An important problem in modern imaging is the imaging of a plane embedded within a thick specimen, without the image being degraded by defocused light scattered from outside the plane of focus (the out-of-plane scattered light). This problem arises particularly in microscopy, where for example, mounted specimens may be thin, yet thick compared to the depth of focus of the imaging system. In biological microscopy, this process has been called optical sectioning.
The light scattered from out-of-plane objects reduces the contrast of the image and adds spurious structure, i.e., noise, to the image. It is important to remove or significantly reduce the effect of this spurious light. One long-known method had its major development in the field of microwave radar: a short pulse is radiated, and the reflected light is separated on the basis of its time of return. Alternatively, the light could be, instead of a short pulse, a long pulse of short coherence length, such as a chirp pulse, a long pulse coded so as to have a narrow autocorrelation function, or simply a cw pulse of short coherence length. The process works in reflection, but is less effective in transmission. One of the most successful applications of this technique to optics is optical coherence tomography (OCT).
Confocal imaging is another method for optical sectioning. This method can be used in either the reflection or the transmission mode, although the reflection mode is more common. In this method, a double scanning process occurs. A point source is imaged onto the object plane; this point image then scans the object plane. In a second cascaded process, the illuminated spot is imaged onto a detector, which scans the image plane in synchronization with the object-plane-illuminated spot. Analysis shows that although each of the two scanning processes individually contributes no out-of-plane suppression compared to a nonscanning process, the two scanning processes in tandem produce significant suppression.
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