Security lighting: Effects of illuminance and light source on the capabilities of guards and intruders†

An important function of perimeter security lighting is to enable guards to detect and recognise possible intruders. The experiments presented here were designed to systematically assess the performance of guards at these two tasks. The behaviour of intruders was also studied. Two experiments were undertaken on a large open field on four clear, moonless nights. In the first experiment, eighteen guards tried to detect and to recognise one of four intruders walking along a known path. In the second experiment the guards tried to detect an intruder moving in an unspecified manner across the open field. In the first experiment, the intruders provided data on the locations where they believed they had been detected and recognised. Their ability to see an acuity target located at the guard's position was also assessed. In the second experiment the intruders simply reported when they believed they had been detected. Both experiments were performed in the dark and under four different perimeter security lighting installations. Two light sources, low-pressure sodium and high-pressure sodium, and two lighting distributions, floodlighting and street lighting, were employed. The distances at which the guards detected and recognised the intruders were measured. The distances at which the intruders believed they had been detected and recognised were also measured. Measurements of vertical illuminance were obtained for the four security lighting installations, throughout the large open field. The main conclusions derived from the results are as follows. The presence of security lighting increases the distance at which intruders can be detected and recognised. A vertical illuminance in the range 4-10 lux will usually ensure a high level of detection and recognition. The low-pressure sodium discharge lamp is as effective as the high-pressure sodium discharge lamp for the detection of intruders and the recognition of their faces. Guards' and intruders' preferences for the different lighting conditions are consistent with the effect of those conditions on their ability to perform their respective tasks.