Measurement Geometry Basics: Hemispherical
|d:8 hemispherical geometries present the sample with diffuse illumination and approximately normal detection. Diffuse illumination is accomplished by the use of an integrating sphere. Light introduced into the sphere is sent thorugh a series of reflections off the diffuse white sphere wall, quickly resulting in the diffuse illumination incident on the sample.|
There are two standard configurations of diffuse geometry: de:8° and di:8°. The former excludes the specular component and the latter includes it. How this is accomplished is described in the movies below depicting the light paths in each respective configuration.
The movies below require a quicktime plugin or some other means to view a MOV file. If you have trouble in your browser, try downloading the files by right-clicking on these links: de:8° or di:8°. Also, some browsers start the movie in reverse. Clicking the image should get the aorrows going forward.
|Schematic light path showing de:8° specular excluded geometry (SCE).|
|The diagram above shows the incoming light, scattering diffusely off the sphere wall. The light incident on the specular port is absorbed, and cannot be measured by the detector. Consider that a specularly-reflected photon must bounch off the sphere wall at a point equal and opposite the detection port (equal and opposite relative to the sample normal). If the specular port absorbs any such photon, then there is no specular reflectance getting to the detector.|
|Schematic light path showing di:8° specular included geometry (SCI).|
|In a similar fashion, you can see that the (white) specular port reflects light back into the sphere, and there is now a path for specularly-reflected photons to be detected.|