Iodine absorption filters for Doppler global velocimetry

Abstract

The Iodine absorption filter is a critical component in the implementation of the emerging flow measurement technique of Doppler global velocimetry. The aim of this paper is to discuss the various factors influencing the performance of this type of frequency sensitive optical filter and to suggest how its design should be optimized. This advice, based upon the authors' own experience and a surprisingly large body of literature, is designed to help prospective users of Doppler global velocimetry. Some of the material may also have relevance to other forms of optical measurement which employ absorption filters.

Doppler global velocimeter — Development and validation with measurements in a round free jet

“…1 gives an overview of the whole set-up (emission and receiver parts in the top and in the bottom respectively). To reduce costs and avoid synchronization problems, our receiver uses only one camera, like Chan's [13] or Ainsworth's [14]. It is based on the Mach-Zehnder interferometer design except that the second beamsplitter (cf.…”

The application of laser measurement techniques to aerospace flows

“…If using three views, they should ideally be orthogonal. Also the accuracy of measurement is dependent on uncertainties in the laser frequency, the iodine cell [174] and sensitivity vector error at each pixel. Current state-of-the-art DGV systems quote errors in measurement of between 0.5 and 1 m/s [173,175,176].…”

Development and Testing of the Virginia Tech Doppler Global Velocimeter

“…The filter dimensions vary depending on the type of measurements. Chan [31] presents a detailed investigation of the use of iodine filters for DGV and gives recommendations for the best cell size ranges. He notes that choosing the length of the iodine cell represents a design compromise: longer cell bodies contain more iodine vapor and theoretically perform better, however, practical issues such as minimizing the temperature gradients in the cell body require shorter lengths.…”

“…The molecular iodine filter used in the Virginia Tech DGV was selected based on the experiences of previous researchers and recommendations of Chan [31]. The filter, manufactured by Opthos Instruments, has a 5.08 cm long cylindrical main body capped on either end with \ wave flat^ 5.08 cm diameter windows.…”

“…As previously discussed, the vapor pressure is set by controlling the temperature of the iodine cell stem. Chan [31] shows that 1% stability of the vapor pressure requires 0.04% stability in the temperature of the cell stem. Assuming this level of control on the stem temperature, the main body temperature is much less critical.…”