Security Cameras
One
of the most misunderstood terms in digital imaging is wide dynamic range.
Wide dynamic range is needed to capture image features when the field of
view includes both bright and dim areas. Problems with standard range cameras
include reflections, glare, sunlight, and car headlights.
Ø Dynamic range is a ratio of the ability of the system to capture simultaneously the brightest and darkest features in an image.
Imagine
two people, one in the shadow of a building, the other in direct sunlight. When there are severe
variations in illumination in the field of view, the capture of both feature
areas in the same frame is a problem.
When the human eye views a scene, with both dark and
bright features, the brain teams with the eye to discern the entire area. With conventional technology, the camera
adjusts itself to the high average brightness, and the less bright features
will be less visible. The output
voltage from an image sensor of a camera is proportional to the number of
photons that come into contact. Digital image sensors are linear. This may not be ideal for our purpose.
Photographic
artists sometimes take a set of photographs of the same target with different
parameters, and combine the set into a single image with imaging software. A digital camera with wide dynamic range
can render comparable results in real-time. Automatic wide dynamic range
is accomplished by cropping the maximum of the dynamic range (e.g. direct
sunlight), and utilizing an “S conversion curve” (non-linear
transform) that compresses the range of luminance.
This technique, called adaptive luminance control, uses highlight suppression and back light compensation (HSBLC) to darken selected highlights and brighten darker portions of an image.
This feature is crucial for
certain tasks. For example,
capturing an image of a vehicle’s license plate when the headlights are
part of the image, can be impossible without a wide
dynamic range system. A
conventional camera would be blind to all areas of the image other than the
headlights.
The CCD sensor is the heart
of many security cameras. An
example is the SONY
EX-VIEW HAD CCD
(Hole-Accumulation Diode Charged Coupled Device). This device improves light efficiency by
including near infrared light region as a design component of the sensor.
For some difficult applications, security cameras should be able
to capture details in both bright and dark areas, within the same scene or
during rapid changes in lighting conditions. The wide dynamic range camera is
well-suited to this purpose.
Website: BV4Tech.com