White Paper Uniform Luminance Technology What s inside? What is non-uniformity and noise in LCDs? Why is it a problem? How is it solved? Tom Kimpe Manager Technology & Innovation Group Barco Medical Imaging Systems tom.kimpe@barco.com Barco Pres. Kennedypark 35 B-8500 Kortrijk, Belgium
ABSTRACT LCD technology has improved a lot in the past years. However, there are still some aspects that raise questions as to the usefulness of Liquid Crystal Displays for subtle clinical diagnosis. One major problem is the existence of luminance non-uniformities and color non-uniformities in medical displays. Several studies suggest that this nonuniformity can cause lower observer performance. Uniform Luminance Technology compensates for the non-uniformity and spatial noise. The result is a significant reduction of luminance non-uniformities and color nonuniformities. As an additional benefit, Uniform Luminance Technology ensures that the display is compliant with DICOM GSDF over its complete display area. Copyright 2005 BARCO n.v., Kortrijk, Belgium All rights reserved. No part of this publication may be reproduced in any form or by any means without written permission from Barco. Page 2 of 19
TABLE OF CONTENTS Table of contents 3 1 Non-uniformity and noise in LCDs? 4 1.1 What is non-uniformity and spatial noise?... 4 1.2 Why is non-uniformity a problem?... 6 1.3 How to quantify non-uniformity?...10 2 Solution for non-uniformity: Uniform Luminance Technology 11 2.1 General compensation concept...11 2.2 Real-time implementation...11 3 Performance of Uniform Luminance Technology 13 3.1 Luminance non-uniformity...13 3.2 Color non-uniformity...15 3.3 DICOM GSDF compliance...16 4 Unique proprietary technology 18 5 Conclusions 18 6 References 19 Page 3 of 19
1 NON-UNIFORMITY AND NOISE IN LCDS? 1.1 What is non-uniformity and spatial noise? In medical LCD displays there are two types of noise: temporal noise and spatial noise. Temporal noise can be described as fluctuations over time, while spatial noise is a distortion over the display area that is stable in time. An example of temporal noise is the well-known phenomenon of flicker on a CRT display. Research [1, 2] has demonstrated that in LCD displays temporal noise plays a minor role, but that spatial noise can cause significant problems. An easy method to visualize spatial noise on LCDs is to look at a uniform image of a specific gray level. In case of a perfect display, we would perceive this image as being perfectly uniform and all of the display pixels would have the same luminance value. However, because of the spatial noise for example a typical luminance fall-off near the corners of the display is visible, but also many different shapes are possible. Figure 1: Typical noise pattern of a medical LCD There are several causes of this spatial noise. Non-uniformity of the backlight is responsible for the typical luminance fall-off towards the borders of the display. Backlight non-uniformities introduce a gradual roll-off towards the corner and are less disturbing to diagnostic precision. Of more concern are LCD cell artifacts causing higher frequency, pixel-by-pixel variations in the luminance of the display. Such artifacts are caused by tolerances on the storage capacitor of each LCD pixel, non-perfect rubbing of Page 4 of 19
the LCD alignment layer, tolerances in the conductivity of the column and row conductors, varying characteristics of the cell transistors, tolerances in the driver circuits. The superposition of all of these effects results in spatial noise, seen as a fixed, cloudy pattern on the LCD. LCD manufacturers refer this to as Mura. Spatial noise is sometimes called fixed pattern noise, because it is fixed in space (location) on the display and differs from temporal noise, as it will not vary as a function of time. High frequency spatial noise changes rapidly over the display area creating small pattern disturbances, whereas low frequency spatial noise varies slowly from place to place on the display and creates larger patterns. Figure 1 shows an example of a typical spatial noise pattern of medical LCDs. The scale used in this figure is percent. For example, a value of 80% at a location means that the luminance at that specific location is 80% of the mean luminance calculated over the complete display area. It is very important to note that the lower frequency noise patterns (that vary slowly over the display area) have much larger amplitude compared to the high frequency Gaussian noise. In other words: noise patterns that vary slowly over the display typically result in much higher distortion of the medical image than noise patterns that very rapidly. Figure 2: Spatial noise depends on the drive level (levels 64, 256 and 1023) One very important aspect of spatial noise in LCDs is that the noise pattern is stable over time but does depend on the image contents. Referring to the above-described causes such as the driving circuitry and the pixel transistors, it will be understandable that spatial noise does change as a function of drive level. This dependency of spatial noise on drive level can easily be seen from Figure 2, where the spatial noise pattern for the same display is shown at different drive levels (levels 64, 256 and 1023). Page 5 of 19
1.2 Why is non-uniformity a problem? Clinical relevance of luminance non-uniformity Spatial noise patterns are much more noticeable than the smallest difference in gray scales that can be displayed on the LCD. Therefore, these noise patterns can interfere with (subtle) features in the medical images that are being displayed. Original image Noise pattern introduced by display + Displayed image Figure 3: Effect of spatial noise on a medical image Page 6 of 19
