Aspect Ratio

In general, the line pitch is not equal to the crossweb pitch. The result is that the digital image provided by the frame grabber is not geometrically exact.

For example, if the web speed and the camera line frequency are such that the line pitch is larger than the crossweb pitch, the resulting image will appear somehow compressed in the downweb direction.

The following table displays the result of varying the web speed for our representative example.

	Slow web	Correct speed	Fast web
Web speed	25 cm/s	39 cm/s	50 cm/s
Camera line frequency	1 kHz	1 kHz	1 kHz
Crossweb pitch	0.39 mm	0.39 mm	0.39 mm
Line pitch	0.25 mm	0.39 mm	0.5 mm
Aspect ratio	1.56	1	0.78
Single pixel look
Image look

In this table, we define the aspect ratio as:

This shows that, if the aspect ratio has to be controlled for the application (and usually it has to), the web speed and the camera line frequency should be coupled together somehow in order to maintain the line pitch to a known value.