Daylight responsive dimming systems continuously adjust electric lighting output with an algorithm to maintain target illuminance levels at the workplane in combination with available daylight. A key factor of closed-loop proportional control algorithms is the ratio of photosensor response to daylight illuminance at the workplane. This ratio determines in part the control slope (M) between dimming level and lighting output. Although this ratio is known to vary significantly with sun and sky conditions, it is typically set to a fixed value at the time of calibration. Such practice adversely affects the accuracy and therefore reliability of these systems. To investigate how to improve performance, a solstice-to-solstice experiment was conducted in a conventional private office mockup with a large-area, south-facing window and a fixed height roller shade positioned to block direct sun for most hours of the year. This simple preliminary case was used to explore and define the methods for derivation of improved control algorithms with the intent of studying more complex cases in future work. First, variations in the ratio of photosensor signal to daylight workplane illuminance were analyzed using a divided time period, clearness index (KT) ratio, and the ratio of diffuse solar irradiance to global solar irradiance on an exterior horizontal surface (Id/IT). Second, an improved closed-loop proportional control algorithm was suggested based on least square fits to the experimental data assuming that commissioning would occur at targeted times. The performance of this improved algorithm was compared to a modified conventional control algorithm and found to deliver more reliable, accurate performance when commissioning was performed on two targeted days within the solstice-to-solstice period. For the improved control algorithm when calibrations were performed based on day of the year (June/ December), the measured workplane illuminance levels did not fall below 90 percent of the design level for 98.4 percent of the test period. For improved control algorithm when calibrations were performed based on sky conditions (Id/IT ratio for clear and overcast), the measured workplane illuminance levels did not fall below 90 percent of the design level for 99.5 percent of the test period. This study indicates that improvements to the dimming control algorithm and commissioning practices can improve system accuracy and may increase market adoption of the daylight responsive dimming systems. © 2011 The Illuminating Engineering Society of North America.