ESP particle collector (1) for collecting particles in a particle containing gas stream, comprising an inlet section (4), a collector section (6), and an electrode arrangement (8), the inlet section comprising a flow tube (10) defining a gas flow channel (12) therein bounded by a guide wall (24) extending between an entry end (14) and a collector end (16) that serves as an inlet to the collector section (6), the entry end comprising an inlet (28) for the particle gas stream and a sheath flow inlet portion (26) for generating a sheath flow around the particle gas stream, the collector section comprising a housing (18) coupled to the flow tube, and a collector plate (20) mounted therein having a particle collection surface (23). The ESP particle collector comprises an optical measuring instrument (9) configured to transmit light through the collector plate along a centre axis (A) orthogonal or substantially orthogonal to the particle collection surface for optical analysis of the collector plate particle collection surface to measure particles collected thereon, and wherein the flow tube has a bent portion (15) such that the entry end (14) is positioned out of the centre axis A to allow the light to be transmitted through the collector plate in the direction of the centre axis and to be picked up without interfering with the gas flow or the gas inlet.