An obturator for measuring incident light in a remote situs such as a body cavity which includes a treatment tubular member through which light is delivered to the remote situs and one or more auxiliary tubular members through which incident light in the remote situs is transmitted to an external light detector. The treatment and auxiliary tubular members are attached to a connector element which is in turn connected to a cystoscope lens port. The treatment and auxiliary tubular members are aligned off-center with respect to the central axis of the obturator so that a cystoscope lens can be inserted in the lens port and pass through the axial center of the obturator. Each auxiliary tubular member receives incident light from a different portion of the remote situs. The apparatus is particularly useful for conducting phototherapy in body cavities and has been demonstrated in the phototherapy treatment of superficial cancer in the bladder.