Oxford Laser Imaging
The Laser Imaging method provides instantaneous images of a spray that allow for post-processing of droplet characteristics from the images. The images are spatially calibrated and thus have an associated length scale reference with each image frame in order to measure the droplet/particle size; furthermore, image-pairs may be acquired in order to track droplet movement and determine velocity characteristics. Other characteristics may be determined from these images as well, including sphericity, solids content, and concentration.
While the methodology is nearly identical, there are various commercial instruments available from companies such at Oxford Lasers and LaVision; Spraying Systems Co. currently uses the Oxford Imaging instrument.
Laser imaging uses a diffused laser light source, and an in-line camera to image a small measurement volume (typically one square centimeter). The laser is pulsed to illuminate the droplets for a only a fraction of a second, resulting in an instantaneous spray image. The backlit imaging method allows for clear identification of droplets or particles as they block some percentage of the laser light from reaching the camera. These images are then analyzed to recover droplet size and velocity information (velocity is determined using two images with a known time delay between the pair). Image analysis is used to determine sphericity as well as a number of other characteristics of the investigated spray.
Laser imaging offers many unique benefits due to its full-frame image results; standard or proprietary image analysis may be used to determine nearly any spray characteristic. Because the laser illuminates the droplets from the back-side, bubbles and/or solid content within the droplets is often distinguishable (if present).
The laser imaging system is limited in that each droplet must be visually distinguishable and is therefore best suited for low droplet-concentration sprays.
The Oxford laser-imaging instrument is used where the droplet/particle shape is of interest, or if solid particulates or bubbles are of interest in the droplets. Examples of past projects using the Oxford laser imaging system at Spraying Systems Co. include extensive measurements with the agricultural industry where solid particles are often intended to be contained within the spray droplets, as well as applications where the spray material is expected to cause the droplets to be non-spherical.
Agricultural spray with solids content
Spraying Systems Co. worked with an agricultural company to determine the optimal spray nozzle parameters to necessary to obtain a target drop size with a solids-content containing material. The desired droplet sizes needed to be large enough to minimize drift, but small enough to become evenly distributed on the target. The Oxford imaging system was used in order to visualize the solids within the droplets as well as tune the spray parameters to obtain the desired droplet size and velocity characteristics.
Viscous, non-spherical spray material
The measurement of non-spherical drops can be a very difficult task. Spraying Systems Co. worked with an academic unit to determine the required spray parameters to produce spherical droplets with various rheology materials (density, viscosity, surface tension, other additives). The result was a list of required operating conditions to ensure spherical droplets using various combinations of material rheologies.