  To enhance the resolution of a confocal laser scanning microscope the
additional information of a pinhole plane image taken at every excitation scan
position can be used [C. J. R. Sheppard, Super-resolution in confocal imaging,
Optik 80, 5354 (1988)]. This photon reassignment principle is based on the fact
that the most probable position of an emitter is at half way between the
nominal focus of the excitation laser and the position corresponding to the
(off centre) detection position. Therefore, by reassigning the detected photons
to this place, an image with enhanced detection efficiency and resolution is
obtained. Here we present optical photon reassignment microscopy (OPRA) which
realises this concept in an all-optical way obviating the need for
image-processing. With the help of an additional intermediate optical beam
expansion between descanning and a further rescanning of the detected light, an
image with the advantages of photon reassignment can be acquired. Due to its
simplicity and flexibility this method has the potential to enhance the
performance of nearly every laser scanning microscope and is therefore expected
to play an important role in future systems.
