Backlight Compensation

From CCTV Information
Jump to: navigation, search

Let There Be Light, or: Lux On Your Side

Love them or loathe them, it is increasingly difficult to escape the unblinking attention of CCTV cameras as we go about our business, writes Mike Tennent of Tavcom Training.

The UK in particular has invested enormous faith and impressive amounts of capital in these electronic security systems and the resulting statistics and moral dilemmas fuel fascinating arguments and debate from Parliament to the lowliest saloon bar. Although Britain undoubtedly leads the way in CCTV development, the trend is growing internationally. Ironically, the largest CCTV system in the world is not in the UK but at Singapore Airport. Its 3,000 cameras put the mere 500 or so in Harrods London store to shame! So, CCTV whatever the pros and cons seems to be with us to stay. They will continue to be the subject of enormous expenditure, not only by private enterprise but importantly - by those who hold the strings of the public purse. In these circumstances it is crucial that CCTV systems are deployed to their very best advantage. If they are to become a permanent part of our modern existence we must at least make certain that they earn their keep by doing their various tasks in the most productive and cost efficient ways. Sadly, this is often far from the norm as many systems struggle to perform to their maximum potential in the face of some readily avoidable problems. One of the most common of these elementary failures is the concept of 'fighting light'. In any successful form of photography the camera lens and recording medium must make a friend of light. In some 'creative' spheres this may mean an artistic use of such elements as contrast, grain and flare but in the world of surveillance camera systems nothing could or should be further from the truth. Good, clear illumination is essential to recognition and detection and yet time and again our expensive technology is made to fight against the direction, quality and quantity of the light that it encounters.


BLC may well stand for bacon, lettuce and cucumber in the catering trade but in the security industry's glossary of abbreviations it denotes 'Back Light Compensation'. In a nutshell, this is the camera's method of dealing with a common and often unnecessary problem trying to register detail in a target that is harshly silhouetted against a bright light source. This problem and its 'cure' is almost a tradition in our industry. "My peers did it before so why should we change?" is often the mentality as back light compensation is happily calculated and applied. Sadly, even with the very best technology using well considered algorithms to reduce the silhouette effect it still is a compromise solution when combating the natural effects of light.

Lux2.jpg Lux3.jpg
Fighting light: in too many instances CCTV cameras are not used to their best advantage. When positioned facing the bright outside daylight this camera struggles to record anything other than a dark silhouette as our target individual enters the building Working with light: the same doorway in the same conditions viewed from the opposite direction. The camera uses the abundant daylight falling on the subject as she leaves the building and is able to produce an excellent and informative image

Check In Lane

Why do so many choose to fight light in this way when the solutions are so simple? Why do we always want to see a person coming into a shop, airport, bank or office? Why are the cameras mounted facing the outside sunshine which naturally causes a dramatic silhouette effect? Why do we not locate the camera facing into the building so we can see the culprits, victims or whoever coming out? This would give us an abundance of lovely natural light shining onto people's faces as they came towards the exit doors and provide really excellent picture evidential - information. Why not consider setting up a simple airport-style "checking in lane" for people entering your building? A simple, unobtrusive tape could guide all incoming visitors along a route where well-sited cameras could capture top quality pictures with no excessive contrast.

No Choice

Sometimes there is really no other choice of camera positioning due to operational and logistical issues. Additionally, this simple reversal of positioning the camera would certainly dismay the sales people! They love to sell you their more expensive cameras, festooned with features guaranteed to make your life a nightmare as you struggle to gain valuable pictorial information against all these unnecessary odds! A simple change of mindset could nullify many of these problems and all manufacturers, I suspect, would far rather your cameras produced the very best images of which they are capable and for which they were designed.


