Abstract: An optical structure has a cavity (1) with an opening provided at one end and a first through hole or first gap (21) provided at the end opposite the opening. The cavity is further provided internally with a beam splitter or a dichroic mirror (4). A first LED (31) emits light into the cavity through the first through hole or the first gap, which passes directly passing through the beam splitter or dichroic mirror and exits from the opening of the cavity A second LED (32) emits light into the cavity through a second through hole or second gap (22) in a side of the cavity, which is reflected by the beam splitter or dichroic mirror. It is then fused with the light emitted by the first LED. The light emitted by the respective LEDs have an identical optical path length to the beam splitter or the dichroic mirror.