An optical fluid monitoring system for imaging particles in at least one conduit carrying a flowing fluid, comprising: at least one fluid flow conduit (14,15,16,51,75,77,81,91); and a plurality of optical sensors (10,11,12,30,40,52,53,54,63,64,74,79,82,83), the plurality of sensors being arranged in series along a single fluid flow conduit or being arranged on a plurality of fluid flow conduits (14,15,16,75,77,81,91), each sensor positioned to transmit laser optical energy into a transparent viewing window in a fluid flow conduit in a direction across the direction of flow, each sensor having an optical imaging system for receiving the optical energy after it has passed through the fluid flow and for imaging particles in the fluid flow; characterized in that: when the plurality of sensors (52,53,54) are arranged in series along a single conduit, an upstream one of the plurality of sensors is configured to image a range of larger size particles and a downstream one (53,54) of the plurality of sensors is configured to image a range of smaller size particles, and for each sensor, the conduit thickness at that sensor is such that the particles in the size range to be imaged are in the optical near field of the imaging system, and wherein the upstream sensor conduit has a conduit thickness that is larger than the downstream sensor conduit; and when the plurality of sensors are arranged on a plurality of fluid flow conduits (14,15,16,75,77,81,91), there is a main flow passageway (62,76,80,90) and each of the fluid flow conduits (14,15,16,75,77,81,91) receives a portion of a flowing fluid from the main flow passageway (62,76,80,90), a first of the plurality of sensors (10,64,74,82,92) is configured to image a range of larger size particles and a second of the plurality of sensors (11,12,63,79,83,93) is configured to image a range of smaller size particles, and for each sensor, the conduit thickness at that sensor is such that the particles in the size range to be imaged are in the optical near field of the imaging system, and wherein the first sensor conduit has a conduit thickness that is larger than the second sensor conduit. at least one fluid flow conduit (14,15,16,51,75,77,81,91); and a plurality of optical sensors (10,11,12,30,40,52,53,54,63,64,74,79,82,83), the plurality of sensors being arranged in series along a single fluid flow conduit or being arranged on a plurality of fluid flow conduits (14,15,16,75,77,81,91), each sensor positioned to transmit laser optical energy into a transparent viewing window in a fluid flow conduit in a direction across the direction of flow, each sensor having an optical imaging system for receiving the optical energy after it has passed through the fluid flow and for imaging particles in the fluid flow; characterized in that: when the plurality of sensors (52,53,54) are arranged in series along a single conduit, an upstream one of the plurality of sensors is configured to image a range of larger size particles and a downstream one (53,54) of the plurality of sensors is configured to image a range of smaller size particles, and for each sensor, the conduit thickness at that sensor is such that the particles in the size range to be imaged are in the optical near field of the imaging system, and wherein the upstream sensor conduit has a conduit thickness that is larger than the downstream sensor conduit; and when the plurality of sensors are arranged on a plurality of fluid flow conduits (14,15,16,75,77,81,91), there is a main flow passageway (62,76,80,90) and each of the fluid flow conduits (14,15,16,75,77,81,91) receives a portion of a flowing fluid from the main flow passageway (62,76,80,90), a first of the plurality of sensors (10,64,74,82,92) is configured to image a range of larger size particles and a second of the plurality of sensors (11,12,63,79,83,93) is configured to image a range of smaller size particles, and for each sensor, the conduit thickness at that sensor is such that the particles in the size range to be imaged are in the optical near field of the imaging system, and wherein the first sensor conduit has a conduit thickness that is larger than the second sensor conduit. when the plurality of sensors (52,53,54) are arranged in series along a single conduit, an upstream one of the plurality of sensors is configured to image a range of larger size particles and a downstream one (53,54) of the plurality of sensors is configured to image a range of smaller size particles, and for each sensor, the conduit thickness at that sensor is such that the particles in the size range to be imaged are in the optical near field of the imaging system, and wherein the upstream sensor conduit has a conduit thickness that is larger than the downstream sensor conduit; and when the plurality of sensors are arranged on a plurality of fluid flow conduits (14,15,16,75,77,81,91), there is a main flow passageway (62,76,80,90) and each of the fluid flow conduits (14,15,16,75,77,81,91) receives a portion of a flowing fluid from the main flow passageway (62,76,80,90), a first of the plurality of sensors (10,64,74,82,92) is configured to image a range of larger size particles and a second of the plurality of sensors (11,12,63,79,83,93) is configured to image a range of smaller size particles, and for each sensor, the conduit thickness at that sensor is such that the particles in the size range to be imaged are in the optical near field of the imaging system, and wherein the first sensor conduit has a conduit thickness that is larger than the second sensor conduit.