$traceraypath

$traceraypath is the command to output the ray's path. Because recording the ray path could wast a huge space if many photons are ran. Therefore, the software will only output certain photons.
The command is

$traceraypath
N_photons 

N_photons means the program save the ray path every N_photons. 

For example:

$traceraypath
100

The $traceraypath setting in GUI interface is here
Press Ray Tracing, check the box, and fill in the field as needed!