a validation apparatus for testing the measurement accuracy of a phase fluorimeter comprising : i ) a photodetector responsive to the excitation light emitted by a phase fluorimeter ; ii ) electronics connected to the detector which calculates the phase of the light emitted by the phase fluorimeter and causes a light source in the validation apparatus to emit light of a wave length which will stimulate the detector of the phase fluorimeter but which is phase shifted relative to the light emitted by the phase fluorimeter .