EP1342226B1 - Display device comprising luminophors - Google Patents

Abstract

The invention aims at correcting display defects caused by the disparity between luminophors of the display device. The correction is obtained by image processing. The invention concerns a display method for a sequence of video images on a device comprising luminophors including at least two types of luminophors and the device comprising the means for implementing said method. The correction is obtained by calculating an intermediate image between two successive images, then in displaying on one type of luminophor one of the successive images and simultaneously on another type of luminophor the intermediate image.

Description

The invention relates to a display device using luminescent materials for displaying the points of an image. The invention is more particularly applicable to plasma display panels as well as cathode ray tubes using high scanning frequencies.

Plasma display panels (PDPs) and cathode ray tubes (CRT) have on their front face a layer of luminescent material that transforms either UV radiation or electron radiation into visible light radiation. The luminescent material is commonly called phosphor.

For monochrome screens, the same phosphor is used on the entire front face of the CRT or PDP. On the other hand, for color screens, three types of phosphor of different colors are generally used to produce a color synthesis. For specific applications, it is possible to have screens with two or more types of phosphor.

The use of phosphors of different colors have some operating disparities due to the intrinsic characteristics of the materials constituting the phosphors. Among the disparities in operation, the temporal response to an excitation is specific to each type of phosphor.

For CRTs, this defect is generally not perceived on low definition screens, for example of the TV type. Nevertheless, one can perceive slight defects on screens very high definition (for example 1600 x 1200 pixels) using high refresh rates (for example> 120 Hz).

For PDPs, disparities are very important. The figure 1 represents phosphor reaction timelines commonly used in PDPs. The Figure 1A represents an excitation time during which electrical discharges are sent into the panel in order to produce unrepresented UV radiation. The UV radiation is then transformed into visible light by the phosphors. The figure 1 B represents the luminous rendering for a blue phosphor, for example a barium and magnesium aluminate doped with divalent europium. The figure 1C represents the light rendering for a color phosphor red, for example a Yttrium Borate doped with trivalent Europium. The figure 1D represents the light rendering for a green phosphor, for example a Manganese-doped Barium Aluminate.

The Figures 1B to 1D have different vertical scales that match the maximum values of each curve. In reality, the maximum value of blue color is about 4.3 times higher than the maximum value of red color and about 5.5 times higher than the maximum value of green color. However, the luminous energy efficiency is substantially the same for each of the colors. These chronograms make it possible to visualize the distribution, energetic by color. By way of example, it is indicated for a given excitation the durations for which the emitted light becomes less than 10% of the maximum emission value. Thus, less than a millisecond after the end of the excitation, the blue color is practically extinguished while the red and green colors are still close to their maximum level, the extinction of red and green respectively corresponding to 11 and 13 ms.

The figure 1E represents on the one hand the luminous renditions of the three colors with the same scale of luminous intensity and on the other hand the sum of the three luminous renderings which corresponds to a pixel seen by the human eye. If we look at the color corresponding to the sum of the three renderings, we see that the pixel is first blue, then changes from blue to white (or gray depending on intensity), then changes from white to yellow (combination of green and red of substantially the same intensity), and finally changes from yellow to green before going out. In PDPs, discharges are repeated cyclically at the refresh rate of the screen.

In the case of a still image, the retinal persistence of the human eye performs a low-pass type filtering on the color variations that mask this defect.

On the other hand, when one has a moving image, the eye becomes more sensitive to the color variation on moving color transitions. Thus a white object moving on a black background is for example affected by a leading edge of blue color and a trailing edge of yellow color (the green is found to be not perceptible by the human eye in our example).

To remedy this kind of problem, the only known solutions are to find new phosphors in order to use three types of phosphor with similar characteristics.

The invention aims to correct this lack of visualization by image processing. In order to reduce the effects of color drag, we delay or advance the visualization of the images according to the red, green or blue color concerned.

Thus, the invention is a method of displaying a sequence of video images on a phosphor device comprising at least two types of phosphors. In the method, at least one intermediate image is calculated between two successive images, then one of the two successive images is visualized on at least one type of phosphor and at least one other type of phosphor is imaged on the intermediate image.

To optimize the improvement obtained, the intermediate image is calculated with motion compensation

Preferably, the two successive images are a current image and a previous image, and in that the intermediate image corresponds to a lagging image on the current image of a duration defined as a function of the types of phosphor.

To optimize the correction rendering, the defined duration is calculated by making the difference between the moments corresponding to the average light emission centers of gravity of the at least two types of phosphor.

The invention is also a video sequence display device comprising at least two types of phosphor, said device comprising means for calculating at least one intermediate image placed between two successive images and means for displaying on one of the types of phosphor the intermediate image and on the other type of phosphor one of the successive images.

