What is fixture alignment in photometry?

It is setting the fixture on the goniometer so that its own reference axis, its light centre and the photometer line up with the rotation axes. Every angle in the measured data is then measured from the fixture's reference direction, not from wherever it happened to be mounted.

The direction the fixture is designed to be aimed, fixed by its reference surface or marks, and set by the manufacturer. Angles in an isocandela diagram or photometric report are quoted from that axis, so the test must start with it pointing at the photometer at the zero angles.

If the light centre is offset from the axis the fixture turns about, the distance to the photometer changes as it rotates and the beam appears to move. That adds error at the angles where the intensity changes quickly. The light centre is placed on the axis and the distance measured from it.

The whole measured pattern is shifted or rotated: the peak appears at the wrong angle, and intensity at each required point reads too high or too low. A good fixture can then seem to fail, or a poor one seem to pass, because the data no longer describes how it will be aimed on the airfield.

Typically with an alignment aid such as a laser or cross-hair along the reference axis, checked at the zero angles and confirmed from the measured pattern, then locked and recorded. The method and tolerances come from the test laboratory's procedure and the applicable standard.

No. This is alignment on the test bench, so the measurement is valid. Aligning a fixture in its base can on site is covered in the installation and maintenance entries, and both depend on the same reference axis.