What is a compliant surface?

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The compliant surface overlay is used to describe an overlay placed above the acrylic. The compliant surface overlay needs to be made of a material that will "couple" with the acrylic surface under pressure and set off the FTIR effect, and then "uncouple" once the pressure is released. Note that compliant surfaces are only needed for FTIR - not for any other method (DI, LLP, DSI).

The compliant surface overlay can also be used as a projection screen.

Rosco Grey, as described in Jeff Han's paper is a very poor compliant surface overlay despite the fact that it's a good stand alone projection screen.

The Rosco Grey rear projection screen can be placed beneath the acrylic as well, as a diffuser, but the blobs that are detected by the camera get more blurry/less sharp.


Advantages of a compliant surface

  • Protects the expensive acrylic from scratches
  • Blocks a lot of light pollution.
  • Provides consistent results (the effectiveness of the bare acrylic touch seems to be down to how sweaty/greasy yours hands are).
  • Zero visual disparity between the touch surface and the projection surface
  • Pressure sensitive
  • Seems to react better for dragging movements (at least in my experience)
  • Brighter blobs to track, as there is no longer a diffuser between the IR blob light and the camera.

LCD FTIR Difficulties

Developing a compliant surface for a LCD FTIR setup is difficult, as the surface must be absolutely clear and distortion-free, so as to not obscure the LCD image. This difficulty is not present in projection-based FTIR setups, as the image is projected onto the compliant surface itself. To date, no one has successfully built a LCD FTIR setup with a 100% clear compliant surface. However, several individuals have had success with LCD FTIR setups, with no compliant surface whatsoever. It appears that the need for a compliant surface largely depends on how strong your blobs are without one, and your specific setup.

Recommended surfaces

Currently the hunt is on to find a compliant surface overlay that works as both a projection surface as well as a touch surface.

Users have been experimenting with various rubber materials like silicone.


There have been several methods introduced for applying compliant surface material to projection surfaces.

Tinkerman's Method


  • Silicone-based (GE Silicone I used by Tinkerman)
  • Textured surface
  • "Rolling" method of application
  • Thinned with xylol/xylene/toluene/lacquer thinner
  • Multiple coats

Sulky Solvy


  • Easy to make
  • Very cheap
  • Weaker blobs in comparison to other methods

Sorta Clear 40

  • In-between strength blobs


http://nuigroup.com/forums/viewthread/2383 http://nuigroup.com/forums/viewthread/2911/