The opportunities for today's structural architects to play with the nature and direction of light have never been more tempting! Downlighting, uplighting, dimmers and filters, lightbulbs in the rhododendrons and office buildings ablaze with light from within all night and every night! Unfortunately these pleasing aesthetics often cause huge problems for the camera systems that are struggling to offer protection to these beautifully illuminated structures! Low light levels, flare and silhouetting are just a few of the difficulties that inevitably arise. While few people would suggest that all lighting design be undertaken with only security considerations in mind, more thought must be given to achieving a sensible balance between safety and the scenery. If the well-being of our buildings and their inhabitants is thought to be of value, architects and CCTV consultants must work closely together as a matter of course. Only by such close collaboration can we ensure that property is not left accidentally undefended by cameras that are once again 'fighting light'.

Colour Co-ordination

The colour rendition of many types of everyday environmental lighting is another rich source of potential - and often conflicting - problems. For example, in a perfect world, the hard pressed camera sensor might choose to exist in a flood of metal halide (HPI) light. This form of lighting, which is often found at airports, docks and similar facilities, produces a cool clear white light with excellent colour discrimination - perfect for the camera. And yet there is trouble in paradise! In this case the 'fight' can be against the existence of too much light for the comfort of the community. Light pollution is a legitimate concern where metal halide is employed so much consideration must be given to mitigating this negative effect through shielding and careful design if such technically superior systems are to be encouraged. At the other end of the scale low pressure sodium (SOX) is a highly efficient monochromatic light with a yellow tinge which has low running costs and is thus used mainly in street lighting. It is not, however, capable of colour discrimination so most objects in its presence appear as shades of grey. Monochrome cameras which outperform colour cameras in terms of low light ability and resolution or specialist day/night cameras can switch between colour and monochrome operation as lighting levels decrease are the 'order of the day' in these circumstances. CCTV consultants, specifiers and end-users should be discouraged from unnecessarily pitching expensive colour cameras into a fight they cannot win! High pressure sodium (SON) is an excellent compromise and is the dominant light source for lighting over long periods. It has an efficient golden white light source and is capable of good colour rendering. The 'fight' here is to persuade our local authorities and others to extend its use where CCTV coverage is deemed important.


Few areas of science and technology have such a plethora of wonderful definitions and descriptions as those pertaining to light! For example, the basic unit of light within the European Standards is the lux - the amount of visible light per square metre visible on a surface. In Europe this is measured with a lux meter although in the USA you would use a delightfully named foot-candle meter as the lux is also equivalent to 1 lumen per square metre or 0.093 foot-candles. Whichever definition you choose the underlying truth is that CCTV cameras are perpetually engaged in one final fight. Although it may seem simplistic to say so, in addition to needing light of the correct quality from a well-directed source, cameras require light in the right quantity. This is not always easy to find! Available light in our everyday lives varies to a surprising extent. A bright sunny day may generate 100,000 lux while the average operating table will be bathed in 20,000 lux. A hotel kitchen may enjoy 1,000 lux, a normal office 500 lux and a bank counter 200 lux. Compare this to street lighting which is typically between 5 and 50 lux and full moonlight weighs in at 0.1 lux! It takes considerable technical knowledge, skill and experience to ensure that the right camera systems with the right components are properly located and calibrated to take full advantage of these widely varying lighting conditions which are, themselves, subject to fluctuation due to time, season, weather conditions and other contributing factors. The opportunities to get the right camera in the wrong place or the wrong camera in the right place are legendary.

The Light Fantastic

Given light in the correct quantity, quality and emanating from the right direction, the modern CCTV camera is capable of minor miracles in terms of its ability to record events, aid identification and enhance our lives in a myriad of ways, not least the provision of surveillance and security. Conversely, if it is starved of those three simple elements, even the most expensive and technically advanced camera will struggle or fail to produce the results expected of it. For a CCTV camera 'fighting light' can all too easily become a fight for life - both operationally and commercially. With just a little more consideration and some small adjustments to our system design and installation practices we could ensure that far more of our many CCTV systems were functioning to the best of their very remarkable abilities.

Tavcom is one of the UK's leading Security Training Organisations. Click here to find out more about Tavcom Training and the courses they offer.

This article first appeared in Professional Security Magazine. Contact us if you are interested in finding out more about Professional Security Magazine or subscribing to it.