An Explanation of SPIN and SPEX
Written by Kyle Foster
DIRS Measurement Group, Center for Imaging Science
Rochester Institute of Technology, Rochester, NY
Second Draft, 11/14/06
In making reflectance measurements with the Cary 500 spectrophotometer, an
option often presented to the operator in preparing the device is whether to operate under
SPIN or SPEX conditions. SPIN and SPEX are shorthand terms standing for "specular
included" and "specular excluded", respectively, and refer to a specific portion of the
reflection of light from a sample.
The Cary 500 spectrophotometer
Typically, reflectance measurements made by the DIRS Measurement Group on
the Cary 500 spectrophotometer make use of the device's "DRA-CA-5500 Sphere
Accessory", essentially a modular assembly containing a Spectralon-coated integrating
sphere and the requisite optics to employ it within the core measurement assembly.
Differentiation between transmission and reflectance measurements of samples is
handled through the manipulation of the sample beam via adjustable mirrors.
When set to measure sample reflectance, the sample will reflect the incident
sample beam about the interior of the sphere. Different samples demonstrate different
reflective properties, one of which may be the specular, or "glossy", reflection. Page 19
of Varian document AQ-00279-000 (primarily concerning the sphere accessory module)
states that the module "has a port... that corresponds to an 8-degree angle of sample
reflection (i.e., the specular direction). This specular subtraction port normally contains a
removable plug. This port plug may be replaced with a light trap designed to exclude the
specular (mirror-like) component reflected from a sample."
A measurement that is labeled SPIN or is part of a SPIN-baselined group has been
taken with the aforementioned Spectralon plug in place, therefore retaining the specular
component of the reflection (hence "specular included"). A measurement that is labeled
SPEX or is part of a SPEX-baselined group has been taken with the standard plug
replaced by one containing a light trap, effectively removing the specular component of
the reflection (hence "specular excluded").
Above: a side-by-side comparison of the SPIN and SPEX setup components for the Cary
500 spectrophotometer.
SPIN
SPEX
Above: a side-by-side comparison of spectra obtained under SPIN and SPEX conditions,
using the green fabric sample shown below.