Purpose. To validate the German-translated Low Luminance Questionnaire (LLQ), a vision-related quality of life scale assessing mainly mesopic and scotopic functioning, and to determine the relationship between the severity of vision impairment, ocular conditions, and low luminance-related visual functioning. Methods. In all, 274 participants, 184 patients with visual acuity <6/12 or a long-standing symptomatic eye condition and 90 controls, were recruited from an outpatient clinic at a German eye hospital. Participants underwent a clinical examination and completed the German LLQ and VF-14 scales. The validity and psychometric properties of the scales were assessed using Rasch analysis exploring key indices, such as instrument unidimensionality, discriminant ability, and targeting of item difficulty to patient ability. Multivariate analyses of low luminance functioning were adjusted for conventional visual functioning (VF-14 scores). Results. The 30-item German LLQ initially displayed poor fit to the Rasch model. Following Rasch-guided iterative adjustments to the scale, a 23-item LLQ emerged as a valid and unidimensional scale. Visual functioning under low luminance consistently declined with worsening vision loss. Compared with patients with no vision impairment, those with mild or moderate/severe vision impairment recorded significantly poorer low luminance functioning scores (mean change, -6.33 and -16.62; P = 0.032 and P < 0.001, respectively). Age-related macular degeneration and cataract were independently associated with low luminance visual functioning, as was worse self-reported health. Conclusions. Low luminance functioning is considerably compromised in visually impaired patients even at the mild spectrum of visual acuity loss. Additionally, the impact of age-related macular degeneration and cataract on patients' low luminance functioning is substantially independent of vision impairment. © 2011 The Association for Research in Vision and Ophthalmology, Inc.