Amplitude of density fluctuations in the inner 200 kpc (radius) region in the simulated relaxed galaxy cluster (CL21 cluster with total mass M_500c = 6.08 × 10¹⁴M_⊙ at r_500c = 1215.2 kpc; see Zhuravleva et al. (2014b) for details) obtained from the 3D density information (dash black curve) and recovered from the image of the X-ray surface brightness (hatched orange/red region). The width of the hatched region reflects the uncertainty of the recovered 3D amplitude of density fluctuations due to variations of the converting geometrical factor with the projected radius. Notice that the true (from the 3D data) amplitude of density fluctuations is within the hatched region. This confirms that the observational procedure of measuring the amplitude of density fluctuations from the X-ray images recovers reasonably accurate the amplitude of the 3D density fluctuations.