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.
This study took a sample of every traceable occupation which occurred in Britain in the six months 1 January to 30 June 1975. These findings were supplemented by a postal survey of 100 of the ‘top 400’ companies, to obtain a more general view of attitudes towards occupations — replies were obtained from about 40 companies.
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