Widefield microscopy illuminates a large area of the sample with light using a collimated beam (one that has parallel rays of light). When fluorescent samples are imaged this can produce background fluorescence from the other planes of the sample, which can interfere with the resolution and optical sectioning. This can cause considerable problems for thicker samples which inherently have more out-of-focus signal, these samples may be better suited to a confocal microscope or to structured illumination microscopy (SIM) for higher resolution.