As the security industry and law enforcement bodies become ever more reliant on CCTV to increase the scope of surveillance, the quality of the images becomes increasingly important. Continual advances in camera technology are certainly doing their bit to achieve this but it is also important to take into account the environment that the cameras are observing.
In particular, the lighting of that environment can make an enormous difference to the usefulness of the camera images, as well as contributing to other aspects of security. Yet, all too often, a huge sum of money is allocated to the CCTV installation in terms of equipment and staff and the lighting is almost thrown in as an afterthought.
Furthermore, in situations where the lighting is designed in from the start, it is not uncommon for that lighting design to be downgraded to save money as other parts of the project go over-budget. But there are very good reasons for giving lighting a higher priority within the project budget.
Both inside and outside the buildings there are a number of factors that have to be taken into account to achieve the right conditions. This is why it is so important that the needs of the cameras are taken into consideration right from the start of the lighting design.
Indoors, for example, the lighting designer or architect may be more concerned with achieving decorative lighting effects that create a particular ambience or corporate image. Electrical engineers may put priority on reducing glare on computer screens and minimising the energy consumption per square metre – because that was their brief from their client – your company.
So it may require some forcefulness on the part of the security manager to get his or her say in this process. Fortunately security managers are not known for being backward in coming forward and it is perfectly possible to satisfy all of the requirements with little or no compromise. Nobody should be expecting security managers to become lighting experts but by appreciating the significance of lighting it’s easier to argue for security considerations to be taken seriously.
The effectiveness of the lighting is dependent on a number of factors and there’s no end of lighting boffins out there who are more than happy to make your head ache with technical terminology and fancy graphs. And, to be fair, it is the work of these boffins that has led to a greater understanding of the interaction between CCTV and lighting. Understanding that has led to practical advice on what sorts of lighting will give the best results.
The key factors relate to the vertical light levels, or illuminance, in relation to the position of the camera; the horizontal illuminance at the height where the camera is looking and how much light the various surfaces reflect back into the room. Other factors, such as how well the type of light shows up colours, also need to be taken into account.
Illuminating considerations
Research by Thorn Lighting into lighting schemes commonly used in banks and building societies has produced some very practical design advice that can be applied to just about any interior CCTV installation.
The research found that spot downlights, which are often used for decorative effects, create spots of high light intensity that distort facial features and alter colours, giving a false impression of the subject. These effects are most marked directly beneath the spotlights and improve if the subject is to the side of the lights.
As computers have become a standard desktop or counter top item, recessed fluorescent light fittings with louvres to prevent glare on visual display terminals have become almost ubiquitous. Although these ‘VDT’ louvres produce a wider distribution of light than spotlights, so there are fewer intense spots of light, they do not provide sufficient levels of contrast to give a good image. However, there are now special louvres, called CCTV louvres, which can be used to meet the needs of the cameras and the computer users.
One of the best schemes uses uplighters, rather than downlights, so the light is projected upwards and reflected off the ceiling to light the space. This gives completely even lighting across the space and very good image quality.
Uplights can also be added to standard VDT schemes to provide a more diffuse light and reduce the shadowing created by the downlights – a cheaper alternative than renewing the entire lighting scheme. Unfortunately, uplighters are not able to overcome the harsh shadows caused by spotlights because the differences in illuminance are too great.
Another way to improve the results of an existing installation is to use the characteristics of the lighting scheme to determine how people are moved through the space, if that is relevant. In a situation where people are queuing, for instance, the queuing area can be defined by ropes to ensure that customers/suspects are standing in the best picture quality area – between rows of spot downlights, for example, rather than directly beneath them.
Similarly, the layout of displays or furniture can be used to channel people so that they move through the areas that are going to give the best images.
Reflecting on surfaces
Another consideration that impinges on the whole interior design of the space is how the surfaces reflect the light that falls on them – known as their reflectance. Paper or matt emulsion surfaces will scatter the light in all directions to give predominantly diffuse reflections. At the other extreme, a mirror or shiny surface reflects light in distinct directions. “A shiny surface reflecting strongly toward the camera could of course be a disturbing feature of an installation and produce flare in the lens,” points out Thorn’s John Hugill.
The amount of light reflected will also vary considerably. For example, a matt white ceiling could be as high as 90 per cent, while a blue pile carpet could plummet to 4 per cent. Thus, the choice of materials has to be made very carefully – combining comfortable conditions for the workforce and customers with the requirements of the cameras.
In contrast
Contrast between the subject and the background also affects the clarity of the image. Contrast is determined by the amount of light in the space, how it’s reflected from various surfaces and the eye’s capacity to adapt to different lit environments.
A person in dark clothes (low reflectance) against a light background (high reflectance) will be perceived as a dark silhouette because of the high contrast between the two. With that same person against a darker background there will be less contrast but the person’s face will appear brighter because the adaptation of the eye has been reduced by using a dark background. The automatic gain in a CCTV camera has a similar effect on the face in the picture.
A colourful subject
Depending on the use of the camera, it may be necessary to ensure that colours of clothes or vehicles are easily identifiable and the type of light used will affect the way that those colours are perceived. Anyone who has tried to make out the colours of cars in a car park lit by yellow sodium lighting will realise how much difference the lighting can make.
This ability to reveal colours is known as the colour rendering and measured using a colour rendering index or Ra – a symbol you will see on many fluorescent tubes and other light sources. An Ra of 90 or more will provide very accurate colour rendering and an Ra around the 80 mark will be perfectly adequate for most security camera applications.
Light sources also have a colour appearance, or colour temperature, determined by the amount of different colour wavelengths there are in the light output. Some light sources appear warm because they have a high level of red light, while colder looking light sources have more blue light.
This colour appearance is important for cameras because a mix of different coloured lights in the background will affect the quality of the picture. So when a bulb or tube has to be replaced, it is important that the maintenance staff replace it with one of the same colour temperature. Colour temperature is shown on the light sources as a number followed by ‘K’ (e.g. 2700K, 5500K) so they can always look at the old light source before replacing it.
In this respect, high levels of daylight entering the space can cause major problems with colour appearance, so in these circumstances a light source that is close to the colour temperature of daylight should be chosen to reduce that conflict. Fluorescent or metal halide light sources will often be the best choice and as these are also energy efficient this is a choice that will often suit the energy manager and the financial director as well.
While the research cited above has concentrated on interior CCTV applications, many of the same principles can be applied to exterior camera installations. Often, these cameras will be more concerned with crowd or traffic control, where the facial detail is less important, but where detail is required the choice and positioning of light sources is just as important.
In many outdoor applications there will be less choice of siting positions but clever use of column or building mounted lanterns and floodlights can help to improve the performance of the cameras. Where such detail would only be needed in the event of trouble, such as in a football stadium, additional lighting can be switched remotely.
Given the size of the investment involved in a sophisticated CCTV system, the lighting only forms a small part of the overall cost – in terms of capital costs and cost of ownership. So making good use of lighting is rarely a matter of having to find a lot more money, it’s more a matter of recognising its importance and going to the trouble of making the lights and cameras work together.
