“…In the measurement problem described in the present article, linear movements of the surface under study are possible along all the axes, in connection with which, the use of the widespread measurement scheme with one QPD is not possible, because a linear displacement of the laser beam without changing its angular position will lead to a shift in the projection spot on the QPD surface, which will be falsely interpreted as an angular deviation. To solve this problem, the authors propose the use of approaches close to photogrammetry [11,12]: using two beam projections (Figure 2) to determine the spatial location of the light source, including linear (x, y, z) and angular deviations (αx, αy, αz) in the global coordinate system (X, Y, Z), using the coordinates of two projections (x′, y′ and x″, y″) in the QPDs coordinate systems, (X′, Y′) and (X″, Y″), respectively. To solve this problem, the authors propose the use of approaches close to photogrammetry [11,12]: using two beam projections (Figure 2) to determine the spatial location of the light source, including linear (x, y, z) and angular deviations (α x , α y , α z ) in the global coordinate system (X, Y, Z), using the coordinates of two projections (x , y and x , y ) in the QPDs coordinate systems, (X , Y ) and (X , Y ), respectively.…”