With any lens image brightness is reduced towards the edges. The cause of this shading can be split into two groups; those primarily caused by the lens and those caused by the sensor.
Mechanical vignetting occurs when the light beam is mechanically blocked, normally by the lens mount, and causes shading towards the edges of the image. It is seen mainly when the image circle (or format) of the lens is too small for the size of the sensor.
All lenses are influenced by what is called Cos4 vignetting which is caused by the fact that the light has to travel a further distance to the edge of the image and reaches the sensor at a shallow angle.
Most camera sensors used for machine vision applications include micro lenses that gather light and focus it onto the active pixel area. However, under certain circumstances, when the incident light falls on the sensor at a very shallow angle, some of the light is refracted such that it no longer falls on the active area of the pixel, as can be seen in the diagram.
Sensors with asymmetric active pixel areas (as shown here) are usually due to associated on-chip electronics. These non-uniform pixels affect the images that are produced, due to the way that light falls on the active pixel area. More light collects along one axis producing an undesired shaded effect.
As the next diagram shows, the angle at which light from a 'wide angle' lens strikes a sensor is often large, causing severe sensor shading due to more of the light falling on the non-sensitive area of the pixel.
If sensor vignetting is an issue, you can consider using a camera with a smaller sensor, using a longer focal length lens, or using a back-sided (imagesided) telecentric lens which removes this effect