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Published on: September 16, 2025
Validation of 3nethra specto, a wearable digital subjective refractor versus conventional subjective refraction for
Kaushik Murali1, Namratha Hegde2, Afia S3
1Department of Paediatric Ophthalmology and Strabismus, Sankara Eye Hospital, Sankara Academy of Vision, Sankara Eye Foundation India, Bengaluru, Karnataka, India.
Background:
Refractive errors cause significant preventable visual impairment globally, particularly in underserved areas. Conventional subjective refraction remains the gold standard but proves examiner-dependent, time-consuming and challenging in high-volume settings. The 3nethra Specto, a wearable digital subjective refractometer, offers rapid refractive assessment. This study evaluates its accuracy and repeatability versus conventional methods.
Methods:
A prospective comparative study was conducted on 135 subjects who underwent both conventional subjective refraction and 3nethra Specto assessment. Spherical (S), cylindrical (C) and axis measurements were analysed using power vector transformation (J₀, J₄₅). Primary predictive validity was assessed using univariate general linear models (GLMs) (IBM SPSS V.25.0) with age/sex as covariates. Model fit was evaluated via F-statistics, R² and residual SD. Instrument repeatability used GLM Repeated Measures, agreement assessed via Bland-Altman plots, keeping significance at p<0.05. Patient and examiner feedback on the instrument was captured using the User Experience questionnaire.
Results:
Of 135 adults (mean age 31.5±8.9 years; 60% male), compound myopic astigmatism predominated (45%). 3nethra Specto measurements closely matched subjective refraction (sphere: -1.02 D vs -0.99 D; cylinder: -0.49 D vs -0.83 D; M: -1.26 D vs -1.24 D). GLM analysis showed excellent predictive validity (all p<0.001; R² 0.914 to 0.994; residual SD 0.084 D to 0.228 D). Bland-Altman plots confirmed 95% limits of agreement of -0.39 D to +0.47 D for M and -0.62 D to +0.64 D for J₀ and -0.32 D to +0.35 D for J₄₅. Device repeatability was high (all F>96, p<0.001; R²≥0.956; residual SD≤0.148 D). Patients rated subjective refraction higher for comfort/enjoyability but found 3nethra more inventive/exciting; examiners preferred 3nethra for efficiency, speed, innovation and ease of use.
Conclusions:
The 3nethra Specto wearable liquid lens refractor demonstrated excellent agreement and repeatability with traditional subjective refraction across all refractive parameters, confirming clinical equivalence. Positive patient and examiner feedback highlighted its innovation, efficiency and usability. This device offers a reliable alternative for routine refractive assessment in clinical settings.