Figure 3 illustrates this problem: the upper left image is the noise-free medical image that we would like to display. However the display suffers from spatial noise, which is shown in the upper right image. Therefore, the image that will actually be perceived by the radiologist is the sum of the original medical image and the noise pattern. This resulting image is shown in the lower part of figure 3. There are strong indications that this spatial noise can have a negative influence on the accuracy of medical diagnosis [3, 5]. It is well known that [4] the presence of noise makes it much harder for a human observer to discriminate a (subtle) image feature from the surrounding background. In other words: spatial noise increases the risk of false negatives (clinically relevant features that remain undetected). On the other hand, it is also possible that a human observer confuses the spatial noise pattern with a clinically relevant image feature. An example is that the spatial noise pattern of the display could be interpreted by a radiologist as actually being part of the medical image. Such confusion can result into false positives. Clinical relevance of color non-uniformity Since spatial noise is present in all LCD displays, it also impacts the performance of color displays. More and more color displays are being used in medical imaging. In some medical applications color is essential because it has a clinical meaning. Therefore, the precise reproduction of these colors is absolutely necessary. In color displays, spatial noise will not only result in luminance errors over the complete display area, but also in different color reproduction across the display area. In other words: the same color will look differently depending on the exact position on the display area. It is obvious that this is not acceptable for applications where precise color reproduction is important (such as PET/CT fused images). Page 7 of 19
Figure 4: Effect of spatial noise on accuracy of color reproduction Figure 4 gives an idea of the magnitude of the problem. Three colors were displayed on a color display. For each of the three colors the exact color coordinates were measured at multiple positions on the display area. If the color reproduction were perfect then Figure 4 would consist of only three points. Because of the spatial noise the color reproduction is dependent on the position on the display area. As can be, the measured differences are several times larger than the smallest color difference that a human observer can perceive (0.005 distance between color in the (x, y) diagram). DICOM-compliance In medical imaging compliance to DICOM GSDF [7] is very important, as it will guarantee that all grayscales in a medical image are effectively visible on the display and that the display is perceptually linear. Compliance to DICOM GSDF is achieved by calibrating the display system. With a photometer the exact transfer curve of the display is measured at one point on the display and then a Look-Up table is generated so that the display will follow the DICOM GSDF. Page 8 of 19
Figure 5: comparison of spatial noise at several video levels (video levels 64, 256, 512 and 1023) However, since all LCDs suffer from spatial noise, the DICOM calibration will only be correct for the exact location where the transfer curve was measured. This can be seen in Figure 5. In this figure, the spatial noise patterns of the same display are shown for different video levels (gray level 64, 256, 512 and 1023). It is obvious that the spatial noise patterns differ significantly for different video levels. This also means that the transfer curves for different positions on the display area are different. Consequently, if one only measures the transfer curve for one position on the display area, then the DICOM GSDF calibration will only be correct for that specific position on the display surface. Figure 6: DICOM GSDF compliance across the display surface Figure 6 confirms that the distortion (in JNDs) compared to the DICOM GSDF target is extremely large. This plot was created by first calibrating a display with a sensor in the Page 9 of 19
center of the display and then re-measuring the GSDF compliance at non-center display positions. The values in the plot show the average (over the 256 gray levels) distortion in Just Noticeable Differences (JNDs) compared to the target GSDF curve the display should follow at that position. In the center the display is well calibrated, since the sensor was placed there. On other display positions however the average distortion can range from a few JNDs to over 25 JNDs. It is obvious that such poor compliance to DICOM GSDF will result in much lower image quality. 1.3 How to quantify non-uniformity? Since the spatial noise pattern of a display depends on the drive level, there will be drive levels that have more non-uniformity than other drive levels and vice versa. Therefore, it is of no use to define a uniformity-metric of a display system based on measurement of one single drive level. A meaningful metric to quantify non-uniformity of a display system should take into account at least several driving levels, for example the 25%, 50%, 75% and 100% driving level. In Figure 7 the native display transfer curve (128 drive levels) is shown for multiple locations on the display surface. From this plot the peak-to-peak non-uniformity for each drive level can be read. This peak-to-peak non-uniformity corresponds to the difference between highest and lowest luminance value for a specific drive level. Figure 7: Display transfer curves for multiple locations Page 10 of 19
