  We analyse basic thermal cloaks designed via different geometric transforms
applied to thermal cloaking. We evaluate quantitatively the efficiency of these
heterogeneous anisotropic thermal cloaks through the calculation of the
standard deviation of the isotherms. The study addresses the frequency regime
and we point out the cloak's spectral efficiencies. We find that all these
cloaks have comparable efficiency irrespective of whether or not they have
singular conductivity at their inner boundary. However, approximate cloaking
with multi-layered cloak critically depends upon the homogenization algorithm
and a large number of thin layers (at least fifty) is required to reduce
substantially the standard deviation of the isotherms.