• la figure 1 représente des chronogrammes de réponse des luminophores,
• les figures 2 et 3 illustre le principe d'image intermédiaire calculée selon l'invention,
• la figure 4 illustre un mode préféré de réalisation d'un dispositif de visualisation à luminophores selon l'invention, et
• la figure 5 illustre une variante du mode préféré de réalisation de l'invention.

The invention will be better understood, and other features and advantages will appear on reading the description which follows, the description referring to the appended drawings among which:

• the figure 1 represents chronograms of phosphor response,
• the Figures 2 and 3 illustrates the intermediate image principle calculated according to the invention,
• the figure 4 illustrates a preferred embodiment of a phosphor display device according to the invention, and
• the figure 5 illustrates a variant of the preferred embodiment of the invention.

Having noted the disparities between the types of phosphor, it is first necessary to study the possible solutions. It was found that to minimize the defect, it was preferable to shift the light emission for the three types of phosphor. Unfortunately, other material constraints do not allow to separate the ignition corresponding to each type of phosphor. For a CRT, the three electron beams corresponding to each of the colors are driven simultaneously. For PDPs, the cells are addressed line by line and each line has the three types of phosphors.

According to the invention, the information to be displayed is shifted. As we have seen previously, blue phosphors have a much lower remanence time than red or green type phosphors, and red type phosphors have a lower remanence time than green type phosphors. We will thus display on the blue and on the red intermediate images instead of a current image, noted Image 1 on the figure 2 . Thus during the display of the image I, the visual information displayed corresponds to the image I for the green color and to two intermediate images for the blue and red colors.

The calculation of the intermediate image can be done according to different techniques. Those skilled in the art can refer to the publications relating to the image calculations implemented to make the change of the image frequency 50 / 60Hz or 50 / 100Hz.

Preferably, it is desired that the intermediate image is as close as possible to the image that should be viewed at this time, particularly with regard to moving objects. To calculate the best possible image, the intermediate image must be calculated with motion compensation.

avec Tt la durée séparant deux images et Tri la durée séparant l'image courante de l'image intermédiaire.The compensation in motion is done according to a known technique. Motion vectors 1 are calculated from images 1 and I-1 so that each pixel (composed of the three colors) corresponds to one vector 1, as represented on the figure 3 . The intermediate image is calculated by determining the value of each pixel by associating it with the weighted value of the pixels 3 and 4 of the images I and I-1 pointed by an extrapolated vector 2 which passes through the pixel of the intermediate image to be calculated. What is summarized by the formula: $$\mathrm{intermediate\; pixel}=\left(\left(\mathrm{pixel}3\right)\times (\mathrm{tt}-\mathrm{Sorting})+\left(\mathrm{pixel}4\right)\times \mathrm{Sorting}\right)/\mathrm{tt},$$

with Tt the time separating two images and Sort the duration separating the current image from the intermediate image.

The extrapolated vector 2 is for example the mean vector corresponding to the nearest vectors 1. When the extrapolated vector 2 points between several pixels of the image 1, then the corresponding pixel of the intermediate image corresponds to the average of the nearest pixels.

Of course, many other image extrapolation techniques using motion compensation are usable.

So that the compensation can have a real effect, it is appropriate that the time Tri separating the image 1 and the intermediate image is large enough to make a correction but not too important so as not to reverse the visualization fault. It seems quite difficult to precisely determine the ideal time Tri.

A simple calculation method giving an effective result consists in calculating the instant corresponding to the mean center of gravity of light emission for each type of phosphor in its environment of use. The time Tri corresponds to the difference between the instant corresponding to the center of gravity of the slowest phosphor and the instant corresponding to the center of gravity of the phosphor associated with the intermediate image. By way of example, with the phosphors mentioned above, it is possible to take Tr1 = 4 ms and TR2 = 0.5 ms.

By center of gravity of light emission, it is necessary to understand the moment after the excitation of the luminophore which corresponds to the emission of half of the luminous energy. By average center of gravity, it is necessary to understand the average of the centers of gravity corresponding to different conditions of excitation. Indeed the center of gravity varies according to the duration and intensity of excitation. The average of the centers of gravity can for example be made from the extreme cases of condition of use.

The figure 4 represents an exemplary embodiment of a plasma display panel implementing the invention.

In the example shown, the PDP receives a signal of YUV type (luminance + 2 chrominance components) for example extracted from a composite video signal. A motion estimator 10 receives the YUV type signal and provides motion vectors calculated from the received signal and a previously stored image. A format converting circuit 11 converts the YUV type signal into three R, G and B type image signals respectively corresponding to the red, green and blue images to be superimposed to obtain a color image. There are three distinct image signals, but in practice it is also possible to use a parallel or serial bus to carry these three image signals.

