The field of view of a boiling water reactor (BWR) in-core detector is evaluated via a combined R-Z transport and X-Y-Z diffusion theory model. A cell homogeneous X-Y-Z diffusion theory model is sufficient for the evaluation of the adjoint function distribution around an in-core BWR detector. All types of subchannels (i.e., side, center, and corner) contribute to the detector signal fluctuations, and the side subchannels are the dominant contributor. Finally, the measured flow velocity via the cross-correlation method in a BWR is an averaged quantity rather than a localized quantity.