The DICOM Grayscale Standard Display Function (GSDF) is widely used in the medical imaging field to map the image values into luminance emitted by the display. However, the DICOM GSDF is not accurate at very dark luminance levels, and this causes a loss of visibility in the details if an image is viewed on the novel High Dynamic Rance display devices such as the ones based on the Dual Layer LCD technology. In this paper, we describe two experiments we performed in order to measure the eye sensitivity at low luminance levels and thus propose a correction. The first experiment was performed in controlled viewing conditions (dark room and fixed viewing distance) that reproduce as closely as possible those defined in the DICOM specification. The method we chose is a “staircase” procedure and uses the recently proposed “2AFC with denoising” technique, combined with a maximum-likelihood method. The results confirmed that the DICOM model overestimates the eye sensitivity at very low luminance levels. The second experiment was conducted with free viewing distance and with the lights both off and on, in order to simulate more realistic operating conditions. We used the “QUEST” algorithm to conduct the test and to compute the results, using available open-source software. The results show that the free viewing distance can improve the visibility of the details in the dark portions, because the observers tend to move closer to the display. At the same time, the ambient light has a severe impact on the observer's performance in the dark portion, but a negligible, and sometimes even slightly positive effect in the bright portions. The final target of this work is to propose a modified Display Function which offers a good performance in the entire luminance range of an HDR display and can be personalized according to the individual characteristics of each observer and to the level of ambient light.
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