> For the complete documentation index, see [llms.txt](https://simplemetalens.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://simplemetalens.gitbook.io/docs/smd-pro/topical-discussions/focusing-efficiency.md).

# Focusing Efficiency

Focusing efficiency is the power within the *focal area* divided by the [power incident on the lens](#user-content-fn-1)[^1] \[[ref.](https://www.nature.com/articles/ncomms8069)]. In SMD PRO, two shapes of focal area are available: circular and square (see image below). This provides users with options to evaluate their focusing efficiency for different lens aperture shapes. For example, if the metalens was used with a square aperture and the shape of the beam after passing through the lens was also square, then it may be more reasonable to use square focal area.

Available options for L<sub>focal</sub> in SMD PRO:

* 4 X FWHM
* 6 X FWHM
* 8 X FWHM
* Custom

<figure><img src="https://1531699250-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FFaobmSDh9ItlF3pOiSuB%2Fuploads%2Fr95otKdcfb5R1SSYnsOI%2Fimage.png?alt=media&amp;token=4647d7d5-0044-4061-83b5-f5a0499a9b87" alt="" width="563"><figcaption></figcaption></figure>

[^1]: For MOI lenses, incident power is the power incident on the unit cell layer.&#x20;

    For SOI lenses, it's the power incident on the substrate.