Molecular iodine cells are used in spectroscopic applications, precise wavelength calibration and as a frequency reference. The spectrum of molecular Iodine includes discret rotational and vibrational bands with very fine structure in the visible (490-650 nm) spectrum. More recently, Iodine has been used as frequency discriminators in Planar Doppler Velocimetry (PDV) and Filtered Rayleigh Scattering (FRS) systems. When used in PDV/FRS systems, the stability of the Iodine cell transmission-vs-frequency profile has been identified as a significant source of error. This instability is caused by variations in the Iodine number density. The number density (pressure) of the Iodine vapor is a strong function of temperature, and therefore, small changes in cell temperature (~ 0.1 °C) can have significant impact on the transmission-vs-frequency profile.
Planar Doppler Velocimetry (PDV, also known as Doppler Global Velocimetry, DGV) is an image-based technique that is capable of producing three component velocity measurements in a plane. With PDV, one measures the Doppler shift of light scattered by seed particles in the flow, similar to LDV. To perform PDV, the flow is seeded with scattering particles and a laser sheet is used to illuminate the interrogation region. The interrogation region is imaged through molecular absorption filters by several CCD cameras and the images are post-processed using a set of calibration images to determine velocities. With PDV, one measures the Doppler shift of light scattered by seed particles in the flow, similar to LDV. The Doppler shift is dependent on the incident light wavelength, the velocity of the scattering particle, and the observation and incident light directions. With LDV, the Doppler shift is determined using heterodyne detection of the beat frequency between the incident and scattered (Doppler shifted) light. For PDV, a molecular or atomic vapor filter is used as the frequency discriminator.
Planar Doppler Velocimetry Data