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Illumination of dairy barns impacts animal health, milk production, and building energy efficiency. The aim of this study was to assess the existing daylighting and electric lighting conditions of dairy barns located in Southern Sweden. The study specifically examined how different architectural features impact the lighting environment, aiming to identify retrofit potentials in older barns and areas of improvement for new constructions. Ten dairy barns on seven farms were selected as case studies with different construction types, construction year, and size; and were monitored during November 2023 and February 2024. Photometric and geometric measurements were collected, and simulation models were created and calibrated by comparing them with the measurements. The simulation results were used to analyse the lighting performance of the barns. The results indicate that several older barns have lower than recommended illumination from daylighting and electric lighting systems, inappropriate choice of building materials with low reflectance, poor maintenance of building surfaces, etc; thus, requiring maintenance and retrofit measures. Newer barns often showcased improved lighting conditions compared to older barns, primarily due to better daylighting through side openings and skylights (aperture-to-floor ratio: 1-6 % in older barns and 12-20 % in newer designs), underscoring the importance of improving design practices. Older barns consistently showed mean daylight factor values below 1 %, while newer barns exhibited notably improved daylighting. Compared to most older barns having a daylight autonomy below 10 %, newer barns achieved a daylight autonomy higher than 50 %. Mean electric lighting illuminance ranged from 57 to 157 lux across the studied barns. Both new constructions and retrofit projects can benefit from implementing clearly defined design guidelines and lighting requirements for agricultural buildings. Future studies on long-term impacts of illumination on farm animals and human caretakers, environmental and economic assessment of improved barn design and retrofit measures can provide more information on this topic.