Thanks to everybody for your helpful suggestions. The clear winner is 
using the LED 'backwards' as photodiode - simplicity prooves geniality!!
I like the idea of not beeing dependent on an exotic single source 
component although the SFH7221 is an interesting part as well.

Thanks again to all contributers!

Tobias


Mike Harrison schrieb:

>Remember that LEDs can be made to act as photodetectors - even if you need more amplification, the
>cost of this will probably be offset by trivially simple optics (i.e.a narrow-angle LED at each
>end).
>
>  
>

>There is also this IR emitter/sensor from Infineon : 
>http://www.farnell.com/datasheets/11678.pdf
>
>A simple lens on top of this might do the trick. The die are about 1.5mm apart. 
> This is phototransistor - not sure if they make a diode version which you may need for this range, 
>
>  
>
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