Task illuminance is one of the most used parameters in office lighting design and is often used as a ‘single number criterion’ to verify that a lighting design meets the requirements. Although other parameters, such as wall luminance, are often highly correlated with task illuminance, not taking these explicitly into account means critical user criteria such as comfort and satisfaction are left to chance. In this study, we investigated the effect of varying desk illuminance (150 to 1500 lx) while keeping wall luminance constant, isolating the effects of illuminance on the desk and eye on office users’ overall perception of the space, their mental state and their performance (visual and cognitive). While both visual acuity (paper-based) and perceived brightness increased significantly with higher desk illuminance, the room’s attractiveness did not. Even though illuminance at the eye increased considerably with desk illuminance (from 118 lx to 796 lx), only minor effects were found on subjective alertness and cognitive performance. This suggests that focusing on horizontal task illuminance as a design parameter is appropriate in view of visual acuity but has little to no effect on the space’s attractiveness, nor on cognitive performance or mental state of the office worker.