Formation of line profile asymmetry. It demonstrates the summation of Eq. (3). These line profiles are taken from the centre of the front-most thread. The number to the left of each set of three plots is i. Each set of three plots are elements of the values in Eq. (2) and are as follows: I_i − 1 in blue and τ_i(s) in dotted orange (left); I_i − 1exp(−τ_i(s)) in blue (center); I_i (right). The centre and right plots are added together to create the next I_i − 1. The λ subscript denotes that these are wavelength specific intensities. The units on the y-axis are 10⁵ erg s⁻¹ cm⁻² Å⁻¹ sr⁻¹. The last three panels are different, and are as follows: I₁₀, leaving the ten threads (left); the IRIS spatial PSF, in dotted blue, where the x-axis is its value normalised such that its peak is 1 and the y-axis is parallel to the slit and the IRIS spectral PSF, in orange, where x is the normalised wavelength and y is its value normalised such that its peak is 1 (centre); the resulting line profile when convolved with the spatial and spectral PSFs of IRIS, and sampled to IRIS resolution (right). This figure originally appeared in Peat (2023).