Adjusting 2 monitors to have the same brightness/contrast

Adjusting 2 monitors to have the same brightness/contrast

I have a dual monitor setup on Linux, with both monitors being Phillips 109S. One is observably brighter than the other, and I could not make them reach the same level, and it's quite disturbing, e.g. one of them tends to add more 'gray' to the color - pure white is not visible.