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LC displays play an important role in modern infotainment systems. Modern measurement technology can be used to examine the quality of the displays installed.
LC displays play an important role in modern infotainment systems. Modern measurement technology can be used to examine the quality of the displays installed.
( Source: gemeinfrei / Pexels)

quality measurement Displays in the vehicle and how their quality can be measured

| Author / Editor: Silke Kirchner * / Jochen Schwab

The various LC displays in the vehicle cockpit must meet high-quality standards. High-precision measurement technology is required to implement the specifications of the automotive industry. The article gives an overview.

Human Machine Interfaces (HMI) in vehicles are based on AR/VR systems and displays. In the German automotive market, strict requirements apply to safety, user-friendliness and integral aesthetics. The "German Automotive OEM Working Group" of the German Flat Panel Display Forum (DFF) comprises representatives of German automotive manufacturers and regularly publishes the requirements for LC displays for the automotive industry. Before an OEM accepts an LC display, the quality must be carefully tested according to fixed criteria. In addition to know-what and know-how, high-precision optical measurement technology is required.

In addition to technical parameters, it is the quality requirements of a display that are constantly being further developed. Representatives of German OEMs meet regularly at the DFF to discuss, quantify and classify current display performance requirements into pass/fail criteria. The results of the work will be published in the document "Display Specification for Automotive Application" [1].

Checking the quality of the LC display

The overview now includes over 100 test parameter entries, additive references and references to international standards for display testing. The metrological implementation of some test parameter entries is described in the "Display Measurement Specification (DMS) for Automotive TFT-LCDs" [2], which is published by DFF. The aim of both publications is to guarantee objective, constant and high-quality testing of LC displays for use in vehicles throughout the entire supply chain.

Included in the OEM quality requirements is that 100% of the displays are tested end-of-line (EOL). The test time per display is a critical factor for the manufacturer to achieve efficient production throughput. In the EOL test, it must be weighed up which of the more than 100 test parameter entries is the minimum requirement for a sufficient EOL test (know-what). It must also be clear how (know-how) and how quickly the test procedures can be implemented.

A trend for sufficient test procedures:

  • Color
  • Luminance homogeneity
  • Gamma and
  • pixel defects

Other prescribed parameters such as angle dependencies, temperature ramps or mechanical influences are often neglected in the EOL test in order not to exceed the critical time dimension.

The components of the OEM quality assessment

Figure 1: Calculation of dominant wavelength λdom and color saturation according to CIE 1931 with reference point (x, y) = (0.31271; 0.32902).
Figure 1: Calculation of dominant wavelength λdom and color saturation according to CIE 1931 with reference point (x, y) = (0.31271; 0.32902).
(Bild: Instrument Systems)

Zur Realisierung der OEM-Farbvorschriften müssen die dominante Wellenlänge λdom und Farbsättigung gemäß CIE 1931 mit dem Referenzpunkt (x, y) = (0,31271; 0,32902) für Rot, Grün und Blau bestimmt werden, sowie die Farbkoordinaten für Weiß (Bild 1). Es gelten folgende Grenzwerte für einen erfolgreichen Farbtest:

In order to implement the OEM color specifications, the dominant wavelengthλdom and color saturation according to CIE 1931 must be determined with the reference point (x, y) = (0.31271; 0.32902) for red, green and blue, as well as the color coordinates for white (Figure 1). The following limits apply for a successful color test:

  • Red: x = 0,661 und y = 0,306, λdom = 623 +7/-5 nm, saturation >85%,
  • Green: x = 0,298 und y: = 0,662; λdom = 549 +5/-5 nm, saturation >80%,
  • Blue: x = 0,137 und y: = 0,067; λdom = 469 +5/-5 nm, saturation >90%.

The evaluation of the luminance uniformity of a display is a critical part of the OEM quality evaluation since the displays must be perfectly legible even under unfavorable conditions. The OEM requirement is based on the Uniformity Measurement Standard for Displays [3] and requires a percentage of ≥75% for luminance homogeneity in the white image and ≥50% for a black test image.

