A method and an apparatus for displaying 3D images are disclosed. 2D images are projected from multiple points to a surface with narrow diffusion characteristics, thus it is possible to get a 3D light field with arbitrary angular dependence. According to the invention, the projectors (10) do not target particular screen (12) points. The projectors (10) are directed to the screen (12) with no special positioning, however from the bunch of light beams present, pertaining to the large number of pixels in the projected images, it is possible to select and define a pixel-wise precise 3D light-field through a calibration process. A novel calibration method and device are also described for calibrating 3D display systems, the device being equipped with proper image sensor(s) and a control system.