When the Lens Goes Blind

Abstract. A companion note that takes the phase of the Li-Sekatskii detector rather than its modulus. At the matched lens width the phase is exactly -pi/2 independent of the zero's displacement and height, so a planted off-line quartet contributes on a strict cycle of period four: detection at n = 0 mod 4, active masking at n = 2, and no exponential term at all for odd n. Following the rate to the far end recovers Sekatskii's Theorem 3 as the other zero of a single curve, and the detection index is measured to grow linearly in the shift. Two natural refinements are tested and both fail.