Left panel: fit of the DustEM model of Compiègne et al. (2011) for the diffuse ISM heated by the standard interstellar radiation field (ISRF) of Mathis et al. (1983). The black solid line is the model, the squares are the model in the photometric bands at 100  μm, 857, 545 and 143 GHz, and the red solid line is the fitted spectrum. Right panel: relative residuals of the fit. The solid line is the continuous model and the triangles are the model in the photometric bands. The dashed line shows the relative residuals of the fit for the Draine & Li (2007) model. The increase of the residuals at wavelengths below 100  μm is caused by the contribution of transiently heated small particles.