  The VERITAS array is a set of four imaging atmospheric Cherenkov telescopes
(IACTs) sensitive to gamma rays with energies above 80 GeV. Each telescope is
based on a tessellated mirror, 12 metres in diameter, which reflects light from
a gamma-ray-induced air shower to form an image on a pixellated `camera'
comprising 499 photomultiplier tubes. The image brightness is the primary
measure of the gamma ray's energy so a knowledge of the mirror reflectivity is
important. We describe here a method, pioneered by members of the MAGIC
collaboration, to measure the whole-dish reflectivity, quickly and regularly,
so that effects of mirror aging can be monitored. A CCD camera attached near
the centre of the dish simultaneously acquires an image of both a star and its
reflection on a target made of Spectralon, a highly-reflective material, placed
at the focus of the telescope. The ratio of their brightnesses, as recorded by
the CCD, along with geometric factors, provides an estimate of the dish
reflectivity with few systematic errors. A filter wheel is deployed with the
CCD camera, allowing one to measure the reflectivity as a function of
wavelength. We present results obtained with the VERITAS telescopes since 2014.
