Abstract Islamic ornaments are used extensively in different types of buildings, serving both aesthetic and functional purposes. They are used in mosques, offices and other buildings, serving as both interior and exterior elements. One application is their use as secondary skins to cover window openings. This research investigates the effect of Islamic ornaments as secondary skins on interior lighting. The main question addressed is how Islamic ornaments affect the quality of natural lighting. Using computer simulations, scenarios with clear glass and varying ornament distance from the window were assessed under clear sky conditions at different times of the day. The results show that the presence of Islamic ornaments significantly reduces the average daylighting level on interior surfaces compared to scenarios without ornaments. Visual representations using ray diagrams and computer-generated images illustrate the effects of varying ornamental conditions. This research highlights the potential of Islamic ornaments to improve the quality of daylighting in architecture. The findings emphasise their role in optimising daylight indoors, and encourage careful consideration of the impact of spacing on window openings. In particular, the distribution of daylight varies between rooms orientated to the north and west.