necessary intensity of the light emerging from the filter 23 may if desired be changed for each successive frame in a predetermined sequence so that the resultant image contains a plurality of elements of widely differing colors and if necessary intensities the frame rate will be adjusted as appropriate to ensure that a flicker free multi color image is obtained the wavelength range of operation is controlled by the acousto optic tunable filter typically the filter could operate over an optical bandwidth of 0 9 of an octave e g 440 nm to 800 nm with up to 100 separate resolvable wavelengths if desired the head up display can therefore be arranged to operate at a longer infra red wavelength at night so that the visibility of the aircraft is not increased by visible light from the head up display the pilot would be able to read the display by the use of a night vision device sensitive to infra red wavelengths should the light source become defective or develop a flicker the response time of the tunable filter 10 u03bcs is so fast that it is possible to configure the system so that rapid intensity variations can be removed the acousto optic tunable filter described with reference to fig2 is orthogonal however acousto optic tunable filters may also operate in a collinear mode the collinear mode may have some advantages in this application in that it intrinsically has a larger field of view and it should be possible using a collinear filter to make.