The eye has an iris. When we say we have blue eyes or brown
eyes or green eyes, we are referring to the iris. But the iris is more than the
colored circle surrounding the pupil. It acts as its diaphragm, reacting to the
intensity of light, to widen or narrow the pupil which admits and focuses light
on the retina at the back of the eye. Like the image sensor of a camera, the retina
reacts to the light and sends a record of it, via the optic nerve, to the brain
which makes sense of the image. The iris
constricts the pupil in bright light and dilates it in darker conditions.
The camera also has an iris which serves the same function.
The camera iris controls the lens aperture, opening it so as to admit light to
the photosites on the image sensor. Iris control for fixed surveillance cameras
is of three types:
·
Fixed - set at a specific circumference and cannot be
adjusted.
·
Manual - allows you to adjust the aperture by hand.
·
Automatic - adjusts the iris according to the prevailing light
If the camera is mounted indoors where the lighting remains
the same, one can get by with either a fixed or manual iris since there is no
reason to adjust the iris. However when using an outdoor camera, automatic
iris control is needed.
The iris setting affects
image sharpness and depth of field. Depth of field refers to the distance both
in front of and behind the focal point where objects share the same degree of
sharpness. The deeper the field, the greater the portion of the scene that is
visible. This is especially important in surveillance when covering a long
corridor or escalator or a parking lot.
However, in waning light, the aperture may not admit
sufficient light to pass through. The pixels that correspond to the darker
portions of the image may not have enough time to collect sufficient photons, resulting
in a shallow depth of field.
A wide iris opening reduces depth of field while a narrow
one increases it. The term, f-number is used to define the size of the lens
opening, the higher the number, the smaller the opening.
The following chart shows the effect the size of the
aperture opening has on the depth of
field. A higher f-number increases the depth while a lower f-number decreases it.
A smaller opening will also improve image sharpness. This is because any lens will produce some
sort of image aberration if the whole surface is used, so the smaller the
opening, the less of the lens used, and the better the error reduction.
However, and there’s always seems to be a however, too small
an opening can actually blur an image due to what’s called diffraction.
Diffraction arises in bright outdoor conditions when a lens closes its shutter
too much and the light is diffracted or spread over too many photosites,
resulting in loss of detail and a dull light-washed image. Megapixel cameras compound the problem
because not only do they have a large number of photosites but in many cases,
the photosites are small and close together.
The following set of images illustrates diffraction at different iris settings on cameras within a range of
megapixels.
More precise iris control would go a long way in decreasing diffraction, thus increasing sharpness and depth
of field but unfortunately the DC-iris lens
as mentioned above, only controls the iris in response to light
intensity . It does not allow for any finer adjustments that might result in
more accurate photon collection. Axis Communications has developed a solution,
jointly with Kowa, for minimizing diffraction. It is called P-Iris.
P-Iris provides automatic precise control of the iris
opening. Rather than merely regulating the flow of light to the image sensor,
P-iris sets the iris at the optimum f-number at which the central and best
part of the lens will be used. The P in
P-iris stands for precise. Using this preferred setting as the
default insures better contrast,
resolution and depth of field.
But in some lighting conditions, P-iris may not be enough. In those cases, electronic processing is called for - gain (amplification of the signal level), or an alteration in exposure time. Either or both can optimize the image quality by maintaining the best iris position for as long as possible.
In the rare instance when neither the preferred iris
position, nor the electronic processing can correct the exposure, cameras equipped with P-iris will automatically
instruct the camera to change the iris position. Axis holds forth that any network
camera equipped with P-iris will adjust itself to produce crisp high definition
images with good depth of field, no matter what the lighting conditions may be,
and will do it all automatically.
In addition to Axis, other camera manufacturers such as CBC
Ganz and Vivotek are using the technology. If you have any questions about P-Iris
cameras or any IP camera, visit Kintronics at http://www.kintronics.com/neteye/neteye.htmlor fill out a request for information form.
