Difussers are optical elements that are widely used in many fields. They are used, for instance, in photography to homogenise the illumination over the subject. Most diffusers have a completely random structure and so the angular distribution of the scattered does not have a well-defined structure. In contrast, an engineered diffuser is a type of diffuser that can exert good control over the scattering angles. Thus, an engineered diffuser can be design according to the scattering requirements demanded by an specific application. Those requirements can be, for example, that the light after diffuser concentrates within certain angular boundaries and that the radiance is constant across an specific area.
Engineered diffusers can be divided into two other types of diffusers, namely:
- Broadband diffusers.
- Diffractive Diffusers.
The first type of engineered diffusers, broadband diffusers, are similar to a micro-lens array but with added capability that the radius of curvature on each lenslet does not have to be the same all across. This feature is very useful as it adds an extra degree of freedom to the design process that permits obtaining the desired angular distribution. In addition, given that the broadband diffuser is in essence a micro lens array, the performance is considered to be polychromatic.
Although the broadband diffuser can yield good results, there are some limitations in what can be achieved in terms of scattering angular distributions. In these cases, a better alternative is to opt for the diffractive diffuser which is the second class of engineered diffusers. Diffractive diffusers are based on the wave nature of light so they work by enhancing the diffractive effects. Thus, the chromatic dispersion is very high but if the input light is monochromatic this is inconsequential. A diffractive diffuser is very good at obtaining any scattering distribution in any kind of geometrical shape. The scattered light can be arranged on rectangular or circular angular area. It can consist of separated areas as well. To design a diffractive diffuser, the first step is to clearly define the desired shape of the scattered light. Then, by applying propagation algorithms that are based in some form of the diffraction integral, a back propagation calculation is carried out to obtain the array of obstacles or modulating elements at the diffractive diffuser. In practice, the design process is slightly more complicated than that as one has to consider the manufacturing constraints for this type of engineered diffuser. But this description provides a good insight into the nature of diffractive diffusers.
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