Diffuse white is an important concept in all color appearance models. However, there is a lack of research directly about the perceived diffuse white in HDR images. Three experiments were conducted to explore the perceptual estimation of diffuse white in real HDR images when presented on displays. The first experiment showed that the perceptual estimation of diffuse white relied more on the image content than the clipped peak luminance levels. Experiment II used images with different neutral density filters rather than clipping. The normalized luminance levels of the perceptual estimation were similar, depending on image content but not on image absolute luminance levels. Moreover, in both experiments, no significant difference can be found between expert and naive observers. The variance across images agreed with each other between experiment I and II. However, both results demonstrated that the absolute luminance level of observers' estimation is higher than the calibrated diffuse white level. Experiment III focused on exploring the impact of measurement methodologies on the absolute luminance level of the estimated diffuse white. Results of experiment III verified that the absolute luminance level of the perceptual estimation can be manipulated by the measurement methodology but the variance across image content is stable.