From 2007.igem.org

In this arena the fly can just walk in 2D. Two Infrared lights are positionned at two extremities of the arena and allow us to take videos in the dark. The blue light and the red light can be set up to shine the arena. The filter of the camera prevent visible light to pass though hence we can not know when the light (red/blue) is turned on and off. For this reason, another infrared light is connected to the same electrical circuits as the color lights so that we can detect when the blue or red light are swiched on.