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Evaluation of High Dynamic Range Displays for Pathology Detection Tasks
This project is conducted in collaboration with J. Ferwerda, a member of the Munsell Color Science Laboratory, and K. Ogden, the head of Image Science and Informatics at SUNY Upstate Medical University. PhD candidates at the Center for Imaging Science, J. Phillips and R. Heckaman, are also involved.

Background
Liquid crystal displays (LCD) have replaced film as the display of choice in radiologic imaging. Use of LCD displays and associated digital images provide conveniences that film can not, such as improved workflow efficiency and image enhancement options. However, film viewed over a light box in many cases still has a diagnostic advantage due to a larger dynamic range. Traditional light boxes have a peak brightness of around 4000 cd/m², providing a 3000:1 contrast ratio (maximum brightness divided by black offset) [Guarnieri, 2008]. This is well in excess of the 600-900 cd/ m² and 600:1 contrast ratio of diagnostic LCD displays. For this reason it is believed that a display with an increased dynamic range and an increase in the number of perceivably different grey levels (just noticeable differences JND) may provide diagnostic benefits.

HDR System
The Munsell Color Science Laboratory (MCSL) at the Center for Imaging Science, Rochester Institute of Technology has developed an HDR display with a dynamic range of five orders of magnitude commensurate with the fully adapted human visual system and the highest gamut volume possible. The MCSL HDR system (Figure 1) includes a Plus U5-232 DLP projector (2000 Lumens, 2000:1 contrast ratio, XGA (1024 768) resolution, F = 2.6 2.9, F = 18.4 22mm) and an Apple 15 LCD panel with backlight removed (768 x 1024). The luminance channel of the projector is further modulated by the LCD panel, resulting in a bright display with a very low black level. The system includes a 150mm achromatic focusing lens, a Fresnel lens (custom Reflexite 24 inch), and a diffuser (Reflexite BP331), to focus and collimate the projector beam on the plane of the LCD panel.

Figure 1: MCSL HDR Display
Fig. 1 MCSL HDR Display

System Calibration
Following the DICOM standard calibration techniques, the system characteristic curve needs to first be determined. Since the system luminance is multiplicative in nature, two characteristic curves, one for the projector and one for the LCD panel, can be independently measured using a photometer. First the projector is set to white and the driving of the LCD is varied from 0 to 255, then the LCD is set to white while the driving level of the projector is varied from 0 to 255. Multiplying the two curves provides a two-dimensional characteristic surface. Given a desired luminance, this surface can be used to identify the appropriate digital driving levels for both the LCD panel and the projector in order to match the grayscale standard display function defined in the DICOM standard. The display was found to have a low black level and a peak luminance of 1,800 cd/m2, providing a 114,000:1 contrast ratio spanning 900 JNDs.