Head-up display on a vehicle, for controlled brightness of warning light

A head-up display on a vehicle, comprising:

• an image display device for displaying an image taken by a camera built into the vehicle;
• a detecting device for detecting a target object in the vicinity of the vehicle;
• a switching device for manually switching between a display state in which the image is displayed by the image display device and a warm-up state in which the image is invisible;
• and
• a warning device for outputting a control signal based on a result detected by the detecting device, thereby making the image display device display the image, and
• wherein the brightness employed by the image display device for displaying the image according to the control signal is higher than the brightness employed by the image display device for displaying the image according to a manual operation of the switching device.