Lens Parameters
(Part 2 0f 2)
Lens Format – The lens needs to
cover an area as large or larger than the sensor size. Hence, the
lens format should be, at least, equivalent to the camera format.
Most CCD sensors come in sizes of
1", 2/3", 1/2", 1/3" and 1/4".
Different sensor formats require
corresponding lens formats. The lens format has
always to be equal or larger than the sensor format. Larger lens
formats reduce distortion at the outside edges. C-Mount and
CS-Mount CCTV lenses can be used for all area-scan sensor
formats. However, these lenses are not recommended for precision
measurements or three-dimensional part inspection.
Mega-pixel line-scan cameras, due to
their size, need a larger image format than C-mount lenses can
offer. In this case, an F-mount 35 SLR lens can be used. These
lenses can also be used with a C-mount camera when a higher image
quality is required. This requires a C-mount to F-mount adapter.
C-MOUNT - An industry standard
for mounting a lens to a camera where a 1" x 32 thread is
employed and the distance from the image plane is 17.52mm
from the shoulder of the lens. A C-mount lens may be used
with a CS-mount camera with the use of a 5mm-adapter ring.
CS-MOUNT
- A relatively new industry standard for mounting a lens to a
camera where a 1" X 32 thread is employed and the distance from
the image plane to the shoulder of the lens is 12.52mm. A
CS-mount lens may NOT be used on a C-mount camera.
Resolution – Resolution is the ability of
a lens to distinguish two features that are close together. Also, a
lens with high resolution will show an edge transition in fewer
pixels than a lens with low resolution. The resolving power of the
optics for viewing distant objects is:
where N is the f/number and
l
is
the light’s wavelength. The resolving power is limited by
diffraction that spreads each object point to an image spot (Airy
disk) - the
D being the diameter of the inner bright spot. A practical
test for determining the resolution for machine vision uses a target
with multiple bars and various spacing between bars.
This type of target can also be used to
measure contrast and magnification. Resolution of the lens does
not determine detection resolution, which can be effected by lens
distortions and aberrations, lighting deficiencies, or limited
camera resolution.
Contrast - Contrast is the amount of
difference between light and dark features in an image.
The light is the gray level of
the brightest pixel and the
dark is the gray level of the darkest pixel.
A contrast of 1 means transition from
full light to full dark. A higher resolution lens not only resolves
finer features but also images larger features at a higher contrast. A
high-contrast image appears sharper than a lower contrast image even
at the same resolution. The quality of optics and lighting can
affect the contrast.
Distortion and Aberrations – The simple
lens formulas are valid only with ideally thin lenses, no spherical
distortion, no field curvature, and using monochromatic light.
Geometric Distortions – Pincushion and
barrel geometric distortions must be compensated especially in gauging applications.
Distortion can be measured by using a precise dot target, which can
also measure the magnification.
Aberrations – chromaticity, sphericity,
coma, astigmatism, field curvature, and vignetting are corrected by
incorporating multiple lenses that compensate for each other.
However, it is impossible to design a lens that is well corrected
for all distances, FOVs, and wavelengths. The smaller the range, the
simpler the design. Hence, the lens may not perform correctly if
used under different conditions.
For more information, please contact
High-Tech Digital Technical Support.
310-265-8203
support@high-techdigital.com.
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