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.