure the lamp is not behind our target. Otherwise all that light simply creates a silhouette with no facial details because now we measure only 0.3 lux on the target towards the camera.

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If you can’t move the light, try and counteract it by adding frontal lighting from the camera’s perspective. In the final image, below, the very bright backlighting in the scene is effectively counteracted by sufficient infrared from a lamp adjacent to the camera.

I hope these ideas have been useful. You might not think that each of these makes much of a difference to the image clarity that you achieve, but if you employ several or all of them the results will be multiplied together.

That’s how to get from ‘ho-hum’ to ‘wow’. The improvement in clarity could be very worthwhile.

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Simon Lambert BSc (Hons), MIET, MASC, RISC, is a CCTV Consultant at Lambert & Associates. This article was originally published on IFSEC Global

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