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Chromaticity and luminance requirements for colored symbology in night vision goggles Paul R. Havig Gary L. Martinsen David L. Post George A. Reis Eric L. Heft

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Chromaticity and Luminance Requirements for Colored Symbology in Night Vision Goggles Paul R. Havig*a, Gary L. Martinsena, David L. Posta, George A. Reisb, and Eric L. Heftb a

Air Force Research Laboratory AFRL/HECV, 2255 H St., Wright-Patterson AFB OH, USA 45433 b

Northrop Grumman Information Technology P.O. Box 317258, Dayton OH, USA 45437

Recent advances in display technology have made it possible to superimpose color-coded symbology on the images produced by night vision goggles (NVGs). The resulting color mixture shifts the symbology’s hue and saturation, which can impede recognition of the color code. We are developing luminance-contrast specifications for color-coded NVG symbology to ensure accurate color recognition. Keywords: Luminance contrast ratio, night vision goggle, color coding, color recognition, symbology

1. INTRODUCTION There have been several schemes for superimposing symbology on the images produced by night vision goggles (NVGs)1. Recent advances in display technology have made it possible to superimpose color symbology and take advantage of the potential benefits of color coding2. This arrangement gives rise to a problem that is similar to one we have investigated previously in the context of color helmet-mounted displays (HMDs)3,4: The outside scene mixes with the display image, shifting the symbols’ hue and saturation and impeding accurate recognition of the color code. In a prior study3, we tested mixtures of three symbol colors (red, yellow, and green) and five background colors over a range of luminance contrast ratios. Each trial started with a 25 cd/m2 background and a 25.5 cd/m2 symbol/background color mixture that increased by 0.5 cd/m2 every second. The observers pressed a button when they felt they could identify the symbol’s color. The resulting contrast ratios were very low (the average was 1.15:1) but the error rates for some color combinations were high, suggesting that these ratios would not be adequate in practice. More recently4, we determined the luminance contrast ratios needed to obtain 95%-correct color recognition. We used the same stimuli as in the previous study; however, instead of slowly increasing the contrast ratio, we presented random ratios ranging from 1.025:1 to 1.3:1 in steps of 0.025. This procedure allowed us to fit to a psychophysical function to predict the 95%-correct point. The function chosen was the Weibull5, which is widely used in the vision literature6, 7. In addition, observers rated their confidence in their answers, using “not sure,” “somewhat sure,” and “very sure.” In this study, the resulting contrast ratios were higher than before, averaging 1.18:1 at the 95%-correct level. We also found that the observers’ confidence ratings increased with the contrast ratios. For example, trials rated “very sure” yielded the highest calculated 95%-correct ratios. Further, showing two colors at once, to provide a point of comparison, did not affect color recognition. In the present study, we used a green background to simulate the NVG image. Green, yellow, and red symbol colors were selected using a color-naming procedure we have used in earlier experiments8, 9, 10, 11. Luminance contrast-ratio requirements for the symbol colors were then determined using the technique we used in our prior HMD research3, 4.

*

[email protected]; phone 937 255-3951; fax 937 255-8366; http://www.hec.afrl.af.mil/; Crew System Interface Division, AFRL/HECV, 2255 H St., Wright-Patterson AFB, OH 45433-7022

Helmet- and Head-Mounted Displays VIII: Technologies and Applications, Clarence E. Rash, Colin E. Reese, Editors, Proceedings of SPIE Vol. 5079 (2003) © 2003 SPIE · 0277-786X/03/$15.00


