A privacy display apparatus comprising: a display device arranged to display an image, the display device being capable of providing a privacy function in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer; a control system arranged to control the display device; and an ambient light sensor arranged to detect the illuminance level of ambient light, wherein the control system is arranged to control luminance of the displayed image on the basis of the detected level of the ambient light in accordance with a transfer function, and when the privacy function is provided the transfer function maintains a relationship Y max ≤ Y upper, where Y max is the maximum output luminance of the display device and Y upper is given by the equation: Y upper = ρ θ = 45 ° / P θ = 45 ° ⋅ I 10 S min − 1 ⋅ π where the equation for Y upper applies for an observation direction having a polar angle θ of 45° from the normal to the display device at at least one azimuth angle around the normal to the display device, I is the detected illuminance level of the ambient light, the units of I being the units of Y max multiplied by solid angle in units of steradian, ρ ( θ = 45°) is the reflectivity of the display device along the observation direction, P ( θ = 45°) is the ratio of the luminance of the display device along the observation direction to the maximum output luminance Y max of the display device, and S min has a value of 1.0 or more. a display device arranged to display an image, the display device being capable of providing a privacy function in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer; a control system arranged to control the display device; and an ambient light sensor arranged to detect the illuminance level of ambient light, wherein the control system is arranged to control luminance of the displayed image on the basis of the detected level of the ambient light in accordance with a transfer function, and when the privacy function is provided the transfer function maintains a relationship Y max ≤ Y upper, where Y max is the maximum output luminance of the display device and Y upper is given by the equation: Y upper = ρ θ = 45 ° / P θ = 45 ° ⋅ I 10 S min − 1 ⋅ π where the equation for Y upper applies for an observation direction having a polar angle θ of 45° from the normal to the display device at at least one azimuth angle around the normal to the display device, I is the detected illuminance level of the ambient light, the units of I being the units of Y max multiplied by solid angle in units of steradian, ρ ( θ = 45°) is the reflectivity of the display device along the observation direction, P ( θ = 45°) is the ratio of the luminance of the display device along the observation direction to the maximum output luminance Y max of the display device, and S min has a value of 1.0 or more. the equation for Y upper applies for an observation direction having a polar angle θ of 45° from the normal to the display device at at least one azimuth angle around the normal to the display device, I is the detected illuminance level of the ambient light, the units of I being the units of Y max multiplied by solid angle in units of steradian, ρ ( θ = 45°) is the reflectivity of the display device along the observation direction, P ( θ = 45°) is the ratio of the luminance of the display device along the observation direction to the maximum output luminance Y max of the display device, and S min has a value of 1.0 or more.