78 



DESIGN AND USE OF INSTRUMENTS 



which is bright and is protected by an in- 

 strument panel in white on black design 

 (49). There has, however, been no syste- 

 matic study to discover whether this con- 

 trast arrangement results in losses of in- 

 strument reading speed or accuracy when 

 compared ^^th instruments in black on white 

 design. The one report which does refer 

 directly to this contrast problem in relation 

 to instrument reading per se provides no 

 detailed data. It merely offers a summary 

 statement that at high or dayhght levels 

 of illumination white on black dials demand 

 a 25% longer reading time than black on 

 white (89). This observation needs check- 

 ing for both high and low illumination levels. 

 It was showTi in Chapter 1 that visual dis- 

 crimination improves as the degree of con- 

 trast between marking and background in- 

 creases. Apparently the only exception to 

 this generahzation in practice is met when 

 markings of low brightness are seen against 

 a background of essentially "zero" bright- 

 ness. Such a situation is met in the use of 

 black dials vnih self-luminous or fluorescent 

 markings. Operators consistently remark 

 on the difficulty they have in accommodating 

 on these markings which are described as 

 scintillating and jumping. This difficulty is 

 not experienced when visible red is used for 

 instrument illumination, for under such fight 

 the dial background is visible. Thus, in 

 order to facilitate accommodation, it is de- 

 sirable to have less than maximum contrast 

 in instrument displays when used under low 

 illumination. 



Thickness and Minimum Length of 

 Scale Graduation Marks 



Choice of the thickness for scale gradua- 

 tion marks, when the space between marks 

 is fixed, involves a compromise between the 

 demands for mark legibility and the demands 

 of precision of interpolation between marks. 

 Thus, although bold markings must be used 

 on instruments intended for use in low il- 

 lumination, stroke boldness in itself may 

 lead to interpolation errors and lack of read- 



ing precision in the neighborhood of the 

 mark. To date, investigations of gradua- 

 tion-mark dimensions have considered scale 

 reading at high illumination levels only. 



Maier (58), concerned Avith the specific 

 non-interpolation problem of reading a stop- 

 watch to the nearest fifth-second mark, con- 

 ducted an experiment to determine what 

 the optimum mark thickness should be. In 

 eight different scales, mark thickness was 

 varied from 14% to 53% of the scale range 

 occupied by each fifth of a second. Maier's 

 subjects, all of whom had had some training 

 in instrument use, read the different scales 

 in projected tachistoscopic exposures of less 

 than 0.4 seconds. Errors were fewest for 

 the scales on which the graduation marks 

 occupied about 25% of the graduation in- 

 terval. Appreciating the possible limita- 

 tions of his tachistoscopic technique, how- 

 ever, Maier cautions that this result might 

 not be confirmed for other exposure periods. 



The length of scale-graduation marks was 

 considered by Schulz (69). In a not too 

 rigorous, apparently casual way, he recorded 

 reading times for a variety of scaled instru- 

 ments. When the data were analyzed, 

 Schulz concluded that reading was best when 

 the ratio of length of the shortest scale 

 marks to mark separation was in the order 

 of 1:1 or 2:1. Larger ratios, some as high 

 as 6: 1, were considered less favorable to good 

 reading. Unfortunately, Schulz presents no 

 details of his procedure, no information 

 about his subject population, and no specific 

 performance data to support his generaliza- 

 tion. An experiment in this area still needs 

 doing. 



Instrument Numbering 



Numeral Contours and Stroke Thickness. 

 Research on the subject of the design of 

 numerals for use in printed materials has 

 been reviewed in Chapter 2. The studies 

 there referred to. Tinker (75, 76) and Dun- 

 lap (21), and other papers by His Majesty's 

 Stationery Office (94) and by Lauer (46), 

 drew attention to the importance for numeral 



