FrozenBlue

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 system factors:

Figure 1: Fundamental Parameters of an Imaging System

1. Field Of View (FOV):
   The area of the object that will be viewed on the monitor.
2. Resolution:
   The smallest resolvable feature of the object.
3. Minimum and Maximum Working Distances:
   Measurement from objective lens to the object.
4. Depth Of Field Requirement:
   Maximum object depth needed in focus.

Terminology Used in Electronic Imaging

Sensor Size:

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.

Primary Magnification:

Defined as the ratio between the sensor size and the FOV, PMAG is the "work" done by the lens.

System Magnification:

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:

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.

F-Number (f/#):

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.

C/CS-Mount:

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 spacer (#03-618).

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 accordingly.

Figure 1:

Figure 2: Exaggerated example in which a pair of red squares are (a) not resolved and are (b) resolved.

Figure 3:

(see also Imaging Case Study)

reference :: http://www.edmundoptics.com/technical-support/imaging/understanding-video-and-imaging-equipment/