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EZContrast series description
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| EZContrastXL88 |
EZContrastL80 |
EZLite |
EZContrastXL88W |
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EZContrast key features
Comparison to other technics
Working Distance
Measurement Spot Size
Angular resolution
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High performance Fourier Optics viewing angle instrument
ELDIM has been manufacturing viewing angle instruments based on Fourier optics for more than 15 years. One of their key features is the patented optical configuration which allows controlling the angular aperture of the system independently of the measurement spot size. Extreme grazing angles (88°) are measured with an excellent accuracy thanks to a very high light collection efficiency .
High speed
The full viewing cone is measured with high incidence and azimuth angular resolution within seconds for luminance and chromaticity and minutes for radiance.
High accuracy
Every ELDIM system follows a strict manufacturing and calibration procedures. Spectral response of each CCD sensor is measured and each filter -designed on purpose- is controlled by spectro-photometry.
High efficiency
ELDIM uses a patented optical configuration that allows optimum light collection even at very high angles. This cosine compensation matches the behavior of a spectro-photometer on a goniometric stage.
High reliability
ELDIM is manufacturing on its own all the key components of its systems. The quality of the optics is optimum thanks to advanced technologies such as magneto-rheological polishing or stitching interferometry. Antireflective coatings and optical alignments are also performed in house to reduce straight light and parasitic polarization.
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Comparison with other technics
Viewing angle properties are certainly among the most common characteristics measured on any type of displays. Historically, goniometers were the first equipments used to perform angular measurements. Main drawback of those systems is the "one after each other" nature of the measurements which results in very long acquisition times if more than a few directions are required. ELDIM has introduced Fourier optics instruments in 1993(*). A specific optic is designed in order to convert angular field map into a planar one allowing very rapid measurements of the full viewing cone with high angular resolution. Our systems have been improved throughout years to reach extremely high performances at every level. Some technical insights are reported hereafter. Recently for viewing angle measurements, hemisphere based imagers have been introduced. But they’re suffer from very poor light collection efficiency, tradeoffs between angular resolution and efficiency and important parasitic light (**). Best technical solution for viewing angle measurements is clearly the Fourier optics approach. There is no strong theoretical limitation neither for light collection efficiency nor for angular resolution. The ELDIM optical design that includes cosine compensation allows accurate measurements up to 88° and 6mm spot size.
(*) T. Leroux, "Fast contrast vs. viewing angle measurements for LCDs", Proc. 13th Int. Display Research Conf. (Eurodisplay 93), 447 (1993)
(**) V. Collomb-Patton, P. Boher, T. Leroux, "Comprehensive Survey on Viewing Angle Measurement Devices: A Theoretical Study", SID, San Antonio, 17.4 (2009)
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CNC systems for grinding, polishing and centering
First hemispheric lens on stitching interferometer
Lens alignment in clean room
Electron beam evaporator with ion beam assistance
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Impact of the distance on the light collection with Fourier optics instrument: at optimum working distance OWD all the angles are measured at the same location (top); if the distance is higher than OWD all the angles are not measured at same location (bottom).
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Working Distance
Impact of working distance
An ELDIM Fourier optics system has an optimal working distance in the range of 1 to 4mm depending on its angular aperture. In contradiction with what is generally thought, this working distance is not critical since the system is collecting "plane waves" at each angular direction. Exact focusing is not required and the system can easily work at larger distances. The impact on the measurement is only that measurement spot centers are not perfectly superposed while varying the incidence angle under focus. This behavior can be related to misalignment of a goniometer axis and has exactly the same type of impact on the measurement results.
Optimum working distance and angular aperture
Even if a Fourier optics system is generally a quite complex combination of lenses, the first front lens dimension obeys to very simple geometrical considerations. The front lens diameter depends on the maximum angle achievable by the instrument and on the optimum working distance. An optimum working distance as high as possible is generally preferable for practical considerations. Nevertheless for very large angle of view instruments the requirements in terms of lens diameter becomes critical. A system working up to 80° can be made with a working distance of 5mm quite easily. On the contrary, if the maximum angle is pushed up to 88° a working distance of 1mm gives already quite large front lens (60mm).
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Measurement Spot Size
Why a large spot size is needed for displays?
Accurate characterization of the optical emission of displays requires light integration from a given number of pixels. Facing a continuous increase in display size, the pixel has also been enlarged. Using a simplified geometrical model and the MSR parameter (Measurement Spot Ratio) we have shown that a measurement stability better than ±1% is assured if MSR is above 6 (*). We can thus easily predict which minimum measurement spot diameter (MSD) is required depending on the display diagonal and definition. A 6mm MSD is for example sufficient up to 80" diagonal for 16/9 format screens with a 1080x1920 definition.
MSR versus display diagonal for 1080x1920 resolution
How to achieve a large spot size with Fourier instrument
Achieving simultaneously a wide angular aperture and a large spot size is a technical challenge. The main constraint lays into the conservation of the geometric étendue. This general principle is the consequence of the energy conservation and can be expressed as:
Ss and Sd are respectively the surfaces of the source and the detector, and Ωs and Ωd the solide angles of the light on the source and on the detector. If we need at the same time a large spot size on the source and a high angular aperture, a very big detector is required otherwise the whole angular range will not be covered by the sensor. ELDIM uses 16M pixels CCD sensors for EZContrastXL80W and EZContrastXL88W taking interest more in their large size than in the great number of pixels.
(*) P. Boher, "ELDIM’s Fourier optics enhance characterization of large LCDs", Display Devices, summer (2006)
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Relation between front lens diameter and OWD: Front lens sizes for ELDIM system are indicated as red points.
Definition of MSR
Conservation of the geometric entendue
16M pixels CCD are 37x37mm large
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Angular resolution measurements for 3 angles and 4 spot sizes
cross sections for 4 spot sizes (300µm, 500µm, 2mm and 6mm) and at normal incidence
Angular resolution measured for the 4 spot sizes on EZContrastXL88
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Angular Resolution
The capacity to separate two light rays with very close directions defines the angular resolution. In Fourier optic systems it depends on the optical design, the CCD resolution and the quality of the optics. The theoretical angular resolution of the EZContrast instruments is calculated from their optical design. It is supposed to be ±0.17° for the XL88W model that works up to 88° of incidence with a maximum spot size of 6mm diameter.
The practical angular resolution has been measured at various incidence angles using a goniometer with a collimated beam, as summarized in the neighboring table. It is very close to the theoretical value demonstrating the good quality of the optics.
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ELDIM at Photonics West 2012
