Relative Effects of Raster Scan Lines and Image Subtense on Symbol Legibility on Television

Abstract

This experiment examined the relative effects of (1) image size and (2) number of TV raster lines making up the image upon an observer's ability to identify 16 different geometric symbols on TV. Four raster-line values per symbol height were each tested at three image angular subtenses. Eight subjects were told to identify 25 symbols for each of the 12 conditions; all had 20/12 near and far visual acuity or better. The forced-choice method was used; no limits were placed on response times. The results showed that (1) at least eight raster lines per symbol height and (2) a symbol subtense of 10' of arc are necessary to obtain good symbol legibility on TV. An equation is developed from these and other data which quantifies the tradeoff between line number and angular subtense for different levels of performance.

Psychophysics of reading—I. Normal vision

“…these studies are difficult to evaluate because the angular sizes of letters at the observer's eyes were usually not specified. For recognition of geometric symbols, Hemingway and Erickson (1969) found a trade-off between angular subtense and the number of raster lines per symbol, suggesting the importance of visual angle. We measured the effects of matrix sampling on reading over a wide range of character sizes.…”

Visual Simulation Requirements for Aircraft Aspect Recognition at Real World Distances

“…The data from the present study are encouraging i n t h a t they support the findings of others who have studied related questions of visual requirements of TV presentations of information (cf. Erickson & Hemingway, 1969, Erickson, 1978. Thus, i n our study contrast , 1 umi nance and resol uti on a1 1 provide improved performance i n monotonic and predictable ways.…”

Prediction and Cross-Validation of Video Cartographic Symbol Location Performance

“…This curve is important because it is affected by other parameters of the system as shown in studies by Shurtleff, Marsetta, and , -323 Showman (1966 and Baker and Nicholson (1967). Hemingway and Erickson (1969) conducted a similar study and combined their results with the results of the two previous studies. The curves from this combination show that performance is a function of both visual angle of targets on the display and the number of TV raster lines per symbol height.…”

“…Shurtleff and Owen (1966) used this definition to investigate legibility requirements for alphanumerics and found resolution to influence both accuracy and time required to identify symbols. Resolution requirements for other symbols, such as stars, hexagons, rectangles, and circles, were studied by Hemingway and Erickson (1969). Resolution was also studied by Johnston (1969) in a task requiring pilots to locate and identify targets on a terrain model presented on a closed-circuit TV monitor.…”