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Color Vision and the Railways: Part 2. Comparison of the CN Lantern Used on the Canadian Railways and Railway LED Lantern Tests

Dain, Stephen J.*; Casolin, Armand; Long, Jennifer

doi: 10.1097/OPX.0000000000000461
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Purpose There are two currently available lantern tests that have their design based solely on the practices of the rail industry. These are the CN Lantern (CNLAN) used on the Canadian Railways and the Railway LED Lantern Tests (RLLT). In the same way that the signaling practices differ in the two jurisdictions, the design of the lanterns also differ. The CIE (Commission Internationale de l’Éclairage)-recommended color vision standards for transport predate both lanterns. The study was undertaken to assess the appropriateness of these lanterns in CIE Color Vision Standard 1. CIE Standard 1 is called “Normal color vision” but some very mild anomalous trichromats may pass the specified “lantern test that presents a high level of difficulty.”

Methods The lantern tests were undertaken by 46 color vision–normal and 37 color vision–deficient subjects.

Results Subjects made more errors on the RLLT, and the pattern of errors is different, partly because there are blank presentations in the RLLT and “no light” is an acceptable response. The two lanterns showed agreement on the pass/fail categorization of 73 of the 83 subjects. The RLLT fails more color vision–normal subjects.

Conclusions Despite the different construction principles, the RLLT and CNLAN have pass/fail levels that are comparable with the Holmes-Wright Type B lantern, which is nominated in CIE Color Vision Standard 1 but is no longer commercially available. The higher failure rate of color vision–normal subjects on the RLLT has been addressed by changing the intensities of the two darkest red lights.

*PhD, FAAO

MBBS, MSciTech(Occ Med)

MSafetyScience, PhD

School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia (SJD, JL); and Sydney and NSW Trains, Sydney, New South Wales, Australia (AC).

Stephen J. Dain Optics and Radiometry Laboratory School of Optometry and Vision Science University of New South Wales Sydney, New South Wales 2052 Australia e-mail: s.dain@unsw.edu.au

This is the second part of a three-part paper describing the Railway LED Lantern Test (RLLT).1 In the first part,1 the historical context of color vision testing on the railways was reviewed, the construction of the RLLT was described in the context of the recommendations2 of the CIE (Commission Internationale de l’Éclairage) regarding a practical test, and performance data were reported for color vision–normal (CVN)1 and color vision–deficient (CVD) subjects.1 In this part, the RLLT,1 carried out at a 6-m test distance, will be compared with the CN Lantern (CNLAN) used on the Canadian Railways, and their appropriateness as lanterns for color vision standard 1 of the CIE was considered. In part 3,3 the RLLT, carried out at a 3-m test distance, will be compared with the Farnsworth Lantern and the OPTEC 900, which are recommended as appropriate lanterns for CIE Standard 2.

There are two currently available lantern tests that have their design based solely on the practice of the rail industry. These are the CNLAN4 and the RLLT.5 Although other lantern tests have been used in the rail industry,6–8 their designs have not reflected railway signaling practice in the same way. The CNLAN is based on Canadian signaling practice, incandescent signals, and a three-aspect display (vertical array), whereas the RLLT is based on New South Wales, Australia, signaling practice; LED signals; and a two-aspect display (vertical pair). Both these lanterns are intended for the assessment of locomotive drivers and represent a lantern “with a high degree of difficulty” in the Standard 1 Normal Color Vision of the CIE recommendations for the transport industry2 where only the Holmes-Wright Type B lantern is currently listed as an example. The Holmes-Wright Type B lantern was designed for maritime and not for railway use.9 Only the Holmes-Wright Type A (Aviation) lantern has been assessed with respect to rail applications and found to be unacceptable.10

CIE Standard 1 is called “Normal color vision” but some very mildly anomalous trichromats may pass the specified “lantern test that presents a high level of difficulty.”

Given the differences in design principles but their common application to the rail industry, this study was undertaken to compare the performance of CNLAN and RLLT.

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METHODS

Subject Selection

Subjects were selected as in part 1 of this paper.1 They comprised 16 (7 male and 9 female) CVN and 37 (34 male and 3 female) CVD subjects who were not RailCorp employees and did not have significant experience with identifying colored signals in commercial work and 30 (26 male and 4 female) CVN RailCorp employees who had significant experience with identifying colored signals.

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Procedure

A general color vision examination (including the Ishihara Test [several 24 plate editions no more than 10 years old] and Neitz anomaloscope for all subjects and the F-M D-15 and Farnsworth Lantern for CVD subjects) was carried out using the routine procedures of the color vision clinic of the School of Optometry and Vision Science, University of New South Wales.

The CNLAN and RLLT tests were interleaved with other lantern tests that are not reported here. Whether they did the CNLAN or the RLLT first was randomly assigned.

The subjects were seated in a normally lit room (≈300 lux) without significant glare (including no visible windows). They viewed the lanterns binocularly at a distance of 4.6 m from the CNLAN and 6 m from the RLLT.

