Bastian Piepers

Power Consumption and Temperature Distribution in WRGB Active-Matrix OLED Displays

In this paper, the power consumption of a white-red-green-blue (WRGB) active-matrix organic light-emitting device (OLED) display and the resulting temperature distribution across the display are analyzed as a function of the applied image and the luminance of the emitted light. It has been shown previously that temperature directly impacts the picture quality of an OLED display. Luminance, spectral radiance, power and temperature measurements are performed on a 55-in WRGB OLED display with a resolution of 1920 ×1080. A power model is presented that allows calculating the displays power consumption for a given applied image. This involves the dependency of the efficiency of the white OLED on the current density, the wavelength dependent transmission of the color filters and the contribution of each of the subpixels in producing the displays nominal white. The output of the power model is used as input for a basic thermal model that simulates the temperature distribution across the display. The thermal model is based on 3D computational fluid dynamics analysis framework (FloEFD). A good agreement between the simulations and measurements on the sample WRGB OLED display is obtained.

Perceptual uniformity of commonly used color spaces

Use of color images in medical imaging has increased significantly the last few years. Color information is essential for applications such as ophthalmology, dermatology and clinical photography. Use of color at least brings benefits for other applications such as endoscopy, laparoscopy and digital pathology. Remarkably, as of today, there is no agreed standard on how color information needs to be visualized for medical applications. This lack of standardization results in large variability of how color images are visualized and it makes quality assurance a challenge. For this reason FDA and ICC recently organized a joint summit on color in medical imaging (CMI). At this summit, one of the suggestions was that modalities such as digital pathology could benefit from using a perceptually uniform color space (T. Kimpe, “Color Behavior of Medical Displays,” CMI presentation, May 2013). Perceptually uniform spaces have already been used for many years in the radiology community where the DICOM GSDF standard provides linearity in luminance but not in color behavior. In this paper we quantify perceptual uniformity, using CIE’s ΔE2000 as a color distance metric, of several color spaces that are typically used for medical applications. We applied our method to theoretical color spaces Gamma 1.8, 2.0, & 2.2, standard sRGB, and DICOM (correction LUT for gray applied to all primaries). In addition, we also measured color spaces (i.e., native behavior) of a high-end medical display (Barco Coronis Fusion 6MP DL, MDCC-6130), and a consumer display (Dell 1907FP). Our results indicate that sRGB & the native color space on the Barco Coronis Fusion exhibit the least non-uniformity within their group. However, the remaining degree of perceptual non-uniformity is still significant and there is room for improvement.

Influence of Temperature on the Steady State and Transient Luminance of an OLED Display

In this paper, the influence of temperature on the luminance of an organic light-emitting device (OLED) display is investigated. Luminance, temperature, and power measurements are executed on a 55-in white-red-green-blue active-matrix OLED display with a resolution of 1920 × 1080 and an oxide-thin-film-transistor (TFT) backplane, under a controlled, static temperature environment. The measurements indicate a strong influence of temperature on the luminance of the display, resulting from the temperature dependence of both the TFT and the OLED. The influence of temperature on the luminance of an OLED display is also investigated in a dynamic context. Measurements show that temperature changes resulting from losses in the display have an important influence on the luminance stability of the display. The measurements linking luminance and temperature in a static temperature environment allow estimating the change in luminance in a dynamic context. Finally, this paper presents the results of a number of experiments that were set up to show scenarios in which the temperature dependence of the displays luminance has a direct negative impact on the picture quality of the display. The results of this work show that the thermal behavior of an OLED display must be taken into account when working towards a high-performing OLED display.

Requirements, desired characteristics and architectural proposal for a visualization framework for digital pathology

Use of color images in medical imaging has increased significantly the last few years. One of the applications in which color plays an essential role is digital pathology. Remarkably, as of today there is no agreed standard on how color information needs to be processed and visualized for medical imaging applications such as digital pathology. This lack of standardization results into large variability of how color images are visualized and it makes consistency and quality assurance a challenge. For this reason FDA and ICC recently organized a joint summit on color in medical imaging. This paper focuses on the visualization and display side of the digital pathology imaging pipeline. Requirements and desired characteristics for visualization of digital pathology images are discussed in depth. Several technological alternative solutions and considered. And finally a proposal is made for a possible architecture for a display & visualization framework for digital pathology images. The main goal for making this architectural proposal is to facilitate discussion that could lead to standardization.

