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.
