Figure 9.9 — Backward scattering position detection Figure 9.9 — Backward scattering position detection. (a) Schematic of a backward position detection set-up. (b) losses can be reduced using a polarising beam splitter (PBSqpd) in combination with a linear polariser (Pol) and a quarter-wave plate (QWP). (c) Images of objective back-focal-plane fields for various particle radii a and transverse displacement C = [Cx , 0, 0]. The circles delimit the field captured by the condenser lens with NAo = 0.20, 0.65 and 1.20 (solid line) from the centre outwards. (d) Signal from a QPD as a function of Cx for a = 400 nm (dotted line), 600 nm (dashed line) and 1000 nm (solid line) for an incident beam with power 1 mW and detectionNAo = 1.20.