This is a close up of the front of the PIN photodiode assembly.
The photodiode is mounted in the center of a 2 inch PVC pipe cap.  This
will allow a section of PVC pipe to be installed to form an ambient
light shield around the photodiode to improve noise immunity.
Additionally, a lens can be glued to the front of the pipe section to
focus additional optical energy onto the photodiode to improve the
sensitivity.  Optionally, a spectral filter could also be glued onto the
pipe section to filter out unwanted optical frequencies in order to
improve the noise immunity.