When it comes to dimensional measurement, positioning, and defect detection, it is a matter of camera accuracy.

1 Resolution

Common camera pixels are 500,000, 2 million, 5 million and 21 million, and the corresponding prices vary greatly, so it is important to choose a camera with the right pixels.

Relationships between Pixels, Resolution, Precision, and Tolerances

The resolution of the camera is related to the field of view, the larger the workpiece, the larger the field of view, and the resolution decreases.

Relationships between Pixels, Resolution, Precision, and Tolerances

For example, the size of the product I want to look at is 30mm×10mm, using a 2 million pixel (1600pixel*1200pixel) camera. Because the product is a long bar, in order to put the product into the field of view, we should consider the long side correspondence when calculating the resolution, and the resolution is

Relationships between Pixels, Resolution, Precision, and Tolerances

2 Accuracy

The unit of accuracy is mm. Depending on the surface and lighting conditions of the product, we can distinguish the number of stable pixels by zooming in on the image to obtain accuracy. If the conditions do not allow the actual test observation, the general rule is that if the front light is used, the effective pixel is 1, and if the backlight is used, the effective pixel is 0.5. In this example, we take 1 Pixel and get an accuracy of 0.019mm, which is about 0.02mm.

Accuracy=Resolution x Pixel

3 Tolerance

In general, the correspondence between accuracy and tolerance is as follows:

Minimum detectable size 10 times accuracy (accuracy is an order of magnitude smaller)  Tolerance band range in this example: 0.2mm
Appearance tolerance 4 times accuracy  in this example The minimum detectable blemishes or stain size is 0.08mm

For a project, we first read the tolerance requirements from the drawing. Then, based on the above relationship and the table below, we can reverse how many pixels we need for the camera.

Camera type 1 Pixel resolution 3 Pixel resolution 5 Pixel resolution 10 Pixel resolution
21 Million Pixels 5120×4096 0.006 mm 0.018 mm 0.029 mm 0.059 mm
5 Million Pixels 2432×2050 0.012 mm 0.037 mm 0.062 mm 0.123 mm
2 Million Pixels 1600×1200 0.019 mm 0.056 mm 0.094 mm 0.188 mm
300 Thousand Pixels  640×480 0.047 mm 0.141 mm 0.234 mm 0.469 mm

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