DE102016000629A1 - Measurement method for image interference on screens - Google Patents

Abstract

The invention relates to a measuring method for the quantitative determination of image interference (sparkle) on screens, which determines the values of the parameter for image disturbance (sparkle) by numerical processing and evaluation of images of screen and image interference inducing layer with an electronic camera, wherein the characteristic a weighted summation is determined from the Fourier amplitudes of the recordings.

Description

The invention relates to a measuring method for picture interference on screens according to the preamble of claim 1.

State of the art

The need for antireflection of screens for ergonomic reasons and the corresponding prior art is detailed in the article "Optimizing Contrast, Reflection and Sparkle in Touch-Sensitive Screens", Photonics 4/2012. In this Review, a measuring device for the quantitative detection of a type of visual image disturbances, in English as "sparkle" (glittering), introduced. Here, the term sparkle means visually conspicuous effects that interfere with or disturbingly superimpose the displayed image information, a statistical distribution of intensity or color modulations across the screen area, usually with a strong change in the appearance of the disturbance with the sighting.

• • für den Fall, dass der Bildschirm und die Bildstörung induzierende Schicht untrennbar verbunden vorliegen, durch räumliche Filterung einer einzelnen Aufnahme der elektronischen Kamera mit einer lokalen Mittelwertbildung mit Gauß'scher Bewertungscharakteristik (siehe M. E. Becker, et al.: Optimieren von Kontrast, Reflexion und Sparkle bei berührungsempfindlichen Bildschirmen, Photonik 4/2012, 46–49. ) oder durch Maskierung im Fourierraum (siehe: M. E. Becker, J. Neumeier: Optical Characterization of Scattering Anti-Glare Layers, SID'11 Digest, 1038–1041 );
• • für den Fall, dass Bildschirm und Bildstörung induzierende Schicht getrennt vorliegen, durch ein Differenzbildverfahren, bei dem aus zwei Aufnahmen mit einer Verschiebung der die Bildstörung induzierenden Schicht durch Differenzbildung eine Bildinformation (Differenzbild) berechnet wurde, die als Grundlage zur Berechnung des Sparkle-Wertes (Wert der Kenngröße) dient (siehe: M. E. Becker, J. Neumeier: Optical Characterization of Scattering Anti-Glare Layers, SID'11 Digest, 1038–1041 ).

The measuring device described captures an image of the z. B. by sparkling (sparkle) disturbed image content and separates the noise components (Sparkle) from the remaining intensity modulations by numerical processing of the image content. Depending on the conditioning of the objects involved in the measurement (screen and the image-disorder-inducing layer), this processing was carried out in two ways:

• In the case where the screen and the image distortion inducing layer are inseparably connected, by spatially filtering a single image of the electronic camera with a local averaging with Gaussian evaluation characteristic (see ME Becker, et al .: Optimizing Contrast, Reflection and Sparkle in Touch-Sensitive Screens, Photonik 4/2012, 46-49. ) or by masking in Fourier space (see: ME Becker, J. Neumeier: Optical Characterization of Scattering Anti-Glare Layers, SID'11 Digest, 1038-1041 );
• In the case where the screen and image distortion inducing layer are separated, by a difference image method in which image information (difference image) was calculated from two images with a shift of the image distortion inducing layer by subtraction, which is used as the basis for calculating the sparkle value (Value of the parameter) is used (see: ME Becker, J. Neumeier: Optical Characterization of Scattering Anti-Glare Layers, SID'11 Digest, 1038-1041 ).

The parameter for describing the level of the image interference (sparkle) relevant here is calculated analogously to the speckle contrast as a quotient of standard deviation and mean value (see: Darran R. Cairns and Philip Evans: Lasers Speckle of Textured Surfaces: Towards High Performance Anti-Glare Surfaces, SID'07 Digest, 407-409 ).

In DE 10 2013 011 359 B4 describes a method which (1) with an improved filtering method ensures the required one-to-one correspondence between determined characteristics and the results of the ranking established by visual inspection and (2) introduces further processing of the difference image by filtering, again in order to correlate the measurement results to improve with visual ratings. This is achieved by spatial filtering (convolution) with a core of rational dimensions.

