  Transformation and detection of photons in higher-order spatial modes usually
requires complicated holographic techniques. Detectors based on spatial
holograms suffer from non-idealities and should be carefully calibrated. We
report a novel method for analyzing the quality of projective measurements in
spatial mode basis inspired by quantum detector tomography. It allows us to
calibrate the detector response using only gaussian beams. We experimentally
investigate the inherent inaccuracy of the existing methods of mode
transformation and provide a full statistical reconstruction of the POVM
(positive operator valued measure) elements for holographic spatial mode
detectors.