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2. EXPERIMENT 1 2.1 Method The purpose of the first experiment was to determine the best CIE chromaticity coordinates to represent green, yellow, and red. 2.2 Participants Five volunteers participated in Experiment 1, including three of the authors and two others who were naïve as to the purpose of the study. All observers had normal or corrected-to-normal visual acuity and normal color vision, as indicated by the Ishihara pseudo-isochromatic plates and the Farnsworth F2 tritan plate. 2.3 Apparatus and stimuli Observers input their responses via a computer keyboard. All stimuli were presented on a 21-inch EDL CRT monitor, viewed from a distance of 8 feet. Selection of the stimuli’s CIE chromaticity coordinates was a two-step procedure. First, we calculated coordinates for 58 stimuli that covered the monitor’s green-to-red range, as shown in Figure 1 (the black diamond shows the coordinates of the green P43 phosphor used in NVGs). Second, we calculated the coordinates resulting from a 1:1 mixture of each of the 58 stimuli with P43, yielding a 2:1 luminance contrast ratio in each case. We chose this ratio because our previous research has shown that it yields essentially 100%-correct color recognition3. The coordinates resulting from these calculations are shown in Figure 2. All colors were calibrated each day during the experiment using an LMT model 1210 colorimeter and a computer program that automatically measures and adjusts colors to a tolerance of 0.0015 on the CIE 1976 UCS chromaticity diagram and ±0.1% cd/m2 in luminance. The stimulus for the experiment was a representative HMD target designator (TD) box, shown in Figure 3.

CIE 1976 UCS Chromaticity Diagram 0.6

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Figure 1. Stimuli before color mixture calculations

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CIE 1976 UCS Chromaticity Diagram 0.6

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Figure 2. Stimuli before and after color mixture calculations

Target Degrees Before Break Lock

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Identify Friend or Foe

Sizes Target Designator Box 1.4 degrees of visual angle Identify Friend or Foe 14 arcmin of visual angle

Target Range

Shoot Cue 34 arcmin of visual angle Target Altitude Shot Cue

Alphanumerics 19w x 28h arcmin of visual angle

Figure 3. Stimulus configuration used in the experiment

2.4 Experimental Procedure Observers were seated in a dark room and used the numeric keypad on the keyboard to input their responses. An experimental session consisted of 290 trials (58 color stimuli x 5 repetitions) and each observer participated in 2 sessions, giving a total of 580 trials per observer and 2900 trials overall (580 trials x 5 observers). Observers initiated each trial by pressing the “5” key on the keypad. The symbology was presented for 500 ms and the observers then indicated whether it was green, yellow, or red. They gave and rated their responses as follows: For “very sure,” the “7,” “8,” and “9” keys were used to indicate “green,” “yellow,” or “red,” respectively; for “somewhat sure,” the “4,” “5,” and “6” were used; and, for “not sure,” the “1,” “2,” and “3” keys were used.



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2.5 Results We analyzed the proportion of times each color was named “green,” “yellow,” or “red” and the associated confidence ratings. Table 1 shows the results for each row of Figure 2 (row 1 is the top row in the figure). The numbers under each color name are the frequencies with which that name was used for a stimulus. For example, in row 1, stimulus 5 was called “green” 26% of the time and “yellow” 74% of the time. The boxes in each column designate groupings where the specified color name was used more often than the other two (e.g., stimuli 1-4 were called “green” more often than “yellow” or “red”). The Confidence column shows mean confidence ratings, where we assigned a 2 if the observer used “very sure,” a 1 for “somewhat sure,” and a 0 for “not sure.” It can be seen that the confidence ratings add useful information to the color-naming frequencies. For example, stimulus 21 was always called “red” by the observers and had a mean confidence rating of 1.96, so we can conclude the observers were very certain the stimulus was red. Stimulus 12, on the other hand, was called “red” 92% of the time but its mean confidence rating is only 0.32, so “red” was usually their best guess but they were uncertain.

