Particle Image Velocimetry (PIV) expresses a vast field with varying techniques and data acquisition methods. However, the main goal is providing an optical method of flow visualization. The exact information obtained depends on which method is used, with new algorithms and approaches being discovered constantly.
There are generally two ways data is obtained PIV and Particle Tracking Velocimetry (PTV) which can then be broken down into many other methods based on how exactly the data was obtained and the processing done to said data. PIV measures the velocity field of a fluid based on a Eulerian method where stated locations are observed over time to determine the flow. While PTV tracks the movement of singular particles over time, a Lagrangian approach. This provides a plot of the particles movement and by relation information about the fluid flow. They each use the same tracer particles however they look at them in different senses. If logs in a river are representations of our seed particles then PIV looks at the river and sees the logs moving through it determining how the river flows based on this information. While PTV watches the movement of individual logs to obtain similar information. Which leads to the assumption that tracking particles must be easily visualized.
Visibility being an important aspect of tracer particles is a given but how those particles are visible is where differences can come about. Tracers can be visible if they block light from reaching the visualization mechanism (eye, camera, etc.) essentially being visible as a shadow. This method is known as backlit shadowgraphy where the flow is placed between an illumination source and a camera allowing for the absence of light (shadow) caused by tracer particles to be tracked.
Another approach to assuring particle visibility is using highly reflective spheres that will reflect in the direction of your camera set-up allowing them to appear as dots of high intensity light, of the illumination source used. Lasers are most commonly used as the illumination source for this form of particle visibility. As lasers have high power, high collimation, and a relatively tight emission bandwidth. Recently LED?s are also being used as the illumination source for reflectivity visualization methods as well as backlit shadowgraphy. LED?s may not currently have the power or collimation abilities of lasers but are consistently growing in power. LED?s also have a very limited emission spectrum as well as their ease of use and low cost compared to lasers.
Finally, some tracer particles can emit their own light which allows them to be an easily distinguishable wavelength from the illumination source which can often flood the visualization area. One of the most common examples of this would be fluorescent spheres. Which when excited by the illumination source will emit a different wavelength of light. This allows the wavelength of light used as your illumination source to be filtered out providing an image with just the light from tracers. Phosphorescent spheres fall into a category similar to fluorescent particles as phosphorescence emits light similarly to fluorescence. However, phosphorescence emits over longer periods of time. Another significant difference of phosphorescent materials is their unique temperature variance which allows for them to be used as a form of temperature sensor.
With both PIV and PTV having their strengths and weaknesses there is no clear superior method. However, with advances in technology PTV is becoming more feasible and thus may overtake PIV methods due to its ability to provide greater data varieties. Visibility options also have their unique aspects that ensure their necessity in specific cases. Shadowgraphy is gaining traction in areas due to its reduced cost requirement and ease of use. While, fluorescent tracers remain as an ideal option for applications where shadowgraphy can not quite meet the necessary criteria.