In the practice of metrology has been shown that visually acceptable sparkle levels should usually be below 5%, the method after DE 10 2013 011 359 B4 itself has a lower detection limit of about 1%, mainly by low-frequency components in the recording, z. B. by unevenness of the backlighting of the measurement object or by unevenness of the illuminance on the detector array of the camera (eg vignetting) caused. The remaining range of about 4% along with unavoidable measurement uncertainties makes it difficult to reliably produce rankings at low sparkle levels.

A method for the quantitative description of sparkle based on a summation of Fourier coefficients ( T.-W. Hsu, et al .: Novel Evaluation Method of Sparkle for LCDs with Different Anti-Glare Films, Proc. IDW 2014, 953-955 ) makes the possibility of designing the result by corresponding weighting of the Fourier coefficients unused.

Disclosure of the invention

The invention has for its object to provide a measuring method for determining sparkle values in screens, with the highest possible resolution and sensitivity is achieved.

The solution to this problem is obtained with the features of claim 1.

To carry out the measuring method according to the invention, a measuring device may be used, which preferably comprises the following devices: An electronic camera with a suitable objective for imaging the test object, consisting of a screen and a layer inducing the image interference to be measured (light-scattering matted anti-reflection layer, AG layer), either separately or permanently connected, an electronic control and data acquisition unit (usually in the form of a personal computer), and a software program for evaluating the recordings for the purpose of determining the values of the characteristic.

Prior to performing the measurement (i.e., taking the image with the electronic camera), the lines of the pixel matrix of the screen are aligned parallel to the lines of the detector array of the electronic camera of the measurement system. The design of the optics ensures a sufficiently high sampling rate (typically more than 5 camera pixels per display pixel). The inclusion of the scattering anti-glare layer suppression layer and screen matrix involves periodic intensity and color changes through the screen matrix and statistically distributed intensity and color changes underlying the Sparkle image noise to be measured.

In contrast to the separation of periodic and statistical intensity modulations by spatial filtering (convolution), the present invention is based on the description of the intensity modulations directly by their Fourier components, ie the harmonic components in the frequency domain. By a weighted sum of the frequency components, a parameter for describing the sparkle level with the required properties is formed.

In this case, the link between the fundamental frequency given by the pixel matrix of the screen, f ₀ , and the spatial frequency at which the human visual system is most sensitive to luminance modulation, is formed in such a way that a test arrangement can always be designed without restriction of generality in that the human observer can recognize the image disturbance (sparkle), but the screen matrix itself lies beyond the threshold of perception, ie is not visible at the moment. In the course of perceptual thresholds as a function of spatial frequency, as determined by different authors with different experimental setups, the range of highest sensitivity is about 5 ± 2 periods per degree visual angle and the visual resolution limit of the normal-sighted observer is 30 periods per degree visual angle. As a result, the fundamental frequency f _{0 is set} equal to a frequency of 30 periods per degree of visual angle.

The invention will be explained in more detail with reference to diagrams and figures shown in the drawing.

Show it:

1 a representation of a typical course of a perception threshold as a function of the spatial frequency,

2a to 2c Power spectra obtained with 2-dimensional discrete Fourier transform from camera images,

3 a region in the plane of the spatial frequencies with small proportions of the periodic modulation by the screen matrix (hatched square) together with a circular arc constant spatial frequency, and

4 the course of the Fourier amplitudes from three transforms 2a to 2c as a function of the spatial frequency together with a band-pass function defined by three straight line sections with a damping-free range between 3 and 7 periods per visual angle.

1 shows a representation of a typical course of the sensitivity of the contrast perception of humans as a function of the spatial frequency according to Mullen [ Kathy Mullen: "The contrast sensitivity of human color vision to red-green and blue-yellow chromatic gratings", J. Physiol. (1985), 359, pp. 381-400 ]. The perception threshold for a visual acuity of 1 for black and white structures is 30 periods per degree visual angle [c / °], for black-green grid structures 60 periods per degree visual angle [c / °]. Depending on the test conditions, the maximum sensitivity is between 2 and 10 periods per degree of visual angle [c / °].