A first image calculation circuit 12 receives, on the one hand, the blue image signal and, on the other hand, the motion vectors. The first image calculation circuit 12 operates for example as indicated above or according to another image calculation algorithm with motion compensation. The signal B 'supplied by the calculation circuit corresponds to the intermediate image in advance of the time Tr1 with respect to the current image for the blue color.

A second image calculation circuit 13 receives, on the one hand, the red image signal and, on the other hand, the motion vectors. The second image calculation circuit 13 is of the same type as the first image calculation circuit 12 but using the duration Tr2 for the intermediate image. The signal R 'supplied by the calculation circuit corresponds to the intermediate image for the red color.

An image memory 14 receives the green image signal for storage during the calculation of the intermediate images. The memory 14 and the calculation circuits 12 and 13 can in practice be connected to a bus for receiving the R, G and B signals or to provide the signals R ', V and B'.

A sub-scan encoding circuit 15 receives the signal V from the image memory 14, the signals B 'and R' from the image calculation circuits 12 and 13 and a synchronization signal from a Synchronization circuit 16. The encoding circuit 15 provides a series of control bits to a column driver 17 for performing column addressing of the plasma panel 18 (also referred to as the plasma panel panel). A line control circuit 19 allows selection by line or group of lines. The synchronization circuit 16 sends the synchronization signals to the encoding circuits 15, column control 17 and line control 19 to ensure correct addressing of 18. The skilled person can refer to various documents of the state of the art to perform the circuits 15 to 19.

The embodiment can support many variants. For example, the figure 5 represents a simplified variant. Those skilled in the art may find that in the example chosen, the operating disparities between the green and red phosphors are not perceptible to the human eye. In this particular case, the correction made on the red does not bring any visible effect. It is then possible to replace the second calculation circuit 13 with an image memory 20. This makes it possible to have a circuit that is less complex and therefore less expensive. However, such a simplification is not possible if the operating disparities between all the phosphors are important.

It is also possible to use a circuitry using a microprocessor and a single memory to perform format conversion, intermediate image calculation and storage of unmodified images. The represented architecture will then be realized by programming.

As indicated above, the invention can also be used for a CRT device. In this case, the three guns of the CRT receive the signals R, G and B 'through shaping circuits.

In the embodiment shown, the intermediate image (s) is (are) between the current image and the previous image. It is also possible to place the intermediate image between the current image and the next image. In this case, the current image corresponds to the fastest phosphors and the most advanced intermediate image corresponds to the slowest phosphors. However, such a variant requires the delay of the image flow to be viewed from an image, which makes it necessary to have larger image memories.

Other adaptations are expected depending on the different variants mentioned throughout the description.

Claims (10)

1. Method for displaying a sequence of video images on a phosphor device comprising at least two types of phosphors (blue, green, red), characterized in that at least one intermediate image between two successive images (image I, image I-1) is computed, then in that one of the two successive images (image I) is displayed on at least one type of phosphor (green) and the intermediate image is displayed simultaneously on at least one other type of phosphor (blue, red).
2. Method according to Claim 1, characterized in that the intermediate image is computed with motion compensation.
3. Method according to either of Claims 1 and 2, characterized in that the two successive images are a current image and a previous image, and in that the intermediate image corresponds to an image delayed from the current image by a defined time period (Tr1, Tr2) as a function of the types of phosphor.
4. Method according to Claim 3, characterized in that the defined time period (Tr1, Tr2) is computed by taking the difference between the instants corresponding to the mean centres of gravity of light emission of the at least two types of phosphor.
5. Method according to one of Claims 1 to 4, characterized
   in that three types of phosphor are used, and in that an intermediate image is displayed on at least one type of phosphor.
6. Device for displaying a video sequence comprising at least two types of phosphor, characterized in that it comprises means (12, 13) adapted for computing at least one intermediate image placed between two successive images and means (14 to 19) adapted for displaying the intermediate image on one of the types of phosphor and one of the successive images on the other type of phosphor.
7. Device according to Claim 6, characterized in that it comprises a motion estimator (10) adapted for extrapolating motion of the intermediate image.
8. Device according to either of Claims 6 and 7, characterized in that it comprises three types of phosphors, and such that it is adapted to allow an intermediate image to be displayed on at least one type of phosphor.
9. Device according to Claim 8, characterized in that the computing means (12 or 13) are adapted for computing the intermediate image only on the colour component which corresponds to the type of phosphor used to display the intermediate image.
10. Device according to one of Claims 6 to 9, characterized
    in that the device is a plasma display panel.

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