If a scanning illumination spot is combined with a detector array, we acquire a 4 dimensional signal. Unlike confocal microscopy with a small pinhole, we detect all the light from the object, which is particularly important for fluorescence microscopy, when the signal is weak. The image signal is basically a cross-correlation, and is highly redundant. It has more than sufficient information to reconstruct an improved resolution image. A 2D image can be generated from the measured signal by pixel reassignment. The result is improved resolution and signal strength, the system being called image scanning microscopy. A variety of different signal processing techniques can be used to predict the reassignment and deconvolve the partial images. We use an innovative single-photon avalanche diode (SPAD) array detector of 25 detectors (arranged into a 5x 5 matrix). We can simultaneously acquire 25 partial images and process to calculate the final reconstruction online.