I got some points after reading all the postings in the forum about the junior soocer robots .

The competition rules are as follows :

Playground of The competiotn is as follows :
87 cm by 119 cm (oversize A0)
The floor of the playing field is covered with a printed, matte greyscale. prefect grayscale from white to black.
around the playground covered with a 14 cm wall ,The walls are painted matte black.
Two goals each side (Color is Gray :25%black+75%white)

The problem is : no one can use any infrared light transmitter ,because the ball itself transmits IR, so anything that sends IR will be recognized as ball and ...

From my point the best way to find the goals can be achieved with using a LED (Visible light) to send light and a LDR to receive the transmitted light .
But i am not sure this don't effects the other robots to find the ball ,
thus i think i have to use Passive LDR .

Any thought , help would be appreciated.

Farshid J.H.