When a lightguide rod extracts partial discharge (PD) light from gas-insulated switchgear (GIS), losses introduced by the rear-section fixture and aperture mismatch with the fiber entrance limit the usable optical output. Ray tracing was used to examine how a parabolic taper affects optical transmission and reception through a finite aperture. The selected design increased the mean radiant flux received at the fiber entrance to 7.26 times the baseline value. In integrating sphere tests under uniform illumination, the sample with a parabolic taper provided a higher detector output than the other three samples under all tested conditions. In needle–plate corona tests in a laboratory GIS model, the main optical responses corresponded in time to the ultrahigh-frequency (UHF) envelope and shared the same temporal features. The optical channel also recorded clusters of low-amplitude responses during the negative half-cycle that were not clearly resolved by the UHF channel; these responses were accompanied by activity in the auxiliary ultrasonic channel. These results identify rear-section transmission loss and finite-aperture reception as constraints on lightguide design and provide experimental evidence for coupling lightguide rods to transmission fibers in optical PD detection in GIS.