AbstractBackgroundSimulated daylight photodynamic therapy (SDL‐PDT) is a new treatment alternative for actinic keratosis. The aim of this study was to show how the illuminance that reaches the target skin area during SDL‐PDT depends on the spatial positioning of the patient.MethodsIn this technical validation study, illuminance from the SDL‐PDT system IndoorLux© was measured at different angles, directions, and distances from the light sources corresponding to potential target skin areas. Using two different photometers, data from 63 measuring points at seven specific distances from the ceiling were collected at 0°, 45°, and 90° angles, respectively. Illuminance levels ≥12,000 lux were regarded as adequate. Hotspots were defined as adequate measurements in all directions at a specific measuring point at distances of 1.3, 1.5, and 1.8 m from the light sources (i.e., the most common patient treatment positions).ResultsAdequate illuminance levels were more common with photometer 1 (73%) than photometer 2 (57%). Almost all illuminance levels were adequate at a 0° angle with both photometers. Adequate illuminance levels were observed at 82–93% of the measuring points at a 45° angle and 22–47% at a 90° angle. Hotspots were registered with both photometers at all measuring points at 0°; 59–79% of the measuring points at 45°; and 0–21% at 90°.ConclusionPatient positioning is important during SDL‐PDT. Adequate illuminance is achieved if target skin areas are positioned at 0°–45° angles relative to the light sources, but not at 90° angles.