A good HUD needs to have a wide contrast ration as it must display an image bright enough to be clearly seen on a sunny day, but also dim the image so that it does not obscure the driver’s vision at night.
For a HUD image to be clearly visible, the HUD contrast ratio should be between 1.15 and 1.54 where contrast ratio is defined as :
HUD contrast ratio = (Display Luminance + Ambient luminance)/(Ambient luminance)