SIM

SIM (Structured Illumination Microscopy), or Structured Illumination Microscopy, is a type of super-resolution microscopy, the principle of which is analogous to the imaging of moiré patterns formed by superimposing two distinct gratings. SIM uses a light beam with a striped pattern produced by diffraction on the grating to illuminate the sample. As the resolution of the optical system decreases, the rasters themselves can no longer be imaged, but the moiré effect remains visible. Due to this effect, information about the existence of rasters with spatial frequencies higher than the optical system can directly image is retained in the image. By computing the moiré effect (using the Fourier transform), it is then possible to calculate what the structures that caused the effect looked like, resulting in an image with much finer detail and a resolution of around 100 nm. A super-resolution image is therefore created by an image restoration process that uses a series of frames captured at different phases and moving in the direction of the stripe pattern.

The SIM method can be applied both to 2D objects (monolayers, TIRF microscope images of cells, etc.) and to 3D objects - by capturing a series of images focused at different depths of the sample, which is illuminated by a three-dimensional raster.