Temperature gradients inside the laser-heated diamond cell are not measured in the axial direction, along the optic axis, yet can bias estimates of the peak temperatures achieved in optically thin (dielectric) samples such as Mg-silicate perovskite. Thermal diffusion calculations show that the observed thermal-radiation spectrum is not significantly affected by axial temperature gradients. The apparent (measured) temperature underestimates the actual peak sample temperature by 350--850 K for 4000<T<6000 K. ¿ American Geophysical Union 1996