Abstract: The present random access PCR reactor for biological analysis, comprises of a number of PCR reactors held on a platform, and one optical system to be shared by all of the PCR reactors on the platform. The optical system is held on a traverse mechanism to move it over any one of the PCR reactors that are ready to be imaged. Other PCR reactors on the platform can be accesses and replaced. The optical system has a lightpipe and a lightguide that distributes a uniform light over all the samples held on the reactor. The lightguide of the present optical system has a set of light reflecting structures that are strategically located to uniformly reflect an incoming light towards all the samples held in the PCR reactor that is being tested.