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Numerical Aperture (Wikipedia)
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Point-spread function (PSF)
In fluorescence microscopy a point source of excitation light is focused on the specimen as a diffraction limited spot, which, in the case of a high NA oil objective (NA=1.4 or above),for example, will have a diameter of 200-300 nm, depending on the excitation wavelength. The blurred extended diffraction image is described by a point-spread function, and is a measure of image degradation. It can be determined theoretically using the microscope control software or commercial algorithms, or by taking image stacks through sub-resolution fluorescent beads mounted in the same medium and emitting at a similar wavelength as fluorophores as the specimen.
Working Distance (WD)
The working distance is the distance from the front of the lens to the nearest surface of the coverslip when the specimen is in sharp focus. For objective lenses designed for use without a coverslip the surface of the culture vessel or specimen in used.
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