They were instructed as follows:

  • CNLAN
    •  You will be shown three lights, one above the other for 2 seconds. I want you to tell me what colors you see. Tell me the top one first, then the middle then the bottom. The only colors you will be shown are red, yellow and green and these are the only color names that you may use.
  • RLLT
    •  You will be shown a pair of lights, one above the other for 2 seconds. I want you to tell me what colors you see. Tell me the top one first. The only colors you will be shown are red, yellow and green and these are the only color names that you may use. In some cases there is only one light, in which case respond ‘no light’ again in the order of top first and then bottom.

For each lantern, the colors were displayed once only for 2 seconds and the response was recorded. The RLLT used in this part of the study did not have the two modified red intensities referred to in part 1.1

Misnaming a color, failing to see a color, and (for the RLLT) naming a blank as a color were all recorded as errors.

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RESULTS

The distribution of these errors, as a function of color named and color displayed, is shown in Table 1. Fig. 1 contains both CVN and CVD data for both the RLLT and CNLAN and includes all types of error.

TABLE 1

TABLE 1

FIGURE 1

FIGURE 1

In summary, CVN subjects made errors on 2.2% of the colors on the RLLT (a total of 14 color misnamings, 30 missed colors, and 4 blanks named as a color by 46 subjects from a total of 48 stimuli). The colors that were not seen were mainly red, and this has been addressed in part 1 of this paper.1 The color misnamings were mainly naming red as yellow. This is a relatively common observation in practice11 although the reverse may occur with tinted lenses.12

On the RLLT, CVD subjects made errors on 37.2% of the colors (a total of 486 color misnamings, 156 missed colors, and 18 blanks called a color by 37 subjects from a total of 48 stimuli). The distribution of these errors, as a function of color named and color displayed, is also shown in Table 1. The errors of the CVD subjects are more varied compared with the CVN subjects.

The performance of the CVN subjects on the RLLT was used to establish a pass criterion based on the 95th percentile normal results. This was set as less than or equal to two misnamings, less than or equal to one green or yellow missed, and less than or equal to one blank named as a color. Any missed or misnamed reds constituted a failure given the greater safety implications of missing a red signal on the railways.

Fig. 1 also contains both CVN and CVD data for the CNLAN. Color vision–normal subjects made errors on 0.7% of the colors (10 misnamings and 2 missed colors by 46 subjects from a total of 39 stimuli). Color vision–deficient subjects made errors on 28.6% of the colors (407 misnamings and 5 missed colors by 37 subjects from a total of 39 stimuli). In accordance with Hovis and Oliphant,4 a single miscalling of yellow was not considered a fail, but more than one error is a fail. As a consequence, four CVN subjects failed the CNLAN (n = 1 miscalled green as yellow, n = 2 miscalled two yellows, and n = 1 miscalled three yellows).

It should be noted that a blank presentation is not a feature of the CNLAN; thus, the subjects knew that failure to see a light was an error. On the RLLT, there were blank presentations and, as a consequence, reporting a blank was preestablished as a legitimate response.

Fig. 2 shows the relationship between misnamings on the CNLAN and RLLT. The correlation for the CVD subjects and CVN subjects combined was 0.84 (n = 85, p < 0.01), and that for the CVD subjects alone was 0.57 (n = 37, p < 0.01). There was no significant correlation between the errors for the CVN subjects alone.

FIGURE 2

FIGURE 2

Given the pass/fail criteria for the two lanterns, the comparison of pass/fail performance was derived and is shown in Table 2. The data in Table 2 were analyzed using the Cohen kappa coefficient, κ,13 which is a measure of interrater agreement. The data, which include the six CVN subjects who failed the RLLT but who passed the CNLAN, give κ = 0.81. Values of κ that are greater than or equal to 0.81 are rated as “almost perfect agreement.”14 The predominant error was missing the two lowest luminance reds and the intensities of these lights were later modified, as described in part 1 of this study.1 If the values in Table 2 are amended, given the likely consequence of the modified intensities, and an assumption is made that, perhaps, only one CVN subject will fail the RLLT but pass the CNLAN, then κ = 0.95.

TABLE 2

TABLE 2

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DISCUSSION

Subjects generally made more errors on the RLLT. The patterns of errors for the RLLT and CNLAN (Table 1) are rather different, mostly because the RLLT contains blank signals and reporting “no light” is an acceptable response. This was done partly because, in practice, there are not always two signals showing and partly to minimize the ability of protans to interpret red by its lowered luminous intensity. However, there was no evidence in the results that protans reported a blank more often than other subjects. If the data reporting blanks is removed, then there is a similarity in the misnaming of colors by CVD subjects in Table 1; that is, the most common error for both lanterns is the misnaming of yellow.

