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System Design

Minimal Light Applications

One of the major ‘battlegrounds’ of video surveillance is low light performance. The vast majority of surveillance applications are in action around-the-clock, and given that many crimes take place in the hours of darkness, protection during this period is vital. There are a number of ways to improve on video performance in minimal light situations, but which gives the best results?

By its very nature, security-based video surveillance needs to work 24 hours a day, seven days a week. It needs to be at its proactive when risks of an attack or event are at their highest. In short, when the risk increases, reliance on video surveillance also becomes more important.

In many risk-based scenarios involving crime, attacks or criminal activity, risks increase when light levels fall. It’s a simple formula. Criminals want to evade detection, and therefore there is less chance of them being spotted by passers by or persons at a site if it is dark. Additionally, most sites are less populated – or even unpopulated – during the hours of darkness.

The practice of flooding an area with light has fallen from favour for good reason. Firstly, concerns about light pollution have meant that sites need to take their responsibilities seriously. Secondly, lighting up a site throughout all periods of darkness is not a cost-effective solution. Finally, those viewing a well illuminated site will often assume that any activity is innocuous, as a well lit site is indicative of being operational.

Therefore, video surveillance has to contend with delivering video streams for security purposes – detection and identification – in circumstances that are often far from ideal. Often light levels will be minimal, supplementary illumination will be necessary, as will a wide range of considerations to ensure the cameras can operate to an acceptable standard.

Of course, it’s all too easy to point out today’s cameras with their outstanding performance in low light applications. If a camera can operate in 0.001 lux, how much light is required?

Performance criteria

Light is an essential element for the generation of any video image. If there is no light, then there is no video image. There is no way of getting around that fact. If no light falls onto the chipset of the camera, no signal can be produced. Cameras need light, at a certain level, if they are to produce anything in terms of an image. The minimum amount of light required for a camera to operate is typically quoted as its sensitivity. The question, however, is what degree of operation is achievable?

In the past 20 years, the video surveillance sector has seen a lot of technological advances. Manufacturers have R&D teams working around the clock to deliver an ever higher level of performance to those seeking to deploy video systems. Resolutions have been raised from 330 TV lines to today’s HD and 4K UHD streams, with higher multi-megapixel devices also available.

Resolution hasn’t been the only advance. Functionality has also increased. Previously surveillance cameras offered a few basic image manipulation functions. Additional features such as WDR, privacy masking, multi-streaming, VMD and IVA all started out as select options on top-of-the-range devices, before becoming standard features.

It is fair to say that over the years video surveillance has developed significantly in terms of performance. If you look at the specifications for products over this period of change, you’ll see a significant shift in figures quoted for sensitivity.

This specification should give an indication of a camera’s ability to deliver a video signal in low light, and as such is – or at least should be – a very important piece of information when selecting a device for use in minimal light situations.

Unfortunately, there is no defined or standardised method for measuring sensitivity. In the past there was an industry ‘understanding’ that sensitivity should identify the lowest light level, measured at the viewed scene, which allowed the camera to deliver a 1 volt peak-to-peak signal with all processing switched off. This measurement was taken using an F1.2 lens.

Using this method of measurement, mainstream colour cameras typically had a sensitivity of 8-10 lux. Today, it is not uncommon to see quoted figures of 0.001 lux.

You would be forgiven for thinking that a drop in specifications from 10 lux to 0.001 lux is a sign of great innovation, of rapid developments in technology, and therefore a cause for celebration in the surveillance sector. However, it’s not the full story.

Whilst some advances have been made with regard to low light performance, the main reason for these dramatic reductions is because manufacturers measure sensitivity in different ways.

Sensitivity can be affected by a wide range of different variables: whether measurements are taken at the viewed scene or the faceplate, scene reflectance, shutter speed and frame integration, gain levels, lens aperture, video signal quality, etc..

The significantly grey area of what represents usable video adds to the chaos. While one user will claim ‘usable’ is the ability to identify an intruder, another might see ‘usable’ as being able to tell that something – anything – is there, without any need for identification.

As such, it is not unusual to see sensitivity figures which, when analysed, will very obviously give a final image that simply will not be suitable for surveillance. It must be remembered that sensitivity figures do not in any way imply that the video captured at the stated levels will be fit for purpose.

As processing quality increases, many of the leading camera manufacturers are including specialised technologies which help boost performance in low light applications. Even with these recent advances, the battle remains how to increase the level of light falling onto the sensor’s picture elements. The challenge is amplified by the fact that as resolutions increase, pixels get smaller. This means that low light performance requires more light falling onto a smaller surface area.

