  We suggest a wavelength-dependent figure of merit for transparent conducting
nanotube networks, composed of the sheet resistance and the optical density. We
argue that this would be more useful than other suggestions prevailing in the
literature, because it relies on more realistic assumptions regarding the
optical parameters of real nanotubes: it takes into account the fact that the
dc resistivity depends on the concentration of free carriers, while the visible
absorption is caused by bound carriers. Based on sheet resistance measurements
and wide-range transmission spectra, we compare several commercial nanotube
types and find correlation between metal enrichment and figure of merit. A
simple graphical approach is suggested to determine if the required optical and
transport properties can be achieved by varying the thickness of the nanotube
layer or a more aggressive treatment is needed. The procedure can be extended
to oxide coatings as well.
