This Arduino Shield allows to measure the optical density (OD) aka turbidity through a standardized cuvette. 3-Channels for different wavelengths, using 5mm LEDs. The design fits nicely a TIL-99 phototransistor.
see details on: http://hackteria.org/?p=2194
The shield was made using 3mm MDF and laser cut. The settings for rastering need to be optimized to have the right thickness to perfectly fit the LED in between. Same for the depth of the pin-header row.
To be able to raster from both sides, 2 sequential steps are needed. After the first 2 layers are rastered deeply and everything is cut out, flip over the main shield part to finalize the other side.