PurposeTo demonstrate the practicability of a portable instrument in assessing tear film breakup time (TFBUT): a lens attachment for smartphones (LAS).MethodsBy applying LAS in combination with the iPhone 12 pro, and a recordable slit-lamp microscope, we obtained TFBUT videos from 58 volunteers. The comparison between the conventional slit-lamp microscope and LAS by an experienced ophthalmologist. Moreover, we also invited an ophthalmic postgraduate student and an undergraduate student with no clinical experience to assess TFBUT in those videos. The inter-observer reliability was assessed using intraclass correlation coefficients (ICC).ResultsThe TFBUT of 116 eyes in total was recorded. Reliability indexes were adequate. The Spearman’s correlation analysis and the intraclass correlation coefficient suggested a strong correlation between the 2 modalities (Right eye: Spearman’sr= 0.929, 95% confidence interval (CI) = 0.847–0.963, ICC = 0.978,p< 0.001; Left eye: Spearman’sr= 0.931, 95% CI = 0.866–0.964, ICC = 0.985,p< 0.001;). Between instruments, the majority of TFBUT measurements showed good agreement on Bland Altman plot. A high concordance was observed in TFBUT, when assessed by an ophthalmologist and an ophthalmic postgraduate student (Left eye: LAS ICC = 0.951,p< 0.001; Left eye: slit-lamp microscope ICC = 0.944,p< 0.001).ConclusionsCompared with the conventional slit-lamp microscope, the LAS has sufficient validity for evaluating TFBUT in clinics or at home.