Interlaced Scan

Interlaced scan is used in normal TV and video systems and was designed for Television to reduce bandwidth, and flicker while maintaining resolution.

The image is broken up into two fields: all odd lines (lines 1-3-5....) are imaged in the first field and all even lines (lines 2-4-6....) are imaged in the second field. A full frame image consists of two interlaced fields. This method of scanning is called 2:1 interlaced. The first field is imaged and output before the second field is imaged and output 20msec later (PAL, CCIR).

All analog security cameras and some industrial cameras use interlaced sensors. The benefit is that they are low cost. If however the image is captured and frozen for analysis by a machine vision system any movement of the object during the 20msec gap in fields is appears as blur in the captured frame and so degrades the image.

Interlaced sensors are adequate for machine vision if the object is stationary or moving slowly or if only half he vertical resolution is used by analysing a single field.

Non-interlaced scan or progressive scan was developed for machine vision but is now finding its way into other application markets. In a progressive scan sensor all lines are captured at the same time. This eliminates the motion blur problem evident in interlaced sensors but also doubles the bandwidth required to transmit the signal along a cable.

Progressive scan is non-standard and so cannot be displayed with standard CCTV monitor. The signal must be captured with a frame grabber or in the case of digital cameras via an adapter e.g. GigE, Firewire, USB.

The benefit is that a full frame is available as the result of a single shutter event.

Click here for the second part of this article which discusses the Interline Transfer architecture for CCD’s and how it compares to that of Frame Transfer sensors and Full Frame sensors.

Adept Electronic Solutions are 'The Machine Vision and Imaging Specialists' in Australia and New Zealand. To find out more about sensor types or information on machine vision please email us at: adept@adept.net.au, call us at Perth (08) 92425411 / Sydney (02) 99792599 / Melbourne (03) 95555621 or use our online contact us form.