Subjective Image Quality of CRT Displays under Ambient Glare: Assessing the ISO 9241-7 Ergonomic Technical Standard


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Virginia Tech


This thesis evaluates the ISO 9241 Part 7, Ergonomic requirements for office work with visual display terminals (VDTs) - Part 7: Requirements for display with reflections. The thesis involved two phases of effort that evaluated the photometric measurements required in the ISO standard in terms of subjective image quality judgments. In phase one, seven monitors were evaluated photometrically according to the ISO 9241-7 standard to determine whether they were Class I, II, or III. Additionally, glare filters were attached to monitors to see if they change the ISO classification of the monitor.

The results of phase one indicated that positive polarity always yielded either Class I or Class II because the Large Source, Screen Image Luminance Ratio (LR BDS/FDS) and the Specular Reflection Luminance Ratio (LR BDS/BD) always passed. Conversely, negative polarity always produced Class II or Class III because the Small Source, Specular Reflection Luminance Ratio (LR BDS/BD) failed every time. Also, the AF 150 and HF 300 were the best filters for reducing glare on monitors. The BF 10 and AF 100 or AF 200 ,on the other hand, were the worst because they intensified screen reflections.

In phase two, human image quality judgments were collected to determine if people rated Class I, II, or III monitor-filter combinations differently under different lighting and different screen polarity conditions. Specifically, phase two assessed the effects of seven monitor/add-on glare filter combinations, five ambient lighting conditions, and two screen polarities on subjective image quality ratings. Each participant provided subjective image quality judgments by viewing alphanumeric text on the CRT screens. Subjective scale values also were correlated with ISO classifications and two ISO metrics: screen image luminance ratio (Diffuse, 200 lux) and specular reflection image luminance ratio.

The ANOVA findings indicated that specular glare significantly degrades image quality ratings more than diffuse glare. The author contends that this finding is the result of an experimental context effect. In other words, the specular glare was so influential on subjective ratings of image quality that subjects paid little or no attention to reductions in contrast from the diffuse lighting conditions. The correlation analysis showed that the specular reflection luminance ratio and the negative polarity classifications did index subjective quality ratings.

Finally, this thesis establishes a human factors basis to justify the measurement requirements in the ISO 9241-7 standard. Specifically, the findings show that it underemphasizes the contribution of the specular reflection luminance ratio' and overemphasizes the contribution of the screen image luminance ratio to compliance classification calculations, because the procedure gives equal weight to both ratios.



Glare, CRT, Glare-Filter, ISO 9241-7 Ergonomic Standard