The camera of the CANGAROO-III Cherenkov Telescope is composed with
many photomultiplier tubes(PMTs).
It causes dead space between the PMTs.
Light guides lead the incident photons which might come to
the dead space, to the effective area of each PMT.
We had applied the Monte Carlo simulation to various shapes
of light guides to evaluate their performance, compared with
experimental values, and decided the optimal shape of
the light guide for the CANGAROO-III Telescope.
We calculated the light collection efficiency as a function
of incident angle for various types of light guides using
the Monte Carlo simulation. As a result, the best efficiency of
the light guide was obtained for the following conditions.
Cross sectional shape cut along the center line of the light guide
should have the Winston Cone shape, though
the entrance finestra is hexagonal.
Gap between the light guide and the PMT should be as small as possible.
Good light collection efficiency is very important to decrease
the detectable energy threshold of Cangaroo-III telescope to around
200 GeV. It is very meaningful to observe this energy region
to bridge TeV region observation by the Cherenkov/Air shower ground
based detectors and GeV region observation by satellites.