Purpose : This study aimed to evaluate the effect of various illuminance levels in optometry and ophthalmology examination rooms on pupil size changes in subjects. Furthermore, it analyzed how these pupil size changes affect refractive error measurements using the duochrome test, thereby seeking to determine optimal illuminance levels for vision examination rooms. Methods : Adults with no history of eye disease related surgery, systemic diseases, or ocular diseases affecting vision or pupils underwent basic examinations (color vision test, pupillary function). After subjective refraction for full correction, the monocular duochrome test was performed. Four illuminance levels (10, 50, 300, 630 lx) were set, and pupil sizes were measured after adaptation at each level. Refractive error changes were corrected using a red-green chart, and if changes were present, a spherical lens of 0.12 D was added to quantify the amount of refractive error change. Results : There was a significant difference in the amount of refractive error (duochrome test values) change across each illuminance level (F=37.61, p=0.000*). Furthermore, there was a significant positive correlation between the change in refractive error values according to the increase in pupil size (r=0.178, p=0.006**). In addition, it was found that as the pupil size increased, the change in refractive error values also significantly increased (p=0.006**). In particular, the change in refractive error was most pronounced when transitioning from low (10 lx) to high (630 lx) illuminance, exceeding +0.24 D. Conclusion : This study confirmed that changes in illuminance within vision examination rooms induce changes in examines' pupil size, and this is one of the key factors influencing refractive error (duochrome test values) measurements. Therefore, considering that spectacle wearers typically live in photopic conditions, it is considered appropriate to confirm the final refractive error results at an illumination level at least 50 lx close to the photopic vision state to ensure stable and optimal vision correction.