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The visual quality of color LED matrix displays is depending on two main parameters: the efficiency of the driving method that can produce spatial and temporal artifacts due to the multiplexing; the lack of spatial homogeneity of the display color properties due to the variable emissive properties of the LEDs and there evolution during aging. In th...
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... practical example is reported in figure 13 and 14 before and after adjustment. The three color primaries red, green and blue are measured separately and also the white state. The differences between the panels in figure 13 before adjustment are enhanced artificially to see better the different panels. After adjustment it becomes impossible to distinguish the different panels as shown in figure 14. Figure 12. Principle of the adjustment of panels in-situ on a complete display Figure 13. In-situ measurements of the primary colors and white on a complete LED display including 12 panels before adjustment. Figure 14. In-situ measurements of the primary colors and white on a complete LED display including 12 panels after ...
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... practical example is reported in figure 13 and 14 before and after adjustment. The three color primaries red, green and blue are measured separately and also the white state. The differences between the panels in figure 13 before adjustment are enhanced artificially to see better the different panels. After adjustment it becomes impossible to distinguish the different panels as shown in figure 14. Figure 12. Principle of the adjustment of panels in-situ on a complete display Figure 13. In-situ measurements of the primary colors and white on a complete LED display including 12 panels before adjustment. Figure 14. In-situ measurements of the primary colors and white on a complete LED display including 12 panels after ...
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... practical example is reported in figure 13 and 14 before and after adjustment. The three color primaries red, green and blue are measured separately and also the white state. The differences between the panels in figure 13 before adjustment are enhanced artificially to see better the different panels. After adjustment it becomes impossible to distinguish the different panels as shown in figure 14. Figure 12. Principle of the adjustment of panels in-situ on a complete display Figure 13. In-situ measurements of the primary colors and white on a complete LED display including 12 panels before adjustment. Figure 14. In-situ measurements of the primary colors and white on a complete LED display including 12 panels after ...
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... practical example is reported in figure 13 and 14 before and after adjustment. The three color primaries red, green and blue are measured separately and also the white state. The differences between the panels in figure 13 before adjustment are enhanced artificially to see better the different panels. After adjustment it becomes impossible to distinguish the different panels as shown in figure 14. Figure 12. Principle of the adjustment of panels in-situ on a complete display Figure 13. In-situ measurements of the primary colors and white on a complete LED display including 12 panels before adjustment. Figure 14. In-situ measurements of the primary colors and white on a complete LED display including 12 panels after ...
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... practical example is reported in figure 13 and 14 before and after adjustment. The three color primaries red, green and blue are measured separately and also the white state. The differences between the panels in figure 13 before adjustment are enhanced artificially to see better the different panels. After adjustment it becomes impossible to distinguish the different panels as shown in figure 14. Figure 12. Principle of the adjustment of panels in-situ on a complete display Figure 13. In-situ measurements of the primary colors and white on a complete LED display including 12 panels before adjustment. Figure 14. In-situ measurements of the primary colors and white on a complete LED display including 12 panels after ...
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... principle of color calibration is schematically reported in figure 7. Each module is measured for the states red, green, blue (and white if there is white LEDs). The current applied to each LED is fixed to same value on one entire panel and the color and luminance of each LED is automatically extracted from the measurements. Then correction coefficients are computed for each LED and input inside the module controller in order that each pixel match a targeted gamut and luminance. The targeted values are depending on the type of LEDs and can be approximated by the common virtual gamut observed when many pixel gamuts are reported on the same diagram (cf. figure 8). More precisely, the targeted color gamut and luminance is generally fixed for one type of module as to be reachable by almost 80% of the pixels [14]. The calibration coefficients are then given by: Table I. The distribution of colors and luminance for the red, green and blue LEDs is strongly reduced after calibration. The distribution of luminance and x color coordinate of green LEDs of one module before and after calibration are is reported in figure 10 and 11. As expected, the fluctuations are not completely suppressed but strongly reduced and the calibration improves drastically the aspect quality for the human eye. Color calibration of each module is a powerful tool to homogenize each panel but when the LED displays are arranged it become easier to see the different panels inside the display if they have not been adjusted exactly to the same color and luminance targets. Consequently, when the calibration process of the panels is done, it requires excellent video- colorimeters in terms of color accuracy otherwise some differences can be seen between panels calibrated with different instruments. The evolution of the panels during their lifetime can be also problem. Even if important improvements have been achieved these last years, there is still some reduction of the emissive efficiency of the LEDs while aging and some color shifts in particular for blue LEDs. In these conditions it can be necessary to realize a color and luminance adjustment of the different panels of one LED display during maintenance for example, and this adjustment need to be made in-situ most of the time. The principle is schematically reported in figure 12. It is very similar to the panel calibration except that the measurement images need to be corrected from the parallax and divided in different regions corresponding to the different panels. A set of color calibration coefficients is then computed for each panel and applied to all the LEDs within ...
