Backlighting, Silhouetting, And High Contrast Situations

Remember the experiment we asked you to do with a cellphone camera? If your phone has a camera, experiment with it. If you like taking selfies, take one with a light right behind your head. Then try one with a source of light (table lamp, flashlight, etc.) between you and the camera.


Ok, try this – look at a lamp, now put the phone up so that its above the light and take a pic.

While it would be nice to always have our cameras mounted on the North side of the building, facing North, real life means we need to work with the lighting conditions that are available to us. Fortunately there is a solution for every problem.

When it’s obvious that the camera will be staring into the sun, we recommend going with a high mounted bullet style camera. Bullet style cameras are equipped with a hood (baseball cap style) that can be adjusted. When the bullet camera is pointed down, the angle of the view and the position of the hood can be adjusted to block the sun from the camera’s field of view.

Backlighting arises when the light source is stronger behind the subject than in front of them. A typical situation is looking outward from an ATM, or looking at the entrance doors from the building’s lobby or hallway. When that front door opens, the daylight outside is typically brighter than the light in the lobby, resulting in the subject’s face appearing as a shadow in the image. BLC (Back Light Compensation) is a feature available on many Analog and Digital (IP) cameras that will brighten the darker portions of the image that appear on a bright background.

The drawback of this feature is that the entire area is brightened. Consequently, the brighter portions of the image will appear washed out and unusable.

Sometimes, the scene is just too bright. Whether looking at the South facing wall of a light colored building, or looking down a brightly lit hallway, or there is a light source (e.g. light bulb) in the camera’s field of view. There are a number of ways this can be handled:

• A major manufacturer has released a line of cameras titled Light Fighter
• FLC (Front Lighting Compensation) is a feature present on some modern day cameras
• If the camera supports it, adjust it’s shutter speed up (shortening the duration of each image frame). Of course this will have the side effect of making the camera less useful at night if it is located outdoors
• You can also try adjusting the gain controls down on the camera. It will improve image quality, but with the same drawbacks such as raising the shutter speed.
  

There are times when the situation involves all of the above. A hallway camera may be facing the front entrance of a building, or an outdoor camera may be looking at a bright lot and looking at a spot that sits in the building’s shadow (or a darkened alley). WDR (Wide Dynamic Range) is a premium feature that allows the camera to gracefully handle just about any challenging lighting situation.

WDR – Wide Dynamic Range

WDR falls into two major categories DWDR or Digital Wide Dynamic Range and WDR, Wide Dynamic Range.

DWDR – Digital Wide Dynamic Range

Digital Wide Dynamic Range is a way to process an image containing both light and dark areas with post-capture processing to try to recover a useable image. Basically, an image is captured, analyzed and various portions are darkened and lightened to try to bring the image into an even level of lighting. While it seems to achieve the desired effect, the GIGO (Garbage In Garbage Out) principle applies – the original image is imperfect, so there will be a loss of detail even after image processing.

WDR – Wide Dynamic Range

While missing the word “Digital,” in this case WDR is a much better technology for handling, Backlighting, Silhouetting, and other High Contrast situations. A WDR equipped camera will actually adjust the sensitivity of portions of the image sensor up or down, depending on the portion of the image that that segment is responsible for. The result is a much more even image with little loss of detail.

Once you start using WDR equipped cameras, it’s very hard to go back to anything else.

The major drawback of using WDR is increased noise generation on the image sensor, resulting in loss of finer detail, reducing the camera’s effective range by about 30%.

Next we will revisit IP cameras and some of the more intriguing features.