The TCV tokamak is equipped with a four-channel ultrasoft x-ray monochromator allowing low resolution (λ/Δλ ∼ 30) measurements in the energy range 200–800 eV. Its main purpose is to simultaneously monitor emission from four of the main Kα resonance lines of highly ionized light impurities such as boron, carbon, nitrogen and oxygen. Wavelength selectivity is achieved using synthetic multilayer mirrors having 30–40 alternate layers with layer periods in the range 3–7 nm, depending on the design wavelength range. Recently the instrument was modified for measuring the radial profiles of Kα and Lyman-α lines from He-like and H-like intrinsic carbon impurities to allow estimates of impurity diffusivities by comparisons with simulations using the impurity transport code STRAHL.