Achieving accuracy in spectroradiometric measurements of temperature in the laser-heated diamond anvil cell: Diamond is an optical component

We present theoretical calculations of the optical effects of the diamond anvil on the accuracy of spectroradiometric temperature measurements made with the laser-heated diamond anvil cell. Considering the dual effects of wavelength-dependent index of refraction and wavelength-dependent absorbance, we find that systematic errors can be minimized by using low numerical aperture optics and by characterizing wavelength-dependent absorbance for each anvil as part of system response measurement. Quantitatively, failure to observe these guidelines can lead to systematic errors of hundreds of degrees. This physical effect may be one part of the origin of the discrepancy of experimental measurements of the melting temperature of iron at megabar pressures.