Microscope spectrometer aperture
When imaging the sample with a microscope spectrophotometer, the black square in the center of the image is the entrance aperture for the spectrophotometer. The numerical aperture with respect to a point P depends on the half-angle, θ1, of the maximum cone of light that can enter or exit the lens and the ambient index of refraction. As a pencil of light goes through a flat plane of glass, its half-angle changes to θ2. It can be configured to measure the transmittance, absorbance, reflectance, polarization and fluorescence of sample areas as smaller than a micron.
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