Mc Martin Boschman

Text quality metrics for visual display units: I. Methodological aspects

A comparison of four experimental variables to measure the visual quality of text on visual display units (VDUs) is discussed. Scaled visual comfort is considered to be the most relevant psychological attribute. Visual performance, saccade length and fixation pauses during a search task approximating closely to reading are under the present conditions highly correlated with visual comfort if luminance contrast is the independent variable. All four variables are found to be satisfactory with respect to sensitivity, but it is argued that comfort scaling and the measurement of visual performance will probably suffice. Only small differences between subjects are found, suggesting that in the future a standard observer is feasible. The advantage of a print reference is argued.

Text quality metrics for visual display units : II. an experimental survey

This paper discusses several experiments aimed at evaluating the image quality of visual display units (VDUs). A number of quality metrics are presented, i.e. scaled preference, based on numerical ratings and paired comparisons; search velocity in a letter-search task; and two metrics derived from eye movement recordings. The former two metrics are found to be practical. The latter metrics, which are hard to determine, turn out to correspond with search velocity. With varying luminance contrast the scaled preference correlates closely with search velocity. With increasing resolution, however, this is true up to a moderate bandwidth, beyond which search velocity and the eye movement metrics saturate, while scaled preference continues to increase. Small quality differences across various commercially available VDUs are reflected by both search velocity and visual comfort. The results are consistent within subjects. Scaling differences across subjects can be understood by allowing for a subject-dependent weighting of underlying perceptual attributes like sharpness and brightness contrast. It is shown that visual comfort and performance are supplementary metrics that are both useful for modelling VDU image quality.

DifScal : A tool for analyzing difference ratings on an ordinal category scale

An algorithm for analyzing difference scaling results is described. Frequency data on ordered categories that represent perceived differences for a unidimensional psychological attribute are modeled according to Thurstone’s judgment scaling model. The algorithm applies the gradient method for the maximum likelihood estimation of the model parameters. Two ways to calculate the start configuration for the model parameters are elaborated. The algorithm also provides asymptotic values for the standard errors of the estimates and three measures for the goodness of the model fit. An additional feature of DifScal is that it is suited to analyze incomplete data.

ThurCatD : a tool for analyzing ratings on an ordinal category scale

An algorithm for analyzing ordinal scaling results is described. Frequency data on ordinal categories are modeled for unidimensional psychological attributes according to Thurstone’s judgment scaling model. The algorithm applies maximum likelihood estimation of model parameters. The Cramér-Rao bounds of the standard errors of the estimated parameters are calculated, and a stress measure and a goodness-of-fit measure are supplied.

Methods for evaluating the perceptual quality of VDUs

The quality of VDUs, as experienced by operators of workstations, should be assessable on the basis of methods which have found general approval. Therefore, methods of assessment and their mutual consistency are investigated. This paper contains a comparison of results obtained by five different methods, luminance contrast being the independent variable. Fixation duration and saccade length of the eye movements during search are compared with search velocity and scaled reading comfort. Reaction times for word identification are also measured. Luminance contrast is varied over a range of about two decades, using both positive and negative polarity. A considerable varialion in the variables was found and a high correlation between the subjective and objective parameters was observed. Variations in fixation duration and in reaction times, reflecting changes in processing time, are consistent. The relative effectivity and the ease of execution of the methods will be discussed. Search velocity and scaled comfort seem to be suitable methods.