It has long been known that the human visual system (HVS) has a nonlinear response to luminance. This
nonlinearity can be quantified using the concept of <i>just noticeable difference</i> (JND), which represents the minimum
amplitude of a specified test pattern an average observer can discern from a uniform background. The JND
depends on the background luminance following a <i>threshold versus intensity</i> (TVI) function.
It is possible to define a curve which maps physical luminances into a perceptually linearized domain. This
mapping can be used to optimize a digital encoding, by minimizing the visibility of quantization noise. It is also
commonly used in medical applications to display images adapting to the characteristics of the display device.
High dynamic range (HDR) displays, which are beginning to appear on the market, can display luminance
levels outside the range in which most standard mapping curves are defined. In particular, dual-layer LCD
displays are able to extend the gamut of luminance offered by conventional liquid crystals towards the black
region; in such areas suitable and HVS-compliant luminance transformations need to be determined. In this
paper we propose a method, which is primarily targeted to the extension of the DICOM curve used in medical
imaging, but also has a more general application. The method can be modified in order to compensate for the
ambient light, which can be significantly greater than the black level of an HDR display and consequently reduce
the visibility of the details in dark areas.
Currently, as a rule, digital medical systems use monochromatic Liquid Crystal Display (LCD) monitors to ensure an accurate reproduction of the Grayscale Standard Display Function (GSDF) as specified in the Digital Imaging and Communications in Medicine (DICOM) Standard. As a drawback, special panels need to be utilized in digital medical systems, while it would be preferable to use regular color panels, which are manufactured on a wide scale and are thus available at by far lower prices. The method proposed introduces a temporal color dithering technique to accurately reproduce the GSDF on color monitors without losing monitor resolution. By exploiting the characteristics of the Human Visual System (HVS) the technique ensures that a satisfactory grayscale reproduction is achieved minimizing perceivable flickering and undesired color artifacts. The algorithm has been implemented in the monitor using a low-cost Field Programmable Gate Array (FPGA). Quantitative evaluations of luminance response on a 3 Mega-pixel color monitor have shown that the compliance with the GSDF can be achieved with the accuracy level required by medical applications. At the same time the measured color deviation is below the threshold perceivable by the human eye.