The RFC has been for a while trying to develop an effective night sight for its night operations. The earliest night sights were ring and bead elements treated with luminous paint, but these were found to be either too bright or too dim, and failed to show up
when there was a moon. After trials with combat pilots at Martlesham Heath, a simple sight designed by Sergeant Albert Hutton, an armourer serving with 39 Squadron based at Hainault Farm, proved more effective than more elaborate designs by Royal Aircraft Factory scientists. Hutton had already been responsible for major improvements to gun mountings and ammunition feeds.
He devised an illuminated sight which could be used on both fixed and free-mounted guns.It consists of a tubular foresight containing a red bulb. At the top of the domed tube was a 0.5 mm (0.02 in) hole through which showed a minute point of red light. The backsight was similar, but had a hollow vee-shaped tube at the top. At the base of the two arms of the vee were holes which, when a green bulb was switched on inside the tube, showed as three green spots in the shape of a vee. The gunner simply aligned the red spot in the centre of the vee and on the target. The bulbs suffered severe vibration when the guns were fired, but they usually lasted long enough for what were often very short engagements. The bulbs were usually fed by batteries taped to the gun body.
Thanks to Robert Hurst over on https://forum.axishistory.com//viewtopic.php?t=17649.