Results . Basic principles . The relevant mechanism for the upconversion imaging typically relies on the sum-frequency generation (SFG) in a nonlinear crystal. The involved fields satisfy the energy conservation law as ? ω u ?=? ? ω s ?+? ? ω p , where ? is the Planck constant, ω s , p , u are the optical angular frequencies for the signal, pump, and upconverted light. Additionally, the efficient frequency conversion requires the momentum cons...