The brightness of an object in an imaging system or the illuminated by a light source is called illuminance. Relative illuminance is used to quantify changes in lighting intensity within a sensor.
The uneven phenomenon of central brightness higher than surrounding brightness will be found when the image is collected from the uniform viewing area. This is due to a combination of lens vignetting and optical propagation attenuation.
This is usually described by a "relative illuminance curve": the percentage of central illuminance (maximum illumination) to peripheral illuminance.
Methods to improve relative illumination:
Reduce your lens aperture;
Use a lens with low vignetting and long focal length;
Select small chip size camera and increase working distance;
Select a higher camera to do digital image processing compensation according to the amount of vignetting
The light source needs to match with the lens and the nature of the measured object (light supplement).