Investigation into screenless 3D TV

If a three dimensional image is to be projected into mid-air in a room with bare walls, then light must follow a curving path. Since this does not happen in a vacuum, a gradient must be introduced into the refractive index of air itself, which can be introduced by varying either the temperature or pressure of air. A reduction from 300 °C to room temperature across the front of a 1 mm wide ray will bend it with a radius of curvature of 3 m. However the temperature gradient cannot be sustained without an unacceptably aggressive mechanism for cooling. The pressure gradients delivered by sound waves are dynamically sustainable, but even powers as extreme as 175 dBm at 25 kHz deliver a radius of curvature of only 63 m. It appears that something will have to be added to the air if such displays are to be possible.