Transformed Normalized Difference Vegetation Index (red, nir channels)
\begin{eqnarray}
ndvi & = & \frac{nir - red}{nir + red} \\
tndvi & = & \sqrt{ndvi + 0.5}
\end{eqnarray}
References
Deering D.W., Rouse J.W., Haas R.H., and Schell J.A., 1975. Measuring forage production
of grazing units from Landsat MSS data. Pages 1169-1178 In: Cook J.J. (Ed.), Proceedings
of the Tenth International Symposium on Remote Sensing of Environment (Ann Arbor, 1975),
Vol. 2, Ann Arbor, Michigan, USA.