Testing the Formula
Let us apply this formula for our representative application.
We assumes that the width of the observed area on the web is 80 cm, and that the CCD sensor is a 2048 pixel type. The pitch of the encoder in our possession is 0.75 mm. We want a visual aspect ratio of 1.
The calculation yields 1.92.
When the speed web varies between 25 and 50 cm/s, the following table shows the system behavior.
|
Slow web speed |
Medium web speed |
Fast web speed |
Web speed |
25 cm/s |
39 cm/s |
50 cm/s |
Encoder frequency |
|
|
|
Rate conversion ratio |
|
||
Camera line frequency |
333 ´ 1.92 = 640 Hz |
520 ´ 1.92 = 1000 Hz |
667 ´ 1.92 = 1280 Hz |
Line pitch |
|
|
|
Cross web pitch |
|
||
Aspect ratio |
|
|
|
This clearly proves that the aspect ratio is maintained to 1 independently of the web speed.