Confocal microscopy without pinhole

Confocal microscopes achieve depth resolution by use of a pinhole. On-axis, in-focus point-source objects are imaged exactly inside the pinhole, and give maximal intensity. Out-of-focus objects, even on-axis, produce extended (blurred) images, and are filtered out by the limited aperture of the pinhole. Theoretically, the depth resolution is optimal when an infinitessimally small pinhole is used [1]. However, such an ad hoc filter does not match perfectly the impulse response of any realistic optical system. In practice, the minimum pinhole size, and, hence, the depth resolution limit, are determined by light efficiency (i.e., the required dynamic range of the measurement) and the broadening of the focal spot by lens aberrations [2]. Using a complex filter, implemented as a thin diftactive element, instead of a pinhole infront of the detector, has been proposed as a means for achieving super-resolution [3, 4].