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Introduction
Besides each camera chip, optics are the
most important components in relation
to resolution and quality in image
processing. Intellisystem Technologies lenses feature
excellent imaging capabilities throughout
the entire image field captured
or photographed. The mechanical
construction of Intellisystem Technologies lenses is rugged,
making them ideal for use in CCTV industry.
Basic Terminology
Reported below we will introduce the most important terms that each technician and system integrators should know before to work in CCTV project. This terminology is basically for understand how to make the choice for the lens to be adopted:
- Contrast: Also called "modulation". The ratio of
the differences to the sum of the maximum
and minimum illuminance of
two self-illuminating or illuminated surface
points. Put in more accurate terms,
the ratio of the difference to the
sum of two intensities.
- Depth of field : The axial plus or minus distance from an
object space, which is reproduced as an
image without any perceptible loss in
sharpness. The receiver-dependent circle
of confusion permitted in the image
plane is decisive for the depth of field.
- F-number:While imaging from infinity, the ratio of
the diameter of the entrance pupil EP to
the focal length of an optical imaging
system f'. Put in more accurate terms,
k = 0.5 / NA (see also "Numerical aperture").
- Image height (max. sensor sizes):
1/4" CCD |
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2u' = 4 mm |
1/3" CCD |
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2u' = 6 mm |
1/2" CCD |
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2u' = 8 mm |
2/3" CCD |
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2u' = 11 mm |
1" CCD |
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2u' = 16 mm |
- Minimum object distance (MOD): Is the smallest possible distance from the
object is given for each lens.
- Modulation transfer function (MTF):Quantitative description of the image
forming power of an imaging system.
To determine MTF, increasingly fine lines
(spatial frequency in line pairs per mm)
of known contrast are imaged by the
optical system and the image modulation
is measured in the image plane.
The ratio of the image modulation to
the object modulation as a function of
the spatial frequency yields MTF.
- Numerical aperture N(A):The numerical aperture NA is a
characteristic value for the widest ray
bundle capable of entering a lens. It is
strictly indicated as a numerical value:
NA = n × sin s (n = index of refraction,
s = half angle ray bundle). NA
(see also "Resolution") defines the
maximum resolution (limited by light
propagation) of an optical system.
- Resolution:The maximum resolution of an image
processing system is ultimately determined
by the pixel dimensions of the
CCD chip. In order to resolve a pair of
light and dark lines, two pixels (see also
"MTF") are needed. Resolution is
limited by light propagation. Because of
the wave nature of light, even an ideal
lens does not reproduce the image
of a point as a sharp dot, rather as a
diffraction disc (Airy disc: Ų Airy = 2.44
× ? × k where ? = wavelength and
k = f-number) with concentric light and
dark rings; according to Rayleigh, this is
the limit of resolution.
- Stop:Limits the ray bundles between the
object and image by mechanical means
during optical imaging. The diaphragm
or aperture affects the brightness of an
image. In addition, the size and position
of the diaphragm or aperture determine
the angle, resolution and depth of field.
- Telecentric system:An optical system where the entrance
and/or exit pupil is imaged to infinity
caused by locating the aperture stop
at the front or back focal point of the
system. Because of this, the principal
rays are parallel to the optical axis in
the image and/or object space. Thus,
it yields a special form of perspective:
telecentry (as opposed to the natural
entocentric perspective). Through telecentric
perspective, objects of the same
lateral distance on both sides appear
to be equal in size at various distances
within specific limits (telecentric range)
in an image.
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Intellisystem Technologies
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