A surgical light – also referred to as an operating light or surgical lightheadis a medical device intended to assist medical personnel during a surgical procedure by illuminating a local area or cavity of the patient. A combination of several surgical lights is often referred to as a “surgical light system”. History Technical development In the mid-1850s, operating rooms were built towards the southeast with windows in the ceiling to benefit from natural sunlight as much as possible. The biggest problem was the dependence of lighting and whether a surgical procedure could be done on the time of day and weather conditions. Furthermore, a doctor, nurse or medical equipment easily blocked the light. The use of mirrors on the four corners of the ceiling to reflect sunlight towards the operating table only slightly alleviated these problems. Attempts were made to use an optical condenser in an indirect light to reduce the heating, but without success. The entrance of electric lights into the operating room in the 1880s was accompanied by problems. With early electrical technology control of the light emitted was poor. Electric light was still moving and diffuse, with great heat radiation. Light-emitting diodes as light sources remove the problem of heat radiation and reduce energy requirements. Terminology and measurements Lux Unit for the amount of visible light measured by a luxmeter at a certain point. Central illuminance (Ec) Illuminance (measured in lux) at 1m distance from the light emitting surface in the light field centre. Light field centre Point in the light field (lighted area) where illuminance reaches maximum lux intensity. It is the reference point for most measurements. Depth of illumination The distance between the points of 20% illumination intensity above and below the center point. From the point of maximum illumination, which is the center of the light field 1 meter from the light-emitting surface, the photometer is moved toward the light until the light intensity measured falls to 20% of the maximum value. The distance between the center and this point is defined as L1. The similarly measured distance in the direction away from the light is L2. The depth of illumination without needing to refocus is the sum of the two distances L1 and L2. In the second edition of the IEC standard, published in 2009, the threshold value was revised from 20% to 60%. Shadow dilution The light's ability to minimize the effect