AQ-00275-000, Rev. 3
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from structured samples. If any of these variations are in the field of view of the detector, a pertur-
bation in the signal level will result. Signal fluctuations as high as 30% have been demonstrated by
rotating a directional reflecting sample about its axis. Similar magnitudes of error have been
observed by measuring samples against references with vastly different bidirectional reflectance
distribution function (BRDF) characteristics.
The effects of wall radiance variations in Labsphere accessories have been reduced through inte-
grating sphere design. The location of sample and reference ports on the sphere, as well as corre-
sponding baffling, have been placed at equal angles from the optical axis of the detector. This
minimizes error due to non-uniformities in angular response of the detector and optical systems.
Port Induced Errors
Theoretically, the reflecting surface of your sample should coincide with the inside surface of the
sphere wall. Most sample surfaces, however, are not spherical in design, so that some small portion
of diffuse reflection may be directional in nature. Using the 8° sample wedge at the sample port
insures this directional component remains inside the sphere and its contribution counted by the accessory
detector.
In any practical integrating sphere, the port will have a finite thickness. As shown in Figure 10, the
edges of the sample reflectance can obstruct radiation diffusely reflected off the sample surface.
This affect is called “sample recess error” and can amount to as much as several percent for
recesses as small as 1 millimeter. Some compensation is provided by mounting the reflectance
standard at the same configuration.
(a)
(b) (c)
Figure 10. Sample configurations that produce port induced errors causing superficially low reflectance
measurements. The example illustrated in figure (a) is the best configuration possible you can expect to
achieve.
Sample
Incident Beam
Sample
Incident Beam
Translucent Sample
Incident Beam