Upper left: image of the reference ion torus corresponding to the parameters listed in Table 1, as observed by an observer on Earth, in inverted colours (the darker, the more luminous). The display is intentionally saturated for a better rendering of the less luminous parts of the image. Upper right: same as in the upper left but with the spin parameter increased to a = 0.9M. Lower left: same as in the upper left but with dimensionless angular momentum increased to λ = 0.6. Lower right: same as in the upper left but with the inclination angle decreased to i = 40°.