Micro lens array diffuser is a special type of diffuser that works using the principle of geometrical optics. Their behaviour can be then understood and fully modeled by just considering rays’ trajectories. Being refractive optics, Micro Lens arrays can operate at a broad spectrum of wavelengths. This is in contrast to diffractive diffusers that have much greater chromatic dispersion, making them more suitable for monochromatic light. The main function of a microlens array diffuser is to scramble the irradiance from an input beam at a given angle. This means that any non-uniformity on the beam is effectively averaged out, and the illumination profile is shaped to the desired shape with controlled divergence angles.
The surface of the micro lens array, as the name indicates, is a succession of small lenses, also referred to as lenslets. Each lenslet can be fully described by the local radius of curvature and, in some cases, by a conic constant as well. When a beam traverses the array, each element on the array forms a focal spot immediately after it. Then, after some small distance, the sub-beams overlap with each other, and a new, more uniform, irradiance pattern emerges. A remarkable fact about the new irradiance envelope is that it resembles a Flat Top beam regardless of the input beam shape. This is due to the fact that each lenslet samples a small portion of the original beam and projects it to a given numerical aperture, and within that patch the irradiance is almost uniform. The shaping is determined by the ordering of the micro lenses, so most illumination shapes produced are square, lines or rectangles, though hexagons are also possible.
There are many applications in which micro lens array diffusers can be used. In medical applications, they can be used as homogenizers for high-power lasers. The same goes for applications in materials processing with lasers. Another interesting application is in digital cinema projectors in which any non-uniformity from the light sources has to be erased before illuminating the imager device.
Micro lens arrays are also widely used in the most common wavefront sensing device of all, which is the Shack-Hartmann wavefront sensor. In this device, a beam of light is sampled multiple times with a micro lens array and a detector is brought forward to the focal plane of the lenslets. The detector is able to capture the precise lateral position of each focal spot which bears a direct relation to the local curvature of the beam area being sampled. With all this information the overall wavefront shape can be reconstructed.