From the functional relationships z. B. between field of view and adaptation luminance, as they are given in the literature, suitable weighting functions for the frequency components of the recordings can be derived according to the described measurement method.

2a to 2c show power spectra (obtained with 2-dimensional discrete Fourier transform from the camera images) of the screen matrix without additional layer (left), the screen matrix together with an AG layer with low sparkle value (center) and the screen matrix with an AG layer with high sparkle Value (right). The amplitude of the measured intensity normalized to the DC component is represented in each point of the diagrams by the gray value of the representation. The spatial frequency at each point of the diagrams is given by the length of the pointer originating from the origin (DC). The frequency f ₀ is drawn in the x and y directions.

To evaluate the sparkle level, an average value of the amplitudes as a function of the spatial frequency is determined from the calculated Fourier transformations from those regions which have no appreciable frequency components through the pure screen matrix. These areas include in particular areas around the diagonal of the representations in the 2a to 2c as in 3 is illustrated by the hatched square area.

From the Fourier transformations after the 2a to 2c For example, the amplitudes are determined as a function of frequency by averaging along arcs or circles around the DC component corresponding to a constant spatial frequency.

4 shows the course of the Fourier amplitudes along circles around the DC component of the transformations of 2 were determined together with the filter function of a bandpass with a non-attenuation range between 3 and 7 periods / degree visual angle, [c / °].

mit
den normierten Fourieramplituden a_i,
den frequenzabhängigen Gewichtungsfaktoren w_i.After weighting with the bandpass function, the amplitudes are squared to form power coefficients and summed to calculate the sparkle index, Ksp.

With
the normalized Fourier amplitudes a _i ,
the frequency-dependent weighting factors w _i .

• 1 Reduktion des Einflusses von niedrigfrequenten Modulationen, wie sie durch ungleichmäßige Hinterleuchtung des Displays oder durch Vignettierung in der Kombination von Kamera und Objektiv auftreten können durch entsprechend geringe Werte (meist Null) für die entsprechenden Gewichtungsfaktoren.
• 2 Selektive Dämpfung derjenigen Frequenzanteile, die zur visuellen Wahrnehmung von Sparkle nicht oder nur wenig beitragen durch Berücksichtigung der Bandpasscharakteristik der Kontrastempfindlichkeitsfunldtion beim Menschen.

The weighting of the Fourier coefficients allows the following design measures in the evaluation:

• 1 Reduction of the influence of low-frequency modulations, as they may occur due to uneven backlighting of the display or due to vignetting in the combination of camera and lens due to correspondingly low values (usually zero) for the corresponding weighting factors.
• 2 Selective attenuation of those frequency components which do not or only slightly contribute to the visual perception of sparkle by taking into account the bandpass characteristic of the contrast sensitivity function in humans.

Case 1: Pixel matrix and anti-reflection layer are separated

When the pixel matrix and image distortion inducing layer are separated, a first image is taken of the overlying interference-inducing layer screen, then laterally displaced the interference-inducing layer a small distance at a fixed position of the screen and a second image taken under otherwise unaltered conditions. From these two shots is a Calculates difference image, in which the images common constant proportions to zero and so the pixel matrix of the screen is partially eliminated.

The difference image is then subjected to the described Fourier transformation and the sparkle characteristic is calculated according to Eq. 1 determined.

The determination of the 2-dimensional frequency range, which contains only small portions of the periodic modulation of the display matrix, can be done from a recording of the display matrix without AG layer.

Case 2: Pixel matrix and anti-reflection layer are firmly connected, not separable

In this case, the electronic camera takes a picture of the firmly connected combination of screen matrix and image-disordering layer. This image is then subjected to the described Fourier transformation and the sparkle characteristic as described by Eq. 1 described determined.

The determination of the 2-dimensional frequency range, which contains only small portions of the periodic modulation of the display matrix, can be made from a recording of the display matrix with AG layer as far as the periodic modulation can be determined by the display matrix. Alternatively, the scattering effect of the antireflection coating can be reversed by applying water or alcohol together with a thin glass layer (eg coverslip for microscopy) to the extent that the periodic modulation of the display matrix becomes measurable.