Interpretation of the rainbow color scale for quantitative medical imaging: perceptually linear color calibration (CSDF) versus DICOM GSDF

Medical displays for primary diagnosis are calibrated to the DICOM GSDF1 but there is no accepted standard today that describes how display systems for medical modalities involving color should be calibrated. Recently the Color Standard Display Function3,4 (CSDF), a calibration using the CIEDE2000 color difference metric to make a display as perceptually linear as possible has been proposed. In this work we present the results of a first observer study set up to investigate the interpretation accuracy of a rainbow color scale when a medical display is calibrated to CSDF versus DICOM GSDF and a second observer study set up to investigate the detectability of color differences when a medical display is calibrated to CSDF, DICOM GSDF and sRGB. The results of the first study indicate that the error when interpreting a rainbow color scale is lower for CSDF than for DICOM GSDF with statistically significant difference (Mann-Whitney U test) for eight out of twelve observers. The results correspond to what is expected based on CIEDE2000 color differences between consecutive colors along the rainbow color scale for both calibrations. The results of the second study indicate a statistical significant improvement in detecting color differences when a display is calibrated to CSDF compared to DICOM GSDF and a (non-significant) trend indicating improved detection for CSDF compared to sRGB. To our knowledge this is the first work that shows the added value of a perceptual color calibration method (CSDF) in interpreting medical color images using the rainbow color scale. Improved interpretation of the rainbow color scale may be beneficial in the area of quantitative medical imaging (e.g. PET SUV, quantitative MRI and CT and doppler US), where a medical specialist needs to interpret quantitative medical data based on a color scale and/or detect subtle color differences and where improved interpretation accuracy and improved detection of color differences may contribute to a better diagnosis. Our results indicate that for diagnostic applications involving both grayscale and color images, CSDF should be chosen over DICOM GSDF and sRGB as it assures excellent detection for color images and at the same time maintains DICOM GSDF for grayscale images.

Contrast sensitivity function in stereoscopic viewing of Gabor patches on a medical polarized three-dimensional stereoscopic display

Abstract. While three-dimensional (3-D) imaging systems are entering hospitals, no study to date has explored the luminance calibration needs of 3-D stereoscopic diagnostic displays and if they differ from two-dimensional (2-D) displays. Since medical display calibration incorporates the human contrast sensitivity function (CSF), we first assessed the 2-D CSF for benchmarking and then examined the impact of two image parameters on the 3-D stereoscopic CSF: (1) five depth plane (DP) positions (between DP: −171 and DP: 2853 mm), and (2) three 3-D inclinations (0 deg, 45 deg, and 60 deg around the horizontal axis of a DP). Stimuli were stereoscopic images of a vertically oriented 2-D Gabor patch at one of seven frequencies ranging from 0.4 to 10  cycles/deg. CSFs were measured for seven to nine human observers with a staircase procedure. The results indicate that the 2-D CSF model remains valid for a 3-D stereoscopic display regardless of the amount of disparity between the stereo images. We also found that the 3-D CSF at DP≠0 does not differ from the 3-D CSF at DP=0 for DPs and disparities which allow effortless binocular fusion. Therefore, the existing 2-D medical luminance calibration algorithm remains an appropriate tool for calibrating polarized stereoscopic medical displays.

Impact of Long-Term Stress on the Light Output of a WRGB AMOLED Display

This paper describes how long-term use impacts the light output of a commercial 55” WRGB AMOLED display with InGaZnO TFT backplane. This covers effects which are known by the terms “aging”, “image-sticking,” and “burn-in.” The focus is on three different observations: permanent change in light output as a function of time, permanent screen burn-in, and permanent shift in color point. From this work it can be concluded that state-of-the-art OLED displays still suffer from light output instability under prolonged stress. The results suggest that the permanent change in light output can be explained by the combination of three different phenomena: a decrease in efficiency of the OLEDs as a function of time for active subpixels, a positive threshold voltage shift of the driving transistor for active subpixels, and a negative threshold voltage shift of the driving transistor for inactive subpixels, if they are illuminated and/or kept at high temperature. To our knowledge, this is the first work that describes and quantifies the permanent change in light output of a commercial WRGB OLED panel with InGaZnO TFT backplane. It sheds light on which effects occur and can be a valuable tool, both in the design and optimization of OLED panels and in the determining the circumstances under which this technology may be applicable.

Subjective contrast sensitivity function assessment in stereoscopic viewing of Gabor patches

While 3D displays are entering hospitals, no study to-date has explored the impact of binocular disparity and 3D inclination on contrast sensitivity function (CSF) of humans. However, knowledge of the CSF is crucial to properly calibrate medical, especially diagnostic, displays. This study examined the impact of two parameters on the CSF: (1) the depth plane position (0 mm or 171 mm behind the display plane, respectively DP:0 or DP:171), and (2) the 3D inclination (0° or 45° around the horizontal axis of the considered DP), each of these for seven spatial frequencies ranging from 0.4 to 10 cycles per degree (cpd). The stimuli were computer-generated stereoscopic images of a vertically oriented 2D Gabor patch with a given frequency. They were displayed on a 24” full HD stereoscopic display using a patterned retarder. Nine human observers assessed the CSF in a 3-down 1-up staircase experiment. Medians of the measured contrast sensitivities and results of Friedman tests suggest that the 2D CSF as modeled by Barten1 still holds when a 3D display is used as a 2D visualization system (DP:0). However, the 3D CSF measured at DP:171 was found different from the 2D CSF at frequencies below 1 cpd and above 10 cpd.