VIDEO IMAGING FUNDAMENTALS

VIDEO IMAGING FUNDAMENTALS

BASIC CONCEPT

When a series of slightly different still images of a subject are displayed sequentially, persistence of vision in the human brain interprets the subject as having motion. With real time video, 60 images per second (ips or fields per second) are displayed and captured if required.

EXAMPLE: 60 ips X 3600 sec./ hr = 216,000 images / hour X 24 hours = 5,184,000 images / day

Were you to play back the recorded images for analysis at the rate of two images per second, it would take thirty - 24 hours days to complete the analysis.

For most observational applications, this is data overload in the extreme.

SOLUTION: Capture only a statistically valid number of images per unit of time.

The time between image captures is call the Temporal Interval (TI) and its choice is dependent on a number of variables.

EXAMPLE:

Speed of Subject
Size of Imaged Area
Size of Subject(s)
Speed of Interactions
Periods of important activity
Need to capture intermittent activity that is very brief
Image capacity of recording media
Availability of power for recorder and camera

Various Time Lapse Recorders have programmable Temporal Intervals from 1/20th second to 90 or more seconds. The important thing to remember is to start off with a shorter TI than you think that you need to make sure that you capture all of your "critical data" and then after careful analysis of the recordings, incrementally increase the TI and reanalyze. This is especially important if you are using a video system to calibrate a "still" image system with limited image capture quantity

CAMERA OPERATION MODES

STANDARD CAMERAS

Standard video cameras operate in a mode known as interlaced imaging. At time zero, Field A with approximately 240 horizontal lines is read off of the CCD camera chip, output to the cable and recorded on the media. One sixtieth of a second later, Field B with approximately 240 lines interlaced with Field B are read off of the chip, output to the cable and recorded on the media. When a still interlaced image combining Field A and Field B is displayed, it is called a Frame. Therefore standard video cameras output 60 fields or 30 frames per second. Various time lapse recorders can record and display various combinations of fields and frames.

PROGRESSIVE SCAN CAMERAS

It is important to remember that if a subject is moving fast, it has moved significantly in the one sixtieth of a second between the fields. If displayed as a still frame, the subject will appear blurred even if the electronic shutter speed for each field was sufficient to freeze all motion. The solution for standard camera imaging is to display only one field at a time but half of the vertical resolution is lost (only 240 instead of 480 lines).

If all of the vertical resolution is required on playback of fast moving subjects, a Progressive Scan (PS) camera is required. PS cameras scan all 480 lines at time zero and immediately out put the lines from field A, the output from line B is delayed one sixtieth of a second to create output that is compatible with standard recorders and displays but without the blur in frame display mode.

FORMATS

There are different video formats unique to countries and various parts of the world. North America and Japan use the NTSC format, Great Britain uses PAL, France uses SECAM, etc. In the early days, great difficulty and expense was experienced in trying to convert from one format to another. Today converters are available for many formats and many monitors and recorders automatically recognize input formats and compensate. We prefer to build systems in the NTSC format because of the tremendous variety of equipment available. Most recordings done with our equipment in the NTSC format can easily have critical portions converted to other formats for presentations or long term storage.

TECHNOLOGY AND TOOLS FOR THE 21ST CENTURY™