Green 1 1 2 1 3 0.94 4 0.66 5 0.26 6 0 7 0.06 8 0 9 0 10 0.02 11 0 12 0 13 0 14 0 15 0 16 0 17 0 18 0 19 0 20 0 21 0

Row 1 Yellow Red Confidence 0 0 1.92 0 0 1.92 0.06 0 1.2 0.34 0 0.28 0.74 0 0.52 1 0 1.16 0.94 0 1.12 0.98 0.02 1.32 0.92 0.08 0.88 0.42 0.56 0.16 0.26 0.74 0.08 0.08 0.92 0.32 0.04 0.96 0.8 0 1 1.56 0.02 0.98 1.52 0 1 1.8 0 1 1.88 0 1 1.88 0 1 2 0 1 1.96 0 1 1.96

Green 22 0.98 23 0.94 24 0.68 25 0.12 26 0 27 0 28 0 29 0 30 0 31 0 32 0 33 0 34 0 35 0 36 0 37 0 38 0 39 0 40 0

Row 2 Yellow Red Confidence 0.02 0 1.76 0.06 0 0.76 0.32 0 0.28 0.88 0 0.4 1 0 1 1 0 0.88 0.98 0.02 0.96 0.66 0.34 0.32 0.32 0.68 0.24 0.14 0.86 0.2 0.02 0.98 0.56 0 1 0.8 0 1 1.44 0 1 1.72 0 1 1.88 0 1 1.92 0 1 1.96 0 1 1.92 0.02 0.98 1.96

Green 41 1 42 0.98 43 0.8 44 0.22 45 0.06 46 0 47 0 48 0 49 0 50 0 51 0 52 0 53 0 54 0 55 0 56 0.02 57 0 58 0

Row 3 Yellow Red Confidence 0 0 1.88 0.02 0 1.04 0.2 0 0.12 0.78 0 0.36 0.94 0 0.68 0.98 0.02 0.8 0.92 0.08 0.48 0.52 0.48 0.2 0.2 0.8 0.12 0.04 0.96 0.2 0.02 0.98 0.52 0 1 1.12 0 1 1.52 0 1 1.72 0 1 1.84 0 0.98 1.92 0 1 1.96 0 1 1.96

Table 1. Proportions for color naming and mean confidence scores

After studying the proportions and confidence ratings, we chose one color from each row to test the effects of color saturation. The colors in the first row are the most saturated those in the third row are the least saturated. Stimuli 1, 22, and 41 where chosen to represent green, stimuli 8, 28, and 46 represent yellow, and stimuli 21, 40, and 58 represent red.

3. EXPERIMENT 2 3.1 Method The purpose of the second experiment was to determine the luminance contrast ratios needed to ensure reliable recognition of the stimulus colors. Our previous research3, 4 has shown that observers can recognize similar colors at

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quite low contrast ratios (e.g., 1.18:1). However, the P43 green background color used in the present study is much more saturated than the backgrounds we used previously and we thought this might increase the required contrast ratios.

3.2 Participants We used the same five observers from Experiment 1. 3.3 Apparatus and stimuli We presented the TD box in the nine symbol colors from Experiment 1, superimposed on the P43 green background. To test the effect of luminance contrast ratio, we calculated color mixtures for each symbol/background color combination at contrast ratios of 1.025, 1.05, 1.1, 1.2, 1.4, 1.6, 1.8, and 2:1. This range enabled us to fit Weibull functions as we did previously4 and, thus, calculate 95%-correct luminance contrast ratios. The stimuli chosen are shown in Figure 4. 3.4 Experimental Procedure An experimental session consisted of 360 trials (9 color stimuli x 8 luminance contrast ratios x 5 repetitions). Each observer participated in 2 sessions, giving a total of 720 trials per observer and 3600 trials overall (720 trials per observer x 5 observers). Observers initiated each trial by pressing the “5” key on the keypad. The symbology was presented for 500 ms and the observers then indicated whether they thought it was green, yellow, or red and rated their confidence, as in Experiment 1. 3.5 Results A full factorial analysis of variance (ANOVA) was conducted on the percent-correct values, using symbol color, saturation, and luminance contrast ratio as the main effects. The analysis revealed significant main effects of symbol color F(2,288) = 14.71, p