A display instrument for aircraft wherein a representation of a data value is recorded and indicated by the position of an illuminated bar alongside a graduated scale. A number of bars are located along the scale, each bar being formed by the emitting end of a respective bundle of optical fibres. A particular bundle is illuminated to illuminate its bar according to a digital value stored in a counting register. This value is variable by the manual rotation of a knob driving an apertured disc so that each time a light beam passes through an aperture as the disc rotates, the beam is incident on an opto-electronic receiver, thus, providing a signal which changes the value stored in the counting register.