Fluorescence measurements (contd...)

A few considerations relevant to measurements are listed below:

1. In gases, particulates may constitute the fluorescence medium. Fluorescent dyes can be added to liquids in small concentrations as markers. A laser sheet is invariably used to fluoresce these chemicals.
   
2. The emission is usually in the visible range and a CCD detector can be used to record the intensity field. The camera is then aligned to image the plane of the laser sheet.
   
3. Based on the discussion of Quantum scattering, regions of high intensity can be associated with regions of high dye concentration (also see Equation 7.5).
   
4. The relative distribution of particulates/dyes in the flow field is a measure of the concentration field, and, indirectly the flow field itself.
   
5. Fluorescence measurements invariably employ a tunable laser, such as a dye laser, so that the frequency of the incident radiation can be matched with the energy levels of the material under consideration.
   
6. In practice, one uses a pulsed laser so that high intensities after scattering are possible. Higher laser power can help in detecting small concentrations of species, particularly in studies related to emissions in gases.
   
7. The pulsing frequency can also be used to distinguish bonafide emissions from the background.
   
8. Ideally, the laser wavelength is tuned so that the fluorescence output corresponds to a change in one quantum level and is, hence, monochromatic.
   
9. In the event of multiple transitions within the material, a fluorescence spectrum is obtained. Such an output may be valuable under many circumstances.

For an example of fluorescence imaging, see movies 1-5 ( courtesy Dr. Arun Kumar Saha, Associate Professor, Department of Mechanical Engineering, IIT Kanpur ). These images show flow from a synthetic jet into a stationary fluid as a function of aspect ratio of a rectangular slot (movies 1 and 2) and nozzle diameter (movies 3-5). The frequency matches the resonance value of the piezoactuator. Synthetic jets take fluid from the surroundings and push them back with added momentum. The resulting flow is pulsatile and has applications in flow control and cooling of electronic equipment. For an introduction to synthetic jets, read article 7 .