  In this paper, we build an apparatus for measuring the optical transmittance
and its uniformity for a semispherical surface at normal incidence; the system
is primarily comprised of a traditional double-beam photometric framework and a
novel custom-made mechanical structure with multidimensional degrees of
freedom. During the measurement process, a key aligning step is adopted to
guarantee that the center point of the semispherical surface stands still in
the light beam while scanning the hemispherical optical element point by point
around the horizontal and vertical axes, which ensures that the laser beam is
always normally incident onto the surface of the hemisphere. The experimental
results show that the uniformity of the optical transmittance for a
semispherical optical glass can be successfully characterized by the system,
with a three-cycle repeatability error of 0.026% being demonstrated. Our system
solves the problem of traditional spectrophotometers when measuring the
spectral property of a hemispherical surface and thus can be popularized in
similar applications.

Optics;Instrumentation and Detectors