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1、GUIRE 4 ，Principal features of the human eye in cross section.Cornea Light passes through the pupil is refracted by thelens, and is brought to a focus on the retina. The ret-A ina receives the light stimulus and transmits anOptic nerve Aqueous humor ”impulse to the brain through the optic nerve. FIG
2、URE 4-2Illustrations of various types of targets used in visual acuity tests and experiments. The featuresto be differentiated in targets a, b, c, and d are all the same size and would, therefore, subtendthe same visual angle at the eye. With target a the subject is to identify each letter; with c,
3、e，and f the subject is to identify the orientation (such as vertical or horizontal); and with b the sub-jectis to identify any of four orientations. With target d the subject is to identify onecheckerboard target from three others with smaller squares，ECum滋inl(aa) Letter target (b) Landholt ring (c)
4、 Parallel bars (d) Checkerboard (e) Acuity 们 Acuitygrating gratingPosition of eyeRah一0一|攻汪HFIGURE 4-3Illustration of the concept of visual angle; H =height of visual stimulus, and D = distance fromthe eye. In this illustration, the visual angle of theSpecific elements of E could be derived (the thic
5、k-ness of the elements would be the H value)，亿) Square wave grating (6) Sine wave grating :三IntensityIntensityPosition PositionFIGURE 4-4(al A square wave grating and its intensity distribution. (b) A sine wave grating and itsintensity distribution. The abrupt changes in intensity of the square wave
6、 grating areseen as sharp contours, whereas the more gradual changes in intensity of the sineWave grating are seen as fuzzy contours.FlGURE 4-5Contrast sensitivity curve using stationary gratings as the targetsThe region between the two dashed lines encompasses contrastsensilivity limits for 75 perc
7、ent of the sample, and the area be-tween the outer solid ines encompasses 90 percent of theSample. (Source: Adapted from Ginsburg, Evans, Cannon,Owsley ana Muvanny, 1984.) 300| 。 95th percentile |200上上 二1o0上 则E 生 了千和 so上FS 二和 55 半5th percentile1o上| 二 吕 1 1 10 3.0 100 30.0Spatial frequency(cycles per
8、 degree)FIGURE 4-6Dimensions used to compute stroke width-to-height and width-to-height ratios. Ratios can also be expressed as a proportion,e.g., 1:10 = 0.10. The letter shown has a stroke width-to-height ratio of 1:5 (0.20) and a width-to-height ratio of 3:5(0.60). StrokeWi Black on white White on
9、 blackheight ratio人ee人A已C 4556ABC456和已CC456和已CC456-ABC 456 ENFIGURE 4-7lustrations of stroke width-to-height ratios of letters and numerals. With reasonably good 让lumination, the following ratios are satisfactory for printed material: black on white, 1:6 to 1:8;and white on black, 1:8 to 1:10 FIGURE
10、 4-8Letter and numeral font of United States Military Specification no. MIL-M-18012B (July 20，1964); also referred to as NAMEL (Navy Aeronautical Medical Equipment Laboratory) or AMELThe letters as shown have a width-height ratio of 1:1 (except for |, J, L, and W). The numeralshave a width-height ra
11、tio of 3:5 (except 1 and 4)TABLE 4-1 EXAMPLE OF GOTHIC STYLESVery light stylesFufura light ABCDEFGHUKLMNOP “1234567890Sans senf lsht ABCDEFGHUKLM 1234567890Vogue light ABCDEFGHIJKLMNOPQR ”1234567890Light stylesFutura book ABCDEFGHUKLMNOPQRST “1234567890Sans serif mediumTempo mediumABCDEFGHUKLYMNOPQ
12、”1234567890ABCDEFGHIJKLMNOPQ ”1234567890 Medium styles Fulura mediumSans serif boldABCDEFGHIHKLMNOP ”1234567890ABCDEFGHI ”1234567890 TABLE 4-2ONE SET OF RECOMMENDED HEIGHTS OF ALPHANUMERIC CHARACTERS FORCRITICAL AND NONCRITICAL USES UNDER LOW AND HIGH ILLUMINATION AT 28 INVIEWING DISTANCE Height of
13、numerals and letters*Low luminance High luminance (down to 0.03 人L) (1.0 人Land above)Critical use, position variable 0.20-0.30 in 0.12-0.20 in(5.1-7.6 mm) (3.0-5.1 mm)Critical use, position fixed 0.15-0.30 in 0.10-0.20 in(3.8-7.5 mm) (2.5-5.1 mm)Noncritical use 0.05-0.20 0.05-0.20(1.27-5.1 mm) (1.27-5.1 mm) * For other viewing distances (Dj, in inches, multiply tabled values by D/28.Source: Adapted from Heglin (1973) and Woodson (1963)
