KEYWORDS: LCDs, High dynamic range imaging, Chest, Eye models, Target detection, Computed tomography, Data modeling, Einsteinium, Signal detection, Mammography
One limitation of visual detection tasks in complex scenes with a large range of luminance values is the decrease in
sensitivity due to veiling glare in the display device and in the human eye caused by unwanted light scattering. We
used our previously measured results regarding the increase in detection thresholds due to veiling glare to formulate an
empirical model for this phenomenon. Our results are based on a ring glare source and a Gaussian target on white noise
using a dual-layer, high-dynamic-range liquid-crystal display prototype. The thresholds, measured using a double-random
staircase technique with added signal-absent images, are modeled as a function of illuminance at the eyes and angular
distance between the veiling glare source and the detection target. In this work, we model increases in detection contrast
thresholds due to veiling glare for any image by calculating the contribution of each display pixel. We validate our model
by determining threshold increases for the set of experimental results previously obtained with human subjects. Our imagedependent
model predicts how the contrast threshold is affected by veiling glare for any target location. Finally, we discuss
the range of validity of our model and show predictions for sample mammography, chest CT, and chest radiography images
displayed on large-luminance-range devices.
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 just noticeable difference (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 threshold versus intensity (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.
Liquid crystal displays (LCDs) are replacing analog film in radiology and reducing diagnosis times. Their typical dynamic range, however, can be too low for some applications, and their poor ability to reproduce low-luminance areas represents a critical drawback. The black level of an LCD can be drastically improved by stacking two liquid crystal panels in series. In this way the global transmittance is the pointwise product of the transmittances of the two panels and the theoretical dynamic range is squared. Such a high dynamic range (HDR) display also permits the reproduction of a larger number of gray levels, increasing the bit depth of the device. The two panels, however, are placed at a small distance from each other due to mechanical constraints, and this introduces a parallax error when the display is observed off-axis. A complex, spatially adaptive algorithm is therefore necessary to generate the images used to drive the two panels. We describe the characteristics of a prototype dual-layer HDR display and discuss the issues involved in the image-splitting algorithms. We propose some solutions and analyze their performance, giving a measure of the capabilities and limitations of the device.
KEYWORDS: LCDs, High dynamic range imaging, Image filtering, Prototyping, Image processing, Distortion, Transmittance, Linear filtering, Visualization, Algorithm development
Liquid crystal displays (LCD) are replacing analog film in radiology and permit to reduce diagnosis times. Their
typical dynamic range, however, can be too low for some applications, and their poor ability to reproduce low
luminance areas represents a critical drawback. The black level of an LCD can be drastically improved by
stacking two liquid crystal panels in series. In this way the global transmittance is the pointwise product of the
transmittances of the two panels and the theoretical dynamic range is squared. Such a high dynamic range (HDR)
display also permits the reproduction of a larger number of gray levels, increasing the bit depth of the device.
The two panels, however, are placed at a small distance one from each other due to mechanical constraints, and
this introduces a parallax error when the display is observed off-axis. A complex, spatially-adaptive algorithm
is therefore necessary to generate the images used to drive the two panels.
In this paper, we describe the characteristics of a prototype dual-layer HDR display and discuss the issues
involved in the image splitting algorithms. We propose some solutions and analyze their performance, giving a
measure of the capabilities and limitations of the device.
KEYWORDS: LCDs, Medical imaging, Telecommunications, Visual system, Visualization, Data communications, Field programmable gate arrays, Colorimetry, Modulation, Medicine
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.
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