PURPOSE: To assess the severity of glaucoma and its worsening using Spaeth/Richman contrast sensitivity (SPARCS). To evaluate its correlation with retinal nerve fiber layer thickness (RNFL) and perimetry. METHODS: In this prospective observational study, 90 eyes of primary open-angle glaucoma were categorized into mild, moderate, and severe glaucoma (30 each). All eyes were subjected to contrast sensitivity measurements by using the Pelli-Robson (PR) chart and SPARCS, Humphrey automated perimetry, and optical coherence tomography (OCT) RNFL, along with routine ophthalmological evaluation at baseline. Contrast sensitivity was repeated at 1, 3, and 6 months. Other investigations were repeated at 6 months. The correlation between contrast sensitivity and other parameters was analyzed at baseline and 6 months. RESULTS: Total SPARCS showed a significant positive correlation with visual field index, pattern standard deviation, mean deviation, OCT RNFL, and a negative correlation with best corrected visual acuity. Central SPARCS and PR scores exhibited strong positive correlations. Both total and quadrantic SPARCS significantly reduced from mild to moderate to severe glaucoma. The quadrant-wise SPARCS also correlated well with opposite-side RNFL thickness. At 6 months, SPARCS showed a significant reduction along with RNFL measurements preceding any significant changes in visual field parameters. CONCLUSION: This study establishes SPARCS as a reliable and reproducible tool in assessing the deterioration of visual function in glaucoma patients even before significant perimetric changes. The specific relationship of quadrantic SPARCS with opposite-side RNFL is a novel yet expected finding. The findings advocate integrating SPARCS into routine glaucoma assessment for timely detection of any worsening and prompt intervention, improving the visual outcomes in these patients.