Quantitative two-dimensional shadowgraphic set-up for high-sensitivity   measurement of low-density laser-plasmas
  We present a quantitative shadowgraphic method which can measure the density of a laser-generated plasma in air with sensitivity and resolution comparable or better than traditional interferometric techniques. Simultaneous comparison of both shadowgraphy and interferometry has been carried out allowing the experimental evaluation of the reliability of the shadowgraphic method. 
Amrutha Gopal, Stefano Minardi and Michael Tatarakis