Emission Measures Demystified

We review the terms spectral radiance and spectral irradiance, and show how their precise definitions are crucial for interpreting observations made with different instruments covering widely different energy or wavelength ranges. We show how the use of column and volume emission measures in different solar physics and astrophysics communities has led to confusion in relating measured extreme-ultraviolet and soft-X-ray spectra with theoretical spectra generated, in particular, using CHIANTI. We describe a method for obtaining spatially integrated X-ray line and continuum spectra using CHIANTI that requires a column emission measure when only the plasma temperature and volume emission measure, but not the source area, are known from observations.