SICK GRTB18S-P2421V Photoelectric sensors. SICK GRTB18S-P2421V is a photoelectric sensors in the GR18 family. SICK identifies this exact configuration as manufacturer article 1085745. The official technical record specifies Functional principle: Photoelectric proximity sensor; Functional principle detail: Background suppression; Dimensions (W x H x D): 18 mm x 18 mm x 55.9 mm; Housing design (light emission): Cylindrical; Housing length: 55.9 mm; Thread length: 31.7 mm. The linked SICK data sheet is specific to GRTB18S-P2421V.