Question 6:
I want to inspect small text using 5 pt Helvetica for a pharmaceutical application. What is the camera resolution that I have to use?
Answer to question 6:
There are a lot of expectations and promises when it comes to what vision systems can find. In order to answer this question it is very important to understand first what the camera sees. With other words, how does the image that the camera takes from your web really look like. Then simply apply the following rule: If you cannot recognize the image or read the scanned text clearly, the vision system cannot either.
Here is an example using a 7 pt font:
The camera being used features 800 x 600 pixels and the web width is 240 mm (9.5 in.).This gives a web width resolution of around 84 Dots per Inch (dpi).
In the following picture each camera pixel is represented by a square. As a 7pt font is fairly small characters will not cover the pixel completely.
The next image shows the information the camera is processing. As pixels are not covered completely, the information taken is not 100% black but a level between 0% and 100% gray according to the pixel coverage.
From what the camera sees, the word "Test" can hardly be read. Therefore the inspection system will certainly not be able to make a statement on the print quality. Even a filled "e" will not be recognized.
It can be remarked that the resolution of the camera is very important when it comes to inspecting small text features. The camera has to match your application properly. Check the image of the camera and make sure that every feature can be seen clearly. It is very easy to detect a little dark spot in a white area. But it is extremely difficult to detect partly missing characters (like the dot in an i) in small text areas, especially as the tolerances applied to the inspection are usually set to high values, due to web stretch problems..
Another related issue is the question about validating an inspection system. Check question 4 for further information.
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