In order to come up with a single number that describes the non-uniformity of a display system, one could take the peak-to-peak non-uniformity averaged over a number of drive levels (such as 25%, 50%, 75%, 100%). 2 SOLUTION FOR NON-UNIFORMITY: UNIFORM LUMINANCE TECHNOLOGY 2.1 General compensation concept The general concept of Barco's Uniform Luminance Technology is explained in Figure 8. In a first step, the spatial noise pattern of the display is characterized for all video levels. Using this data, we calculate appropriate correction values for every display pixel and all video levels [2]. The Uniform Luminance Technology correction data is chosen so that it will compensate for the spatial noise of the LCD panel. Uniform Luminance Technology will digitally pre-compensate each image that needs to be displayed before sending it to the display panel. The correction data that is applied is the inverse of the spatial noise of the LCD. When this pre-compensated image is displayed, the stationary spatial noise and the digital pre-correction cancel out. Therefore, the overall perceived image is spatially noise-free. 2.2 Real-time implementation To make the Uniform Luminance Technology completely transparent for the user, we developed a real-time hardware implementation [6] inside the display. There is no need to install any extra software or application at all to benefit from the Uniform Luminance Technology. Everything is handled automatically by dedicated hardware inside the display. This highly complex hardware performs the required calculations at the frame-rate of the display, typically 60 frames per second. This guarantees that at all times the image shown on the display is fully Uniform Luminance Technology corrected. Because the correction is performed in real-time, there are absolutely no motion artifacts introduced by the Uniform Luminance Technology. Page 11 of 19
Original image Correction pattern introduced by Uniform Luminance Technology + Image sent to panel + Spatial noise pattern introduced by display Displayed image Figure 8: Concept of compensating for spatial noise Page 12 of 19
3 PERFORMANCE OF UNIFORM LUMINANCE TECHNOLOGY 3.1 Luminance non-uniformity Uniform Luminance Technology results in greatly improved luminance uniformity. Figure 9 compares the luminance uniformity (for video level 64) without and with Uniform Luminance Technology. Without Uniform Luminance Technology, luminance variations of over 40% are possible (luminance uniformity of less than 60%), whereas with Uniform Luminance Technology the luminance uniformity is increased to over 95%. As has been explained before, the spatial noise pattern of a display depends on the drive level. Therefore, to provide a true solution for spatial noise, it is necessary that the compensation algorithm uses a different correction pattern for each video level. This is exactly what Uniform Luminance Technology does. As an example, Figure 10 shows the luminance uniformity of the same display but for video level 1023. It becomes clear immediately that the spatial noise pattern for level 64 (Figure 9) and level 1023 (Figure 10) differ significantly. However, due to the fact that Uniform Luminance Technology uses a different correction image for each video level, it significantly increases uniformity for all video levels. Alternative uniformity enhancement techniques Other technologies that digitally optimize the uniformity of the backlight's luminance do exist. However, because these technologies use the same correction pattern for all video levels, they only provide optimal uniformity for one specific video level. Typically these algorithms are tuned to provide optimal bright uniformity (best uniformity at maximum video level or full white). However, because these algorithms use the same correction pattern for each video level (while the spatial noise pattern depends on the video level), there will still be significant non-uniformity at other drive levels. Page 13 of 19
Video level 6.25%, without Uniform Luminance Technology Video level 6.25%, With Uniform Luminance Technology Figure 9: Luminance uniformity without and with Uniform Luminance Technology (video level 6.25%) Page 14 of 19
Video level 100%, without Uniform Luminance Technology Video level 100%, with Uniform Luminance Technology Figure 10: Luminance uniformity without and with Uniform Luminance Technology (video level 100%) 3.2 Color non-uniformity In color displays, Uniform Luminance Technology not only improves luminance uniformity, but also color uniformity and color accuracy. To analyze the effect of Uniform Luminance Technology on color uniformity, three different colors were shown on the same display. For each of the three colors the exact Page 15 of 19
