Valley-Peak Modulation in Phase Space: an Exposure-Invariant VPM and its Theta-Function Structure

Abstract: Valley-peak modulation (VPM) was introduced as a metric for quantifying read-noise in deep sub-electron read noise (DSERN) CMOS sensors. In the original amplitude-domain definition, VPM is strictly a function of both read noise and quanta exposure, yet Starkey & Fossum demonstrated exposure-independent approximations that hold in the DSERN regime. In this note we show that these approximations are truncations of a wrapped-Gaussian phase-space VPM that is exactly invariant to quanta exposure. Starting from the standard Poisson-Gaussian model, we apply a phase mapping that quotients out the integer photoelectron count. The resulting phase variable has a wrapped-Gaussian density admitting both lattice-sum and Jacobi theta-function representations parameterized only by the read noise. A closed-form expression for the phase-space VPM follows as a theta ratio, and the inverse mapping (read noise as a function of VPM) is expressible using elliptic integrals. The existing exposure-independent formulas are recovered as truncations of the lattice sums at the valley and peak. A short simulation example illustrates practical computation of VPM in phase space and inversion to recover read noise.