A system for measuring properties of a thin film coated glass having a light source, a spectrometer, at least one pair of probes, a first optical fiber switch and a second optical fiber switch. The pair of probes includes a first probe located on one side of a glass sheet and a second probe located on the opposite side of the glass sheet, directly across from the first probe. The first and second optical fiber switches are adapted to couple either probe to the light source and/or the spectrometer. Because the design of the system is optically symmetrical, calibration may be performed without the use of a reference material such as a tile or mirror.