Optical measurement standards for reflective e‐paper to predict colors displayed in ambient illumination environments

Abstract

Color displays are viewed in ambient conditions ranging from indoor lighting to bright sunlight. For emissive displays such as phones or tablets, color and readability are disturbed by ambient light reflected from the screen. However, ambient light is required to view e‐paper, so color measurements must be reflection‐based. This article describes how a bidirectional reflectance distribution function‐based methodology for measuring the disturbing reflection on emissive displays has been applied to measure the useful reflection from e‐paper displays. These measurements have been embedded in recently developed optical measurement standards for e‐paper that contain color measurement specifications for spectral reflection and—if a front light is present—spectral radiance of each displayed color. Then the measurements are combined with ambient illumination models to predict how display colors would appear in realistic lighting conditions. Although developed for e‐paper, this concept can be applied to any type of color display. Examples illustrate the standard measurement methodology, and how to predict displayed colors on both handheld e‐paper and emissive displays in both indoor and daylight viewing conditions.

52‐1: Blue‐Light Eye Safety Predictions under Ambient Light: ePaper Displays with Front Light vs. Emissive Displays

“…Measurements of LQ,em(λ), RQ,dir(λ) and ρQ,hemi(λ) have previously been reported. [19] A CS-2000 spectroradiometer (Konica-Minolta) was used to measure spectral radiance from 380nm to 780nm in 1nm intervals at a 1° measurement field angle. All measurements were made in darkroom conditions.…”

“…The spectral reflectance factors RQ,dir(λ) and ρQ,hemi(λ) were determined from spectral radiance measurements relative to calibrated diffuse white reflectance standards, following previously described standard measurement procedures. [18,19]…”

“…As in previous studies, although EPD reflectance is dependent on the displayed color Q, the LCD reflectance is not. [19] Since the luminous reflectance factors in Table 3 show that the LCD reflects considerably less ambient light than the EPD, the effect of ambient illumination on the LCD's total spectral radiance is expected to be small. Figure 2 shows the total spectral radiance LQ,T(λ) for each display under the reference indoor illumination of Table 1.…”

Evaluating Display Reflections in Reflective Displays and Beyond

“…The methodology outlined so far for predicting ambient contrast and color from a combination of measured display emission and reflection (with models of useful illumination) has been successfully applied to color EPD using CFAs 15 and colored pigments 16 . An example application of this methodology is illustrated for a black and white EPD.…”

Stretchable and reflective displays: materials, technologies and strategies

“…Also, the readability of reflective displays is superior under ambient light [7]. The optical contrast of emissive displays tends to be severely diminished by bright ambient light.…”