used to calculate how much attenuation has been introduced to the light beam fig4 a shows an exemplary photocurrent curve 400 and fig4 b shows the corresponding electronic structure 402 of the assembly 200 of fig2 a what is not shown in fig4 a is that a feed back loop e g a circuit in which the photocurrent can be processed and used to control the movement of the common light blocker 304 to refine the attenuation through some miner blocker position adjustment in another embodiment of the invention the common light blocker 304 is actuated with rotational movement such as by a dc motor or stepping motor fig5 a and fig5 b show respectively two movements which demonstrates that a common light blocker has a rotational axis as indicated the light blocking is performed complementarily in the same way as described above on the left side of fig5 b the common light blocker 502 rotates anticlockwise so that the light beam from the collimator is blocked while the ir beam goes directly to the ir sensor so that a maximum value of the photocurrent from the ir sensor is produced on the right side of fig5 b the common light blocker 502 rotates clockwise so that the light beam from the collimator is unblocked while the ir beam is blocked so that a minimum.