Simultaneous two-wavelength holographic interferometry ina superorbital expansion tube facility

A new variation of holographic interferometry has been utilized to perform simultaneoustwo-wavelength measurements, allowing quantitative analysis of the heavy particle and electron densities in a superorbital facility. An air test gas accelerated to 12 km/s was passed over a cylindrical model, simulating reentry conditions encountered by a space vehicle on a superorbital mission. Laser beams with two different wavelengths have been overlapped, passed through the test section, and simultaneously recorded on a single holographic plate. Reconstruction of the hologram generated two separate interferograms at different angles from which the quantitative measurements were made. With this technique, a peak electron concentration of (5.5 ± 0.5) × 10²³ m⁻³ was found behind a bow shock on a cylinder.