Identification and Recognition Tutorial
Illumination
Illumination greatly affects the ability to identify persons or objects. Shadows, high contrasts and backlit scenes all make identification and recognition more difficult compared to when lighting conditions are more favorable. These examples compare good outdoor lighting with more challenging conditions.
| Indoor surveillance | Outdoor surveillance | Indoor surveillance | ||
| 100 lux partially shaded features | favorable light direction | 100 lux - partial backlight | ||
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85 pixels |
80 pixels |
75 pixels |
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15 meters from camera 50 mm lens |
15 meters from camera 50 mm lens |
At distances between 15-20 m, you will need a 50 mm lens to ensure that a face covers around 80 pixels. However, the examples clearly show that even at this resolution, positive identification is not guaranteed at the 100-150 lux illumination that is typical in an office corridor or subway station. Camera features such as wide dynamic range and sensors that perform well in low light situations can help, but the best results are obtained if these are combined with additional lightning and adjustment of camera positions to avoid backlit situations.
In outdoor surveillance it is important to take into account that the sunlight shifts in intensity and direction through the course of a day. Weather conditions will also affect lighting and reflection. Snow will, for example, intensify the reflected light, while rain and wet tarmac will absorb much of the reflected light. For identification of a human face, balanced illumination in the region of 300-500 lux is recommended. For license plate identification, 150 lux may be sufficient.
Noise
In low light, camera sensors produce significant amounts of noise that can affect the image. This can make identification more difficult. There is always a trade-off between noise, shutter speed, and depth of field at any given level of illumination, where better lighting conditions allow you to improve all of these.
Color fidelity
Color is often an important factor for identification. To ensure color fidelity, camera white balance should be adjusted to suit the color temperature of the light sources used. In outdoor surveillance, the color temperature will change throughout the day, requiring automatic white balancing to keep color fidelity.
Cameras that are compliant with the SMPTE (Society of Motion Picture and Television Engineers) standards for HDTV fulfill stringent requirements on color fidelity.
Next topic: Camera positioning, motion and compression
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