Note on Target Diffuser

Note on flat-top diffuser efficiency

Some applications require uniform (flat-top) illumination of a target region where it is desirable to have most of the energy scattered by the diffuser concentrated at the target.

Efficiency is thus defined here as the “target efficiency,” or the ratio of the energy within the target area to the total energy in the observation plane.

Note that AR-coatings do not affect the target efficiency only the total transmission through the diffuser.

Here is a simple example:

As an example, assume the target distribution is a square region at the observation plane. From the geometrical optics point of view, it is theoretically possible to concentrate nearly all the light incident in the desired domain, in this case a square. In actuality, however, there is always some fraction of the energy that makes it to the observation plane but falls outside of the target area.

Example of measured scatter from an Engineered Diffuser™ designed to produce a 60x60 deg square pattern>

Note the energy outside the target area. The target efficiency is measured at about 84%.

The diffuser is composed of lenslets of size in the range of 40-60 microns.

There are three main factors that affect the target efficiency:

Fabrication technique – induces modifications to the theoretical profile that leads to wide-angle scatter losses.
Diffraction effects – the size of the lenslets that define the diffuser surface determines the intensity fall-off at the edge of the target pattern.
Divergence angles – smaller diffuser divergence requires larger feature sizes to maximize target efficiency.

We have estimated the target efficiency for flat-top illumination (see figure below) as a function of the minimum lens size used to produce the Engineered Diffuser™ for various values of diffuser divergence.

Please use this plot to help guide the choice of diffuser feature sizes for those applications where efficiency is critical.

Also note that if the desired illumination is not flat-top these efficiency estimates do not apply.

The above graphic should serve only as an estimate of flat-top target efficiency for collimated illumination and standard manufacturing and design conditions.

Actual efficiency depends on operating set-up, manufacturing conditions, and design philosophy. It is sometimes possible to improve efficiency by addressing some or all of these items.

A basic rule to keep in mind is that the input illumination should be several times larger than the diffuser minimum feature to ensure best uniformity. At the same time, larger feature sizes are needed to maximize efficiency. Therefore, as it is common in these kinds of problems, one needs to find the best compromise between efficiency and uniformity.