Assessment of light sensitivity using the Ocular Photosensitivity Analyzer: Discomfort threshold, pain threshold, and tolerance in healthy volunteers

Photophobia encompasses a continuum of light-induced experiences from mild discomfort to overt pain and intolerance, yet clinical tools rarely quantify these distinctions, leaving many patients undiagnosed and untreated. This study employed the Ocular Photosensitivity Analyzer (OPA), a compact instrument for quantitative sensory testing (QST) with light, to investigate three perceptual levels in healthy adults: visual photosensitivity discomfort threshold (VPDT), visual photosensitivity pain threshold (VPPT), and an exploratory outcome, visual photosensitivity tolerance level (VPTol). Twenty-nine participants ages 19-34 underwent standardized OPA testing, during which they identified when light first became uncomfortable, painful, and intolerable. After each trial, participants rated the peak pain intensity (PI) and peak unpleasantness (UP) using visual analog scales. Measured illuminance values differed significantly across the perceptual levels (RM-ANOVA, N = 28, F(1.69, 45.6)=37.06, p < 0.001, ηp² = 0.58; Mauchly's W = 0.82, p = 0.072), indicating sensory levels are differentiable. Post hoc t-tests revealed VPTol exceeded VPPT (N = 28, t(27)=-2.29, p = 0.030, dz = 0.43)), and both exceeded VPDT (VPTol: N = 29, t(28)=-7.69, p < 0.001, dz = 1.43, VPPT: N = 29, t(27)=-7.69, p < 0.001, dz = 1.47). Friedman's test revealed UP (N = 28, χ2(2)=17.64, p < 0.001, Kendall's W = 0.32) and PI (N = 28, χ2(2)=22.36, p < 0.001, Kendall's W = 0.40) differences across perceptual levels and post hoc Wilcoxon tests showed UP and PI increased between VPDT and VPPT (UP: N = 29, Z = -4.34, p < 0.001, r = 0.57, PI: N = 29, Z = -4.58, p < 0.001, r = 0.60) and UP increased between VPDT and VPTol (N = 28, Z = -2.92, p = 0.007, r = 0.39), suggesting that both affective and sensory dimensions of pain vary across perceptual levels. Although preliminary, these findings are consistent with a multidimensional framework of photophobia. Additionally, the OPA's ability to differentially quantify VPDT, VPPT, and VPTol offers a novel approach to characterize light sensitivity that may aid future assessment of abnormal light sensitivity.