There is provided an optical engine for a navigation device including a first light source, a second light source, a lens, a barrier structure and an image sensor. The barrier structure has a first space for containing the first light source, a second space for containing the lens and a third space for containing the second light source and the image sensor. The reflected light associated with the first light source propagates to the image sensor via the lens in the second space. The reflected light associated with the second light source propagates to the image sensor via the third space without passing through the lens in the second space.