Standard low-power optics treats visibility loss as a readout problem: the detector still records photon locations, but the measured hit pattern no longer separates cleanly into bright regions and dark regions.This paper controls that readout explanation before the prediction is tested. First, a stable visible interference pattern is made. Then the dark-region background, detector noise, false counts, pixel scale, position uncertainty, and minimum distinguishable bright-dark difference are measured. That calibration sets the measured visibility limit of the tested setup.After that calibration, detector sensitivity is part of the threshold test.This paper predicts that the measurable interference zone shrinks as the beam-waist to opposite-polarity + - coupling-event distance increases, until the setup reaches a measurable-visibility threshold.The predicted measurable-visibility threshold distance is:R_low = z_R sqrt(P/P_low - 1)