Where to Start?
“I would like to use video, but where do I start?” We receive
hundreds of calls each year from people wanting to use video in a
never-ending variety of applications. Over the years, we have found the
best way to help our customers is to have them determine four basic
Figure 1: Fundamental Parameters of an
- Field Of View (FOV):
The area of the object
that will be viewed on the monitor.
The smallest resolvable feature of
- Minimum and Maximum Working Distances:
from objective lens to the object.
- Depth Of Field Requirement:
depth needed in focus.
|FEATURES OF IMAGE QUALITY PARAMETERS||COMPONENTS AFFECTING THE PARAMETERS|
|Resolution: Resolution is a measurement of the
imaging system’s ability to reproduce object detail
|Contrast: Comparison of the shades of gray in
the image that define an object and its background.
|Depth of Field (DOF): The difference between
the closest and farthest distances an object may be shifted before an
unacceptable blur is observed. Sometimes called Depth of Focus, DOF
should not be confused with working distance. This is also applicable to
objects with depth.
|Distortion: Distortion is an optical error
(aberration) in the lens that causes a difference in magnification at
different points within the image.
|Perspective Errors: Also called parallax, this
is a phenomenon in conventional lenses which causes a change in
magnification as it moves in and out from best focus. Closer objects
appear larger than objects further away. Telecentric lenses optically
correct for this occurrence.
Terminology Used in Electronic Imaging
The size of a camera sensor’s active area, typically specified in the
horizontal dimension. This parameter is important in determining the
primary magnification (PMAG) required to obtain a desired field of view.
Note: Most analog cameras have a 4:3 (H:V) dimensional aspect ratio.
Defined as the ratio between the sensor size and the FOV, PMAG is the
“work” done by the lens.
The total magnification from the object to the image on the monitor,
this is the “work” done by the whole system. It is the product of the
PMAG and the camera-to-monitor magnification (the ratio between the
monitor size and the sensor size).
Resolution is a measurement of the imaging system’s ability to
reproduce object detail. Figure 2 illustrates a simplified view of two
squares imaged onto pixels of a CCD camera. In order for two objects to
be resolvable, a certain amount of space between the objects is needed.
A measure of the light gathering ability of a lens. f/# increases as
the lens aperture is closed, see Figure 3. Closing the aperture can also
increase the system’s depth of field (see definition below) but
illumination should be increased to compensate for this.
These are both threaded mounts (1-32 TPI) found on most industrial
CCD cameras and lenses. The flange (back focal) distance to the sensor
is 17.52mm for C-Mount. CS-Mount has a flange distance of 12.52mm.
C-Mount lenses and CS-Mount cameras are compatible using a 5mm
Depth Of Field:
The maximum object depth that can be maintained entirely in focus.
DOF is also the amount of object movement (in and out from best focus)
allowable while maintaining a desired amount of focus. As the object is
placed closer or farther than the working distance, it goes out of focus
and both the resolution and contrast suffer. For this reason, DOF only
makes sense if it is defined with an associated resolution and contrast.
In general, DOF can be increased by closing the lens aperture (i.e.
increasing the f/#), however, the illumination must be increased
Figure 2: Exaggerated example in
which a pair of red squares are (a) not resolved and are (b) resolved.
(see also Imaging Case Study)
reference :: http://www.edmundoptics.com/technical-support/imaging/understanding-video-and-imaging-equipment/