color coordinates were measured for multiple positions on the display surface. Figure 11 shows the results. Without Uniform Luminance Technology, there is a very large variation in color coordinates for the same color depending on the position on the display (left-hand side of Figure 11). This variation is much larger than the smallest difference in color that a human observer can perceive. without ULT with ULT Figure 11: Color uniformity without and with ULT The right-hand side of Figure 11 shows the same measurements, but with the Uniform Luminance Technology activated. With Uniform Luminance Technology the color variation over the display surface becomes much smaller and will typically be less than distance 0.005 in (x,y)-space. This 0.005 distance is the smallest difference in color that a human observer can perceive. Therefore, it is fair to say that Uniform Luminance Technology also significantly reduces color non-uniformity and results in more accurate color reproduction across the complete display area. 3.3 DICOM GSDF compliance Figure 12 shows the positive effect of Uniform Luminance Technology on DICOM GSDF compliance of LCDs. Without Uniform Luminance Technology, the display system will only be perfectly compliant to GSDF at the exact same position where the display was characterized. This can be seen on the left-hand side of figure 12. On a display system with Uniform Luminance Technology however, the display system will be compliant to DICOM GSDF across the entire display surface. Figure 12 also shows that the improvement is significant: without Uniform Luminance Technology, the average (over Page 16 of 19
256 video levels) distortion in JNDs compared to DICOM GSDF can be over 25 JNDs. With Uniform Luminance Technology, the DICOM GSDF compliance is as good for every location on the display surface and average distortion numbers are far below 1 JND. Figure 12: DICOM GSDF compliance without (left) and with (right) Uniform Luminance Technology. Note the difference in scale of the two plots. Right scale is 50x smaller. It is important to note that this improved compliance to DICOM GSDF is the result of making the display luminance uniform for every drive level. Other technologies that only digitally optimize the uniformity of the backlight's luminance exist. Such systems use the same correction pattern (typically the pattern for full white) for all video levels. Because the spatial noise pattern depends on the video level there will still be significant non-uniformity at other video levels. Such systems will therefore still suffer from different transfer curves for different locations on the display surface. Therefore, also compliance to DICOM GSDF will not be guaranteed except for exactly that position where the display was calibrated. Page 17 of 19
4 UNIQUE PROPRIETARY TECHNOLOGY Uniform Luminance Technology is a unique and proprietary Barco technology. Uniform Luminance Technology provides superior luminance uniformity and color uniformity and true DICOM GSDF compliance over the complete display surface. A unique aspect of Uniform Luminance Technology is that the technology simultaneously improves uniformity for all drive levels by using the appropriate correction images for each individual drive level. This is necessary because the spatial noise pattern of the display depends on the drive level. Alternative uniformity enhancement techniques Other technologies that optimize the uniformity of the backlight's luminance exist. These technologies use the same correction pattern for all drive levels and can only provide optimal uniformity for one specific video level (usually full white is chosen). Therefore significant non-uniformity remains for the other drive levels. As a result, technologies that only optimize the uniformity of the backlight's luminance also cannot provide compliance to DICOM GSDF across the entire display area. 5 CONCLUSIONS LCD technology has improved a lot over the past years, but there are still some aspects that raise questions as to the usefulness of Liquid Crystal Displays for subtle clinical diagnosis. One major problem is the existence of luminance non-uniformity and color non-uniformity in medical displays. Barco s Uniform Luminance Technology compensates for the non-uniformity and spatial noise. The result is a significant reduction of luminance non-uniformities and color nonuniformities. Because spatial noise depends on the drive level, Uniform Luminance Technology uses different correction images for every video level. Therefore, Uniform Luminance Technology also guarantees that the display is compliant with DICOM GSDF across its complete display area. Page 18 of 19
6 REFERENCES [1] Roehrig H, Krupinski EA, Chawla AS, Fan JH, Gandhi K. (2003). Spatial noise and threshold contrasts in LCD displays. SPIE Medical Imaging, 15-20 Feb, San Diego, CA. [2] Solution for Nonuniformities and Spatial Noise in Medical LCD Displays by Using Pixel-Based Correction, Tom Kimpe et al, Journal of Digital Imaging, Springer New York, 06.07.2005, vol. 18, no. 3, pp. 209-218 [3] A. Badano, S. J. Hipper, R. J. Jennings (2002), Luminance effect on display resolution and noise, Proceedings of the SPIE, Vol 4681, 305-313 [4] E. Krupinski, H. Roehrig (2002). Pulmonary nodule detection and visual search: P45 and P104 monochrome versus color monitor displays, Academic Radiology, Vol 9, No. 6, 638-645 [5] Roehrig H, Krupinski EA, Fan J, Chawla A, Gandhi K (2004). Physical and psychophysical evaluation of LCD noise. 18th International Computer Assisted Radiology & Surgery Conference, 23-26 June, Chicago, IL [6] Solution for non-uniformities and spatial noise in medical LCD displays by using pixel-based correction, Tom Kimpe, Albert Xthona, Paul Matthijs and Lode De Paepe, Conference proceedings SCAR 2004, Hot topics session, May 20-23 2004,Vancouver, Canada [7] Digital Imaging and Communications in Medicine (DICOM), Supplement 28: Greyscale standard display function (GSDF), published by NEMA Page 19 of 19