Sensor sizes (or formats) are usually designated with an imperial fraction value – i.e. 1/2”, 2/3”. However, the actual dimensions of a sensor are different from the fraction value, which often causes confusion among users. This practice dates back to the 50’s at the time of TV camera tubes and is still the standard these days. Also, it is always wise to check the sensor specifications, since even two sensors with the same format may have slightly different dimensions and aspect ratios. Spatial resolution is the number of active elements (pixels) contained in the sensor area: the higher the resolution, the smaller the detail we can detect on the image.
Suppose we need to inspect a 30 x 40 mm FoV, looking for 40*40 μm defects that must be viewed on at least three pixels.
There can be 30*40/(0.04*0.04) = 0.75x10^6 defects. Assuming a minimum of 3 pixels are required to see a defect, we need a camera with at least 2.25 MP pixels. This gives the minimum resolution required for the sensor, although the whole system resolution (also including the lens resolution) must always be assessed. Table 1 gives a brief overview of some common sensor dimensions and resolutions. It is important to underline that sensors can have the same dimensions but different resolution, since the pixel size can vary. Although for a given sensor format smaller pixels lead to higher resolution, smaller pixels are not always ideal since they are less sensitive to light and generate higher noise; also, the lens resolution and pixel size must always be properly matched to ensure optimal system performances.