Spatial uniformity is one of the most important image quality attributes in visual experience of displays. In conventional researches, spatial uniformity was mostly measured with a radiometer and its quality was assessed with non-reference image quality metrics. Cameras are cheaper than radiometers and they can provide accurate relative measurements if they are carefully calibrated. In this paper, we propose and implement a work-flow to use a calibrated camera as a relative acquisition device of intensity to measure the spatial uniformity of projection displays. The camera intensity transfer functions for every projected pixels are recovered, so we can produce multiple levels of linearized non-uniformity on the screen in the purpose of image quality assessment. The experiment results suggest that our work-flow works well. Besides, none of the frequently referred uniformity metrics correlate well with the perceptual results for all types of test images. The spatial non-uniformity is largely masked by the high frequency components in the displayed image content, and we should simulate the human visual system to ignore the non-uniformity that cannot be discriminated by human observers. The simulation can be implemented using models based on contrast sensitivity functions, contrast masking, etc.