Optical Measuring Methods for Transparent Displays
June 4, 2015, Society for information Display Symposium, San Jose, CA—John Penczek from U of Colorado and NIST talked about the efforts to develop measurement methods for transparent displays. The challenges are defining the appropriate characteristics for transmissive, emissive, and reflective displays.
One of the problems is that ambient illumination can have a significant impact on observed photometric and colorimetric characteristics of emissive or transmissive displays. Reflective displays, on the other hand, do much better in high ambient lighting conditions. All of these display technologies is a proper means to determine performance in specific lighting environments.
A measurement method that uses linear superposition of incident light enables fairly simple evaluation of display surfaces with multiple light sources in the background. These sources can be directional or in a hemispherical diffuse component, and the method allows the display to be measured with certain illumination level and spectra. Using linear superposition, the display optical performance can be evaluated for arbitrary light levels and spectra. This approach provides flexibility in predicting displayed performance for wide range of ambient environments, and supervise testing.
Reflection coefficients are used extensively to describe the reflective properties of surfaces. This methodology has been incorporated into standards. For transmission measurements, however, the terminology and methodologies are still evolving. Traditionally, transmittance is measured as the ratio of the source to detector optical flux through a flat sample of the material.
This method works for clear glass or color filters, but it is not appropriate when the sample has haze or matrix scatter, which scatters light of the specular direction. When the samples exhibit diffuse scatter, the measurement is sensitive to measurement geometry and detector configuration. As a result, transmittance factor must be measured for both hemispherical and directional lamination.
The setup for measuring transmittance requires a two port broadband source. A high reflectance like standard is placed at the light trap port and the emitted light is measured at the sample port. The displays placed in direct contact with the sample port and the luminance is measured again. This provides a measurement of the luminance transmitted through the display. Then the white standard is removed and the luminance of the sample port is measured again to calculate the transmittance factor with specular excluded.This measurement method applies to transparent LCDs. If the display uses OLED, and the transmission characteristics are known, then the OLED can be measured in the off or black state.
The sampling sphere, normally used for reflection measurements of large displays, can also be used to determine transmittance factor. A calibrated quite reflectance standard with no reflectance factor is placed at the sample port of the sphere. A photometer or spectroradiometer measures luminance from the white standard. The detector is then rotated about the detector port and measures while luminance adjacent to the sample port. The light reflected standards and replaced with the display rendering color Q the back of the displays placed against a measurement port.
The sphere wall luminance is then measured again. Finally, the detector is moved in front and perpendicular to the display and focused on the sample port. The detector measures the transmitted luminance at the center of the sample port. The measurement field of the detector should not include the detector port.
Many transparent displays are observed to have significant factors may take scatter. This scatter makes it difficult to obtain reliable reflection and transmission measurements, so a ring light configuration is recommended. The sample plane should be located near the design working distance of the ring light for optimum uniformity. The light should be positions so light rays have a 45° inclination angle to the detector optical axis.
A white reflectance standard displace at the center the measure field facing the light source a photometer measures luminance for the white standard through the center of the ring light. The transparent display then replaces the white reflectance standard in the same sample plane. The detector is moved to the front of the display, which is a line parallel to the ring light face and perpendicular to the optical axis of the detector.
Since LCD displays use color filters in the pixel structure, transmission coefficients have a strong dependence on rendered color. The measurements are repeated with the display rendering white, black, red, green, blue, cyan, magenta, and yellow.