The reporting of blanks for red lights by CVN subjects was associated with the two lowest intensity red lights. Color vision–deficient subjects also frequently missed these reds but always made other errors, which, on their own, would have resulted in them failing the RLLT. A subsequent study was carried out on CVN subjects with the two lowest intensities doubled and 1 subject, out of 106, reported a blank for one red light and no subjects misnamed any colors.1 As a consequence, the production of RLLT includes the two modified intensities. Some or all of the six CVN subjects in this part of the study who failed the RLLT as a consequence of failing to see reds, but passed the CNLAN, might be expected to pass the modified RLLT (as manufactured). Missing a signal does occur for reasons other than color vision, such as attention and concentration. The subjects carried out a number of tests over an extended experiment, such that boredom and concentration lapses might well have intervened.13,14

In the validation study of the CNLAN,4 4% of CVN subjects failed and 3% of CVD subjects passed the CNLAN. In this study, the figures are 2% (1 in 46) and 5% (2 in 37), respectively. Given that the CVD population is unlikely to be exactly the same in both studies especially given that recruitment methods differ, these results are fully comparable. Given this “almost perfect agreement” of the CNLAN and RLLT shown by the Cohen κ, the two lanterns may be seen as being of essentially the same difficulty, despite their different design, signaling practice basis, and pass/fail criteria. Because they are both based on railway signaling practice, it seems reasonable to conclude that, again despite their differences, the difficulty in interpreting Canadian and New South Wales railway signals is also very similar.

If the results of this and the previous study4 are combined, the failure rate of CVD subjects is comparable with the reported fail rate of 98% of the Holmes-Wright Type B lantern.2 They are also more stringent than the Holmes-Wright Type A lantern (77% failure rate), Beyne Lantern (60%), OPTEC 900 (70%), and Farnsworth Lantern (66%), as summarized by the CIE.2 The Holmes-Wright Type A lantern has been identified as being insufficiently stringent for use on the railways.4

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CONCLUSIONS

Despite differences in the signaling practice on which they are based, the RLLT and the CNLAN require very similar levels of color vision to pass. The Holmes-Wright Type B lantern has been identified as “a lantern test that presents a high level of difficulty” and is considered appropriate as a test used to assess applicants for CIE Color Vision Standard 1.2 The proportion of CVD subjects failing the Holmes-Wright Type B lantern (98%) is essentially the same as the RLLT and CNLAN; thus, it is also reasonable to consider the RLLT and CNLAN as appropriate for applicants for CIE Color Vision Standard 1. Unlike the Holmes-Wright Type B lantern, the RLLT and CNLAN are currently commercially available.

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ACKNOWLEDGMENTS

This study was supported, in part, by a grant from RailCorp NSW (now Sydney Trains). The assistance of Clair Taylor in data collection in experiment 1 is appreciated.

The CNLAN is available by contacting Dr. JK Hovis at the School of Optometry and Vision Science, University of Waterloo, Canada. The RLLT is available from ART Electronics (www.rllt.com.au). Sydney Trains own the rights to the RLLT.

Received January 31, 2014; accepted August 19, 2014.

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REFERENCES

1. Dain SJ, Casolin A, Long J. Color vision and the railways: Part 1. The Railway LED Lantern Test. Optom Vis Sci 2015; 92: 138–46.
2. Commission Internationale de l’Éclairage. International Recommendations for Colour Vision Requirements for Transport. Vienna, Austria: Commission Internationale de l’Éclairage; 2001.
3. Dain SJ, Casolin A, Long J. Color vision and the railways: Part 3. Comparison of FaLant, OPTEC 900 and Railway LED lantern tests. Optom Vis Sci 2015; 92: 152–6.
4. Hovis JK, Oliphant D. A lantern color vision test for the rail industry. Am J Ind Med 2000; 38: 681–96.
5. Casolin A, Katalinic PL, Yuen GS, Dain SJ. The RailCorp Lantern test. Occup Med (Lond) 2011; 61: 171–7.
6. Williams CH. Standards of form and color-vision required in railway service. Trans Am Ophthalmol Soc 1897; 8: 227–41.
7. Williams CH. An improved lantern for testing color-perception. Trans Am Ophthalmol Soc 1900; 9: 192–6.
8. Edridge-Green FW. Two new tests for colour blindness. Brit Med J 1890; 1: 72.
9. Holmes JG, Wright WD. A new colour-perception lantern. Color Res Appl 1982; 7: 82–8.
10. Hovis JK, Oliphant D. Validity of the Holmes-Wright lantern as a color vision test for the rail industry. Vision Res 1998; 38: 3487–91.
11. Wood JM, Atchison DA, Chaparro A. When red lights look yellow. Invest Ophthalmol Vis Sci 2005; 46: 4348–52.
12. Hovis JK. When yellow lights look red: tinted sunglasses on the railroads. Optom Vis Sci 2011; 88: 327–33.
13. Carletta J. Assessing agreement on classification tasks: the Kappa statistic. Comput Linguist 1996; 22: 249–54.
14. Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics 1977; 33: 159–74.
Keywords:

accident prevention; visual function; vision standard; medical standard; color vision; color-vision loss; color vision standards; railroad worker; transport; occupational risk; safety standard; lantern tests; practical test

© 2015 American Academy of Optometry