Indoor light quality is the intensity and amount of illumination of interior spaces. Technological developments have led to the utilisation of artificial natural light. Illumination is crucial in the visibility of tasks. Quality of light can influence the visual comfort and ocular health of the students. Although Kenya lacks lighting standards for schools, ISO and CIE provide international guidelines on optimal illumination for various facilities. This study was conducted on public boarding secondary schools in Nairobi City County to assess the visual comfort or discomfort of students in classrooms. The study utilised a cross-sectional descriptive study design. Four hundred twenty-three students were randomly sampled from 8 schools. Illumination levels were measured in 38 classrooms using a calibrated Benetech GM1010 light metre and compared to levels recommended by ISO and CIE. A self-administered semi-structured questionnaire was used to collect data on students’ visual comfort or discomfort. Data was analysed using SPSS version 23. Descriptive data were analysed using frequencies, while inferential data were analysed using chi-square. The study established that 15% of the respondents experience visual comfort during the day, 13.5% at night, and 12.7% both day and night. Fourteen classrooms achieved the recommended illumination level at 7:30, 20 at 13:00 hours, and 10 at 18:00 hours. Recorded levels ranged from 56.71 lux to 1289.93 lux. The study found a statistically significant association between visual comfort and factors such as existing eye problems, presence of blinds or curtains, number of windows, presence of whiteboards, classroom orientation and student sitting positions. The study recommends that public secondary schools need to ensure quality indoor illumination in their facilities to enhance the visual comfort of students in learning facilities.