The best option

If you’re expecting some magical formula that makes defeating minimal light situations a breeze, you might be disappointed. Well, you might not be, because there is a simple cure but it’s the one we are all aware of. Add more light. That’s it; nothing does the job better than additional illumination!

Where light pollution isn’t an issue, white light can allow colour images to be retained. Also, if used in a triggered application, it can be used to make any events obvious to passers-by and onlookers. It can also help with site management issues during the winter months when an application may be open for business in hours of darkness.

Alternatively, if light pollution is a concern, or where a more discreet solution is preferred, infrared illumination offers a viable choice. While the industry is seeing a growing number of cameras with integral illuminators, there are arguments to go for standalone options.

Standalone illuminators offer more flexibility when it comes to the location of the light source. It’s not always the best option to have the light on the same axis as the camera. Also, standalone devices offer a wider choice when it comes to coverage angles and range. Integral illuminators are typically specified with restrictions to size, power and adjustability; they are also built to a cost. Dedicated illuminators can be selected for each individual camera view.

Many standalone illuminators make installation a simpler task. Most manufacturers offer PoE versions of their illuminators, and with a number of cameras offering RJ45 power outputs, connecting an illuminator via an edge device makes financial sense.

Adding illumination is also a very cost-effective solution if you consider the price/performance ratio. As with any performance-based developments, changes at the lower end of the curve inevitably deliver a decent boost for an acceptable cost. However, as you get closer to the top limit of performance, incremental changes can carry increasingly high prices.

Adding illumination can deliver significantly better low light performance to an average camera. The performance gain makes the cost less of an issue. With the other choices that follow, the actual benefits will be less impressive and will only really add value if used with cameras that are very high specification. In short, if you’re looking for that ‘silver bullet’ solution, add illumination!

Other considerations

If you think back to earlier, it was mentioned that getting the right level of lights to fall on the camera’s sensor is more difficult as resolutions rise. This has some grounding in basic physics.

Theory indicates that the low light performance of higher resolution devices will often not be as good as it is with standard definition cameras. The reason that higher resolution cameras could have inferior low light performance has to do with pixel size and density on a chip.

Image sensors require light to fall onto the pixels to create the charge which sets the relative values for each individual picture element. If you consider a 1/3 inch D1 camera chip, it contains around 400,000 pixels. In effect, this means that the surface of the chip is divided up into 400,000 picture elements. Onto each one of these, a sufficient level of light must fall to create a signal every time a frame of video is created. The strength of the light dictates the level of the signal, which in turn decides the value of that pixel. Indeed, the pixels also have separate elements for red, green and blue.

The size of the picture elements affects the low light capabilities of the camera, which is why 1/2 inch sensors are typically better performers in low light than 1/3 inch sensors.

With this in mind, consider that an HD camera with a 1/3 inch sensor has around 2 million pixels and a 4K UHD camera with a similar size chip has over 8 million picture elements. Because the pixels are subsequently so much smaller, getting the required level of light to fall on each element can be a challenge.

The theory that higher resolution video cameras may not match standard definition cameras with regard to low light performance must be put into context. It only considers chip size and pixel density. Resolution is not the only thing to have improved in recent years.

Processing power has also gone through the roof, enabling the development of enhanced feature sets in the better cameras.

Most credible manufacturers will include functions designed to enhance low light performance. These vary from the simplistic through to the complex. It is worth bearing in mind that with any processing, every additional features does has an impact on other areas of performance. However, if the right balance is maintained, then installers and integrators can enjoy some significant benefits.

Processing approaches vary and include intelligent signal boosting and light scavenging.

Other approaches such as frame integration and sens-up are well established, but cannot be used if the system is specified as 4k UHD or HD compliant as the techniques effectively reduce the overall frame rate.

One point to consider is the size of the chipset. Many higher resolution cameras offer a wide variety in chip sizes, with cameras using 1/2 inch or 1/1.9 inch sensors becoming more common. If these cameras are deployed with high quality lenses, the challenge of getting light to fall on the pixels is made less onerous.

In summary

Dealing with minimal light in security environments has been a challenge for many years, and going forwards it will remain a challenge. The bottom line is this: without light you have no video.

That said, the leading manufacturers are pushing low light performance to levels that previously haven’t be possible, and clean colour video at 3, 2 and even 1 lux is a reality. Where consideration is paid to larger sensors and high quality optics, performance improves. Plus, you could always just add more light…

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