Why Wont My CCTV Capture Number plates?

This is a common problem people encounter when trying to review footage and hopefully catch that car of interest or that person running past the property. The problem of blurry number plates and even blurred faces seems even worse in the dark, but why?

The fact is that not many CCTV cameras that are intended for general monitoring and observation of a wide area can't accuratly capture moving objects very well.

It is only partly to do with the manufacturer, resolution, mega-pixel or cost of the camera but is more to do with the set up and expectations of the system.

Of coarse a poor quality camera wont perform as well as a higher grade model but it is not always an indicator of performance in regards to motion blur. Many high-end models can suffer from motion blur if not set up correctly.

Theory Behind CCTV Image Capture

How this all works is described with a small equation called the "exposure triangle". The exposure triangle describes the relationship between the depth of field which is determind by the aperture, motion blur which is determined by the shutter speed and the image noise which is a result of the ISO sensitivity.

A picture illustrating the exposure triangle.

Before we go any further it is important to understand what these settings mean.

Lets Capture Some Car Number Plates

Now we have a handle of some of the jargon and terminology we can explain how all this relates to being able to identify that car that threw rubbish into your garden at one in the morning.

To acheive bright sharp and clear images with CCTV we need to be able to let enough light into the camera sensor to produce these images reliably. Failure to do this results in dark and grainy images. It can also affect the ability for the camera to focus correctly.

Now we know that increasing the shutter speed reduces motion blur and longer shutter times create more blur. It is then easy to think that increasing the shutter speed is the answer. Well not really, since each of the three variables in the triangle have an effect on each other, you can't simply adjust one without the other.

When we increase the shutter speed we actually reduce the amount of light available to the image sensor. Ahhh, so why don't we just increase the shutter speed then increase the ISO to make it brighter.

You guessed it, the image noise increases, you could in theory increase the aperture but most CCTV doesn't have an aperture adjustment. They have a fixed aperture, as having an adjustable aperture would make them prohibitively expensive.

In this circumstance you can make an adjustment to the shutter speed and find a balance between the picture having enough light going on to the sensor to facilitate an acceptable image of the general area but have a fast enough shutter speed to reduce motion blur.

This of coarse can be a compromise so having the correct cameras from the get go can pay dividens.

Select the Right CCTV Camera for the Job

This brings us to initial camera selection and how important it is to understand the intention of the CCTV system.

There are CCTV cameras available with bigger apertures and these can negate some of the issues with cheaper cameras. A good example of a camera that is more suited to capturing a moving vehicle is the Hikvision Darkfighter range of CCTV hardware.

Still, there will be an issue with capturing plates reliably when the vehicle is moving quickly as physics doesn't allow us to escape the exposure triangle so planning is essential.

If capturing number plates is a must, this is where a dedicated CCTV camera that can reliably capture number plate detail is needed. These cameras use the exposure triangle to their advatage, the exposure or "shutter speed" is extremly fast and produces a very dark picture except the number plate. The light is projected back to the camera from the number plates reflective surface allowing the camera to accuratly capture the plate. The motion blur is reduced to a minimum because of the fast shutter speed.

These specialist cameras known as ANPR are coupled with a suitable NVR or "network video recorder" that can decipher the text or digits on the plate and log them as an exportable file.

Very often we couple the ANPR camera with a regular CCTV camera which is known as an "overview" camera. This allows you to capture regular images and identify the vehicle colour etc and other goings on in the area."

Not only can the system log number plates it can perform actions based on number plate reads like send an alarm or open a gate for example. More information on these cameras can be found on our ANPR & vehicular detection page.

How the Enviroment Affects Image Quality

It is important to highlight how critical light is for any CCTV system or any type of camera for that matter. A well lit area will provide far superior images compared to a poorly light area. Improving the available light should be the first step to reduce motion blur from any system.

Cameras that are subject to a lot of enviromental pollution can also get dirty very quickly so ongoing maintenance like cleaning will also help.

Other factors that affect the cameras performance with moving objects can include:

Conclusion

To gather reliable images from a system that can identify number plates, the appropriate cameras need to be selected and they need to be set up correctly.

There are dedicated systems designed purely for capturing car registration plates. What you should be aiming for with a system would depend on the individual needs of the site or area where the cameras are being deployed.