Application Note: Quantitation of Single Beam Substitution Correction In Reflectance Spectroscopy Accessories

The single beam substitution correction, sometimes known as single beam substitution error, is the systematic, predictable, and non-random error inherent in single beam integrating spheres measuring reflectance or transmittance.

The error is caused by the difference in the throughput of the sphere when the reference makes up a portion of the sphere wall and when the sample is substituted for the reference. In reflectance measurements the throughput and corresponding measured reflectance is usually lower when the sample is present since a reference material of high reflectance (nearly 100%) is used. In transmittance measurements the throughput and the measured transmittance is usually higher when the sample is present since an open port (which has zero reflectance when viewed from inside the sphere) is typically used as a reference.

With spectrophotometers that use a chopped signal between sample and reference this error does not occur. In double beam sphere accessories, the sample and reference beam each ‘see' the same sphere. There is an active comparison between throughput with both sample and reference in place, thus there is no substitution error.

When a sample and a reference are of similar reflectance, the substitution correction is very small; at worst it may reach as much as 4-5%. In quality control applications where a threshold value is used, this may not be a concern, as the error can simply be built into the threshold. This is also true if only peak position information is required, as single beam correction only concerns the photometric scale. Submitted By Labsphere, Inc.