Video cameras like low contrast scenes these are location where there are few if any shadows, not very brightly lit areas with lots of shadows. Overcast days will produce better images than bright sunny days with clear skies. While we don’t have any control over the weather we can place and set the cameras in locations to achieve the best image possible. Remember we’re not making a movie so it’s not pretty pictures we’re after it’s surveillance coverage.
The cameras are designed to work on automatic exposure so if something bright enters the frame the camera will reduce the exposure to enable it to be seen more clearly. Likewise if you point the camera at something bright the same thing will happen. In theory this is great but the downside is there is only so much room in the system to handle the brightest and darkest thing in the image. This is known as the Contrast Range. If there is too much contrast in the scene the system will crush the bright and dark parts of the picture, known as Black Crushing and White Clipping.
To be able to see into dark shaded areas of a scene the Brightness control needs to be set as high as possible, and so that the bright white area of the image still retain some detail the Contrast control needs to be set as low as possible. 100% is a good setting for the Brightness control while 40 to 45% is a good setting for the Contrast control.
The Brightness and Contrast settings are accessed by first clicking the mouse on the image you want adjust then selecting the Paint Palette from the blue floating menu. Once in the camera settings menu change the Operation Scene dropdown menu from Auto to Outdoor then adjusted the slider controls for the most even vision levels. Remember to Save your settings before selecting the next camera to adjust.
The placement of the cameras is probable the most important part of the installation and the main consideration is given to what part of the scene is for surveillance, however how the scene will affect the operation of the camera is equally important.
Once you have identified the areas needing surveillance decide from which angle you will cover them. Cable run accessibility is going to be a big consideration but there’s no point in running an easy cable run if the end results produces as useless under or overexposed shot. So the thing to bear in mind is evenness in the lighting and when that’s not possible think about the important surveillance area, … is it in the shade or in the sun in that view? Try to minimise areas with the opposite conditions to those of the surveillance area. This is often hard to achieve as the sun has a habit of moving around. Likewise avoid including lamps that might be switched on in night scenes as they will also alter the exposure.
Finally take into consideration the aspect ratio of the image, it’s wider than high and the greatest converge is what we want even if it means the image is at a strange angle.
There’s not much more we can do than raising the Brightness Control to lift the brightness in the darker areas of scene without adding lights, this is the main job of the Lighting Director on a movie set, however we can often shade light from a scene or reduce it entering the camera lens.
For an example, if a camera mounted on a roof eave that includes in the view a part of white rainwater downpipe; this bright object will cause the auto exposure to pull down the vision level of the rest of the picture making it too dark to see detail in the surveillance area. Painting that section of the pipe a darker colour will stop the auto exposure pulling down the vision level of rest of the picture.
However if it’s not possible to stop light bouncing off an object it may be possible to stop light entering the camera lens, either by using a shield or filter. A lens hood is one way unwanted light can be reduced from entering a lens. Barn-doors or a shade known as a flag are others.
Filters known as ND filters (ND = Neutral density) can be bought in different densities from 1,2 and 3. 3 being the darkest. These have the same effect as wearing sunglasses and can be placed on cameras viewing High
Contrast Range scenes. It’s not the best method but may be the only one in some cases. The downside is not only will the bright area be reduced but so will the dark. However it is often easier to control the dark areas with the camera set-up controls. Sometimes in may be possible to cut the plastic filter film to suit the problem.
An example might be a scene including a bight highly reflective concrete drive adjoining some dark bushes where light reflected from the drive causes the auto exposure to pull down the light in the bushes. Mounting a small piece of ND Filter, which has been cut in way to only cover the image of the bright drive and not the bushes, to the front of the lens will dramatically help the auto exposure control. Be careful not to cover the IR LEDs with the ND Filter.
Of the 4 pictures below A represents the real scene as it might be seen by the human eye, the black vehicle under the tree can be seen relatively easily and so can the detail in the roof of the house opposite. Image B shows how the scene might look when captured on a video camera using auto exposure or with the Contrast set too high. The detail in the roof is gone and vehicle under the tree has become a little harder to see as the Brightness is now too low. Image C shows how the picture might look if the Contrast is correctly set as the detail can be seen in the bright areas but the Brightness is far too low and it’s now very difficult to see the vehicle under the tree at all. The final photo shows the sort of image ideal for surveillance work where the darker areas have been lifted by increasing the Brightness and there is still detail in the brighter area as the Contrast has been reduced; certainly not the settings for a pretty picture but ideal for our application.
Reduced contrast pictures will also help in extending your storage space by producing smaller files sizes, low contrast images also limit the number of false alarms triggered by Motion Detection due to the movement of shadows being less detectable. See here -> Getting the best from Motion Detection
(Base photo courtesy: Drow, at RGBstock free photos)