• • Ein Verfahren zur Ermittlung von Sparklewerten bei Bildschirmen, mit erhöhter Auflösung und Empfindlichkeit im Vergleich zu den bekannten Verfahren.
• • Ein Verfahren zur Ermittlung von Sparklewerten bei Bildschirmen, mit erhöhter Unterdrückung des Einflusses von niedrigfrequenten Frequenzanteilen.
• • Ein Verfahren, das die unterschiedlichen Empfindlichkeiten des menschlichen visuellen Systems für laterale Intensitätsvariationen (hier Sparkle) mit unterschiedlicher Ortsfrequenz berücksichtigt.

The following advantages are achieved in particular with the present invention:

• • A method for determining sparkle values on screens, with increased resolution and sensitivity compared to the known methods.
• • A method for determining sparkle values on screens, with increased suppression of the influence of low-frequency frequency components.
• • A method that takes into account the different sensitivities of the human visual system for lateral intensity variations (here sparkle) with different spatial frequency.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102013011359 B4 [0005, 0006]

Cited non-patent literature

• ME Becker, et al .: Optimizing Contrast, Reflection and Sparkle in Touch-Sensitive Screens, Photonik 4/2012, 46-49. [0003]
• ME Becker, J. Neumeier: Optical Characterization of Scattering Anti-Glare Layers, SID'11 Digest, 1038-1041 [0003]
• ME Becker, J. Neumeier: Optical Characterization of Scattering Anti-Glare Layers, SID'11 Digest, 1038-1041 [0003]
• Darran R. Cairns and Philip Evans: Lasers Speckle of Textured Surfaces: Towards High Performance Anti-Glare Surfaces, SID'07 Digest, 407-409 [0004]
• T.-W. Hsu, et al .: Novel Evaluation Method of Sparkle for LCDs with Different Anti-Glare Films, Proc. IDW 2014, 953-955 [0007]
• Kathy Mullen: "The contrast sensitivity of human color vision to red-green and blue-yellow chromatic gratings", J. Physiol. (1985), 359, pp. 381-400 [0020]

Claims (9)

Measuring method for the quantitative determination of image interference (sparkle) on screens, which determines the values of the parameter for image disturbance (sparkle) by numerical processing and evaluation of images of screen and image interference inducing layer with an electronic camera, characterized in that the characteristic is determined by a weighted summation is determined from the Fourier amplitudes of the recordings. Measuring method according to claim 1, characterized in that the Fourier amplitudes are effected by averaging on circular arcs or circles around the DC component. Measuring method according to claim 2, characterized in that the Fourier amplitudes are determined from an area in the frequency range in which the components of the periodic modulation by the display matrix are sufficiently small. Measuring method according to one of claims 2 or 3, characterized in that the Fourier amplitudes are weighted according to the bandpass characteristic of human visual perception (contrast sensitivity function). Measuring method according to claim 4, characterized in that the given by the display matrix spatial frequency f ₀ - is set according to the experimental arrangement - the perception threshold at 30 or 60 periods per degree visual angle. Measuring method according to claim 5, characterized in that the weighting of the Fourier amplitudes for the DC component and for small frequencies is set to zero or to sufficiently small values in order to avoid long-wave interference due to non-uniform backlighting of the display and due to nonuniformities of the optical imaging (eg. Vignetting). Measuring method according to claim 5, characterized in that the weighting of the Fourier amplitudes for the given by the periodic modulation of the display matrix frequency f ₀ is set to zero so as to suppress the fundamental frequency of the display matrix. Measuring method according to one of claims 6 or 7, characterized in that an index of the value of the image interference (sparkle) is formed as the sum of the squares of the weighted Fourier amplitudes. Measuring method according to claim 1, characterized in that from the Fourier transforms amplitudes by averaging along circular arcs or circles around the DC component, which correspond to a constant spatial frequency, are determined as a function of frequency, and that after weighting with a bandpass function the amplitudes for the formation of Power coefficients are squared and used to calculate the sparkle metric, Ksp, using the equation

where a _{i are} the normalized Fourier amplitudes and w _{i are} the frequency-dependent weighting factors.

Images