  The high quantum efficiencies (QE) of backside illuminated charge coupled
devices (CCD) has ushered in the age of the large scale astronomical survey.
The QE of these devices can be greater than 90 %, and is dependent upon the
operating temperature, device thickness, backside charging mechanisms, and
anti-reflection (AR) coatings. But at optical wavelengths the QE is well
approximated as one minus the reflectance, thus the measurement of the backside
reflectivity of these devices provides a second independent measure of their
QE.
  We have designed and constructed a novel instrument to measure the relative
specular reflectance of CCD detectors, with a significant portion of this
device being constructed using a 3D fused deposition model (FDM) printer. This
device implements both a monitor and measurement photodiode to simultaneously
collect incident and reflected measurements reducing errors introduced by the
relative reflectance calibration process. While most relative reflectometers
are highly dependent upon a precisely repeatable target distance for accurate
measurements, we have implemented a method of measurement which minimizes these
errors.
  Using the reflectometer we have measured the reflectance of two types of
Hamamatsu CCD detectors. The first device is a Hamamatsu 2k x 4k backside
illuminated high resistivity p-type silicon detector which has been optimized
to operate in the blue from 380 nm - 650 nm. The second detector being a 2k x
4k backside illuminated high resistivity p-type silicon detector optimized for
use in the red from 640 nm - 960 nm. We have not only been able to measure the
reflectance of these devices as a function of wavelength we have also sampled
the reflectance as a function of position on the device, and found a reflection
gradient across these devices.
