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Providing lighting for domes has long been a challenge for CCTV professionals as the lighting cannot be fitted to move with the camera, as would be the case with a traditional PTZ system. However, there are three lighting solutions available for fully functional domes:

1. Wide angle illumination: New wide angle illuminators covering 120-180deg allow the full 360deg angle of a dome to be covered with only two illuminators.

2. Target Area Illumination: Illuminators can be used to target points of interest including gates and entry points on pre-set positions.

3. Local Area Illumination: Illumination can be fitted away from the camera to flood the scene. When the camera zooms in the whole area must be lit.

Lighting and PTZ systems

Typically PTZ systems are used in applications where a single camera is designated to secure a large area. At long distances the camera lens is often zoomed in, giving a narrow field of view, and at short distances the camera lens pans to a wide angle. The flexibility of such a system requires the lighting to cover a long distance at both narrow and wide angles. The solution is a twin lighting system using one narrow angle illuminator to cover the long distances, and one wide angle illuminator to cover the shorter distances. Both illuminators should be attached to the PT motor by a bracket. A second solution for shorter range ptz units is to use a single wider angle illuminator such as a 30-60deg model. If the required distance is not too great a wide angle illuminator may be able to cover the maximum distance as well as the wide angle.

Infrared and focus shift

Focus shift is a potential issue encountered when setting up camera systems for 24-hour performance using infrared. The different wavelengths of visible light (400-700nm) and infrared (750-950nm) create different focus points through the lens onto the camera chip. This can lead to a loss of image focus at night, particularly if the camera is set up during the day. The degree of focus shift depends on:

Lens quality

• Wavelength of the IR. Longer wavelengths such as 950nm will provide a more exaggerated focus shift

• IR response of the camera

Focusing the camera correctly for low light performance means the camera will be focused for IR with the aperture fully open. During daytime operation the increased depth of field created by a closing aperture will counter the effects of focus shift. The best solution is to focus the camera using only infrared. This can be achieved by setting up the camera at night using infrared lighting, or using an IR pass filter over the camera lens to simulate.

Techinical TIP: Lens manufacturers such as Computar now provide lenses which overcome the issues of focus shift.

Back focusing

Back focus describes the relationship between the distance of the lens to the camera chip. This distance is critical to maintaining the proper depth of field through changing focal lengths and varying light conditions and setting it correctly can ensure the image stays in focus 24 hours a day. Correct back focus of the camera can be used to overcome the typical problem of a sharp daytime image followed by a blurred image at night. This situation is caused when, during bright sunlight, the lens is closed and the depth of field is very wide, whereas when the light level drops the iris opens and the depth of field decreases causing a lack of focus. For correct 24-hour focusing a camera should be back focused with the lens iris fully opened to simulate the worst possible depth of field.

Neutral density filters can be used to cover the lens during back focusing to simulate lower light intensity on scene. This is an effective solution for colour cameras or cameras using white-light CCTV illumination. However, for mono cameras, or day-night cameras using infrared lighting, the best solution is to place an infrared pass filter over the lens.

High sensitivity cameras and lighting

All cameras require light, sensitivity is simply a measure of how much light they need. High sensitivity cameras require less lighting to produce high quality CCTV images. However, even high sensitivity cameras should be fitted with professional CCTV lighting to provide even illumination. This allows the camera to provide sharp, clear images. Don't compromise the performance of a CCTV system by removing lighting when high sensitivity cameras are used.

Technical tip: High sensitivity cameras can achieve longer distances with CCTV lighting than standard cameras

Megapixel cameras and lighting

Digital cameras record brightness on a per-pixel basis so the greater the amount of pixels the smaller surface area each pixel has available to capture light. The end result is that the greater the resolution the less sensitive the camera.

A megapixel constitutes 1,000,000 individual pixels and these offer a number of advantages including higher resolution, wider angle images, and the ability to digitally zoom images. However, they are less sensitive than standard CCD cameras, meaning they require additional lighting to achieve high quality images at night. When using Megapixel cameras ALWAYS use additional CCTV lighting to achieve high quality night-time images.

Camera integration vs CCTV lighting

A common tactic used to achieve visible images during low light conditions is to use frame integration on the camera. Frame integration overlaps fields of image to "multiply up" the available lighting. For example, x2 integration overlaps two frames of image while x10 integration overlaps 10 frames of image. This allows the camera to collect light over a much longer time period giving brighter images. However, as frame integration overlaps images to multiply lighting, movement in the scene causes blurred images.

TECHNICAL TIP: Integration can provide acceptable images in static scenes but provides blurred images when movement occurs. During "events" this leaves the camera unable to capture high definition images of the target. There is no alternative to providing the camera with dedicated CCTV lighting to provide premium quality images.

Infrared reflectivity

Crucially, different materials reflect IR to different levels. Infrared light inverts the colour of man made fibres. Foliage and man made fibres emit high levels of infrared properties providing very clear, bright IR images.

• Taken from The Complete Guide to CCTV Lighting published by Raytec. For a copy of the complete guide contact the company at sales@rayteccctv.com

• Turn page for a quick guide to typical installations and practical guidelines.