Lateral and Axial Camera Motions Reveal Two Fixed-Error Components in Active Infrared Stereo

Multi-frame depth fusion reduces only errors that vary across observations; a static active infrared stereo camera can retain a repeatable spatial residual field. We characterized a Gemini 336L over 1.5–3.5 m and performed a Gemini 335 check. Residuals after plane fitting comprised averageable temporal and fixed terms. After registration to checkerboard coordinates, sampled fixed fields admitted an exact finite-view mean-centered partition into view-invariant and view-varying constituents. Lateral displacement decorrelated the varying constituent at 1.5–2.5 m; fixed-pose optical substitution supported its speckle association. The invariant constituent persisted across same-range lateral views and showed fractional-disparity periodicity and axial phase-dependent correlation, supporting a disparity-locking association. At 3.5 m, eight lateral views retained 57% of fixed-field mean square, whereas four axial views reduced residual root-mean-square (RMS) from 13.87 to 7.48 mm (ratio 0.540). Median total and fixed-only single-frame disparity-equivalent residuals were 0.068 and 0.056 px. For static acquisition, total error is persistent bias squared plus temporal variance, not a conditional covariance bound. Results motivate combining lateral speckle and axial disparity-phase diversity. Axial fusion was verified for registered planar residual fields at one range endpoint, not across ranges or in full-scene three-dimensional (3D) reconstruction.