82 



DESIGN AND USE OF INSTRUMENTS 



and labeling be cut to a minimum. The 

 first is the rule of keeping markings at a 

 minimum in order to preserve operator dark 

 adaptation when this is important. The 

 second is the rule evolved by Loucks (50, 52) 

 that instrument faces should be kept as 

 "clean" as possible. Neither discussion nor 

 research on this labeling problem has been 

 extensive, however. At the discussion level 

 there is Craik's suggestion (19) that instru- 

 ment titles be abbreviated to two or three 

 letters. At the experimental level there is 

 Loucks' finding (52) that when the words 

 "Up" and "DowTi" are ehminated from the 

 climb and dive indicator, accuracy of read- 

 ing is actually improved. 



Unfortunately, even when instruments are 

 thought to be well labeled, misreading one 

 instrument for another is very easy on a 

 busy panel w4th many indicators. Confu- 

 sions of this sort accounted for 7% of all the 

 serious "pilot errors" reported in a recent 

 survey by Fitts and Jones (27). As Fitts 

 and Jones point out, there is need for the 

 development of a practical system which 

 ^ill insure the easy and positive identifica- 

 tion of each type of instrument. 



From the foregoing discussion it is ap- 

 parent that currently available research in- 

 formation on instrument marking is hmited 

 and that in too many cases the data reported 

 are for very few subjects. Of particular 

 interest for the immediate future will be an 

 extension of the studies of the interactions 

 between the variables of contrast direction, 

 illumination level and stroke thickness for 

 markings. Research papers which approach 

 aspects of this problem and which should 

 be consulted in the planning of future work 

 include those of Berger (6), By ram (13), 

 Ferree and Rand (22, 23), Fry and Cobb 

 (28), Holmes (39), Taylor, (74), Walls (81), 

 Weston (84), and Wilcox (85, 86). 



Instrument Illumination 

 Color and Method of Illumination 

 In Chapter 1 evidence was presented to 

 show that in work situations where opera- 

 tors must remain well dark adapted it is 



desirable to use red lighting. Ultra-violet 

 fighting of fluorescent pigments does not 

 disturb dark adaptation if kept at a minimum 

 level for instrument reading (10), but since 

 red fluorescing pigments are not available, 

 an ultra-violet system is less safe than a 

 red lighting system when the illuminating 

 intensity happens to be set too high. 



Spragg and Rock (73) in a recent study 

 have shown that dial reading speed and 

 accuracy are unaffected when filters are used 

 over the light sources to cut out all but the 

 red end of the spectrum. They compared 

 the performance of subjects reading sets of 

 twelve dials seen under light passed by each 

 of four sharp cut-off filters: wavelengths 440 

 mju and longer, 550 m/x and longer, 620 m^u 

 and longer, and 640 m/x and longer. Ob- 

 servations were made at 0.1 and 0.01 foot 

 lamberts. Further data seem to be needed 

 to clarify the present records for reading 

 under the 640 m/i filter, but up to 620 m^i 

 there was no loss in reading speed or ac- 

 curacy at either brightness level. 



Of the several methods of instrument il- 

 lumination which might be used, flood light- 

 ing and edge or rim lighting are the two 

 deserving of most consideration. In gen- 

 eral, flood lighting is preferred because it 

 provides more even illumination. No speci- 

 fic tests have been reported, however, on 

 visual performance under these two forms 

 of lighting. 



Intensity of Illumination 



When really precise instrument scale read- 

 ing is needed, high illumination intensities 

 should be used. Craik (19) and Bartlett 

 (4, 5) both point to the need for brightnesses 

 above 10 foot lamberts on the basis of gen- 

 eral visual data of the sort reviewed in 

 Chapters 1 and 2. Crouch (20) reviews 

 data on scale reading speed which are to the 

 same effect. These are data of Ferree and 

 Rand, presumably not published elsewhere. 

 They show that the speed of reading a steel 

 vernier rule with black markings increases 

 through the brightness range of 20 to 200 

 foot lamberts. Correspondingly, Grether 



