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We introduce an application of thermionic emission in a PMT photocathode. Because of the nonlinear dependence of thermionic emission on absorbed laser power, a conventional PMT is found to produce a virtual pinhole effect that rejects unfocused light at least as strongly as a physical pinhole. This virtual pinhole effect is exploited in a scanning transmission confocal microscope equipped with a cw laser source. Because the area of the PMT photocathode is large, signal descanning is notdoi:10.1364/ol.33.001345 pmid:18552953 fatcat:aqrbdainkrh55dc7ik7e56elcq