Abstract Aims: The aim of this study was to investigate how brightness and contrast settings of the display monitor and ambient light level (illuminance) in the viewing room affect the clinician’s ability to diagnose carious lesions in digital radiographs. Methods: Standardised radiographs were taken of 100 extracted teeth. Seven observers evaluated the images for approximal carious lesions twice, once under 500-lux and once under 1000-lux room illumination. Radiograph brightness and contrast were varied ±50% and ±6%, respectively, to mimic normal limits of monitor adjustment by an inexperienced user and one optimal setting. Thus, five radiographs of each tooth were made. Receiver operating characteristic (ROC) analyses were performed. Histologic examinations of the teeth served as the criterion standard. A paired t-test was used to evaluate whether differences in the areas under the ROC curves were significant, and kappa was used to evaluate intra-observer agreement. Results: When a monitor with optimal brightness and contrast settings was used to detect approximal carious lesions, ambient light levels less than 50 lux were significantly better than levels above 1000 lux (dentine and enamel lesions, p<0.01; dentine lesions, p<0.02). Increasing the contrast setting of the monitor by 6% did not change these results; 50 lux was still significantly better than 1000 lux (enamel lesions, p<0.01; dentine and enamel lesions, p<0.02) for evaluating radiographs. Intra-observer agreement differed from fair to good. Conclusion: Reducing ambient light to less than 50 lux significantly increased the accuracy of diagnosing approximal carious lesions on a monitor with an optimal brightness setting and an optimal or slightly higher than optimal contrast setting.