Various pixel errors in the display

Figure 2: Technical setup for high-precision 2D measurements at production speed with a spectrally optimized luminance and colorimetric camera.
Figure 2: Technical setup for high-precision 2D measurements at production speed with a spectrally optimized luminance and colorimetric camera.
(Bild: Instrument Systems)

Figure 3: Two-shot gamma measurement. Top: A gamma test image contains nine different gray levels so that 18 gray levels can be determined with only two images. A white image is used to correct spatially determined luminance inhomogeneity. Below: Electro-optical characteristic curves. After the white image correction (right), the electro-optical transfer function of the DUT lies within the tolerance band (blue dotted lines) as required by the OEMs.
Figure 3: Two-shot gamma measurement. Top: A gamma test image contains nine different gray levels so that 18 gray levels can be determined with only two images. A white image is used to correct spatially determined luminance inhomogeneity. Below: Electro-optical characteristic curves. After the white image correction (right), the electro-optical transfer function of the DUT lies within the tolerance band (blue dotted lines) as required by the OEMs.
(Bild: Instrument Systems)

Der relative Leuchtdichtegradient muss bei Normalisierung auf den weißen Bildschirm kleiner als 0,02% pro mm sein. Die Hellempfindung des Auges unter photopischen Bedingungen spiegelt sich im Gamma-Exponenten wieder, der aus der elektrooptischen Übertragungskurve extrahiert wird. Um die OEM-Qualitätskriterien zu erfüllen, muss die Leuchtdichte aller 255 Graustufen im Vollbildmodus nacheinander vermessen werden und ein Ziel-Gammawert γ = 2,2 mit einer Toleranz von δγ = 0,2 erreicht sein. Die Anzahl akzeptierter Pixeldefekte für verschiedene Pixeldefekt-Untergruppen ist von den OEMs streng limitiert. Beschädigte Pixel können zu Sicherheitsrisiken führen. Drei Defekt-Gruppen sind durch die OEM kategorisiert:

The relative luminance gradient must be less than 0.02% per mm when normalized to the white screen. The light perception of the eye under photopic conditions is reflected in the gamma exponent, which is extracted from the electro-optical transmission curve. In order to meet the OEM quality criteria, the luminance of all 255 gray levels must be measured successively in full-screen mode and a target gamma value of γ = 2.2 with a tolerance of δγ = 0.2 must be achieved. The number of accepted pixel defects for different pixel defect subgroups is strictly limited by the OEMs. Damaged pixels can lead to security risks. Three defect groups are categorized by the OEM:

  • Hot pixels: Pixels that are always in a bright state.
  • Weak Bright pixels:only partially allow light to pass through and are never completely black or white.
  • Dead pixels:are stuck in an off state and do not let light through.

The quality is evaluated with precise measuring technology. A spectrophotometric measurement according to ISO 7724-2 is prescribed for the color. This implies a repeatability of the spectrophotometer better than 0.001 (absolute) and an accuracy of 0.002 (absolute). With a spectrometer corrected 2D colorimeter, a 2D color evaluation with spectroradiometric precision according to OEM criteria (see Fig. 2) can be performed quickly.

The combination measurement system tests color, luminance uniformity, gamma and pixel errors according to OEM regulations in 15 seconds per display. Nine test images (Fig. 3) are taken into account and user-specific conditions such as handler time, duration of data transmission or white point calibration are excluded.

This article was first published in German by Next Mobility.

References

[1] German Automotive OEM Working Group, Display Specification for Automotive Application V5.1 (2018)

[2] German Flat Panel Display Forum (DFF), Display Measurement Specification (DMS) for Automotive-TFT LCDs.

[3] German Automotive OEM Working Group, Uniformity Measurement Standard for Displays V1.30 (May 2018)

* Dr. Silke Kirchner is product manager for the Automotive Interior division at Instrument Systems Optische Messtechnik GmbH in Munich.

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