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... principle of color calibration is schematically reported in figure 7. Each module is measured for the states red, green, blue (and white if there is white LEDs). The current applied to each LED is fixed to same value on one entire panel and the color and luminance of each LED is automatically extracted from the measurements. Then correction coefficients are computed for each LED and input inside the module controller in order that each pixel match a targeted gamut and luminance. The targeted values are depending on the type of LEDs and can be approximated by the common virtual gamut observed when many pixel gamuts are reported on the same diagram (cf. figure 8). More precisely, the targeted color gamut and luminance is generally fixed for one type of module as to be reachable by almost 80% of the pixels [14]. The calibration coefficients are then given by: Table I. The distribution of colors and luminance for the red, green and blue LEDs is strongly reduced after calibration. The distribution of luminance and x color coordinate of green LEDs of one module before and after calibration are is reported in figure 10 and 11. As expected, the fluctuations are not completely suppressed but strongly reduced and the calibration improves drastically the aspect quality for the human eye. Color calibration of each module is a powerful tool to homogenize each panel but when the LED displays are arranged it become easier to see the different panels inside the display if they have not been adjusted exactly to the same color and luminance targets. Consequently, when the calibration process of the panels is done, it requires excellent video- colorimeters in terms of color accuracy otherwise some differences can be seen between panels calibrated with different instruments. The evolution of the panels during their lifetime can be also problem. Even if important improvements have been achieved these last years, there is still some reduction of the emissive efficiency of the LEDs while aging and some color shifts in particular for blue LEDs. In these conditions it can be necessary to realize a color and luminance adjustment of the different panels of one LED display during maintenance for example, and this adjustment need to be made in-situ most of the time. The principle is schematically reported in figure 12. It is very similar to the panel calibration except that the measurement images need to be corrected from the parallax and divided in different regions corresponding to the different panels. A set of color calibration coefficients is then computed for each panel and applied to all the LEDs within ...
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... typical structure of an LED standalone display is composed of several panels. Each panel is a combination of different modules which are an assembly of unit components associated with their control electronics on a printed circuit (cf. figure 1). The resolution of these modules and of the displays including them is limited by the size of the components, or at least a few millimeters in the current state of the art. The production of large screens by assembly of different subassemblies or modules is well described in the technical literature and for example in the document published By Avago Technologies [5]. A structure commonly used to realize and control the different pixels of these modules is described in figure 2. It describes an example with three lines of three color pixels each composed of three red, green and blue sub-pixels produced by red, green and blue light-emitting diodes (LEDs), in order to make any color images. This structure is repeated as much as necessary to achieve the desired number of rows, columns and hence pixels of the module. The addressing mode of such a structure uses a single circuit or module for selecting the different lines in succession versus time. In the example of figure 2, where the first line of pixels is selected, the anodes of the LEDs of a same row are interconnected to each other and receive the same positive control voltage. The cathodes of the LEDs of the same subpixel column are connected to each other and to one output of the control circuit associated with the corresponding subpixel color. Then the sequential selection of the lines of the screen makes possible to construct and display any color image in three successive sub-frames. The patterns needed for a white image resulting from the superposition of all the sub pixels of the pixels of the same line are also schematically reported on figure 2. The frequency must be of course sufficiently high to avoid perception of the different images by the human eye (> 30Hz). This example presents a multiplexing rate of N=3. In practice N=2, 4 or 8 are generally used. This driving require N times less command outputs but also a current N times higher for the same visual effect. The operating voltage for the different LEDs used to create the red, green and blue elements are not the same what can create disparate voltage drops between the red, green and blue elements and excess heat generation. In addition, a video of the display taken with a frequency comparable to the sub-frame frequency is not representative of the visual ...
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