Color Vision in the Mountains.

This Lessons from History article uses science, aviation, medicine, and mountaineering sources to describe some of the effects of hypoxia, illumination, and other environmental conditions on the eye, the central nervous system, and light and color perception. The historical perspective is augmented by an analysis of an informal observation of the altered perception of red color on a deck of playing cards while climbing Mera Peak in the Himalaya. The appearance of a grayer red color on the cards was initially attributed to the effects of hypoxia alone. Instead, analysis of cards in combination with the low incidence of protan color vision defects at altitude indicated that glare and contrast effects in the extremely bright lighting environment combined with hypoxia likely caused the perception of a grayer red. The incident provides an educational opportunity for review, analysis, and commentary about some of the complex elements that impact color vision.

Clinical analysis of the Konan-Waggoner D15 color vision test using the Surface-Pro display.

This work expands on our previous comparison of the Konan-Waggoner D15 (KW-D15) and Farnsworth D15 (F-D15). Sixty subjects with normal color vision and 68 subjects with a red-green color vision defect participated in the study. The KW-D15 had good agreement with the F-D15 for both pass/fail and classification across all failure criteria. The agreement was slightly better if subjects had to pass on 2/3 trials compared with just the first trial. The KW-D15 is an adequate substitute for the F-D15, with the caveat that the KW-D15 might be slightly easier to pass than the F-D15 for deutans.

Recommendations and requirements for the wavelengths in Rayleigh equation anomaloscopes.

DIN 6160:2019 is a technical standard that sets requirements for Rayleigh equation anomaloscopes. Table 1 of the standard contains the limits for centroid wavelengths and spectral half power bandwidths (SHBW). The centroid limits are more restrictive than dominant wavelength recommendations. The SHBW limits have no known evidence base and are inconsistent between colors. The spectral characteristics of three commercial anomaloscopes brands were measured using a telespectroradiometer. Only the oculus instruments complied with DIN 6160 Table 1, but all the anomaloscopes complied with published recommendations. All complied with the DIN 6160 bandwidth requirements. This highlights the need to provide an evidence base for such requirements.

Evaluation of the Third edition of the City University Colour Vision Test.

INTRODUCTION: The 3rd edition of the City University Colour Vision Test (CUT) was originally based on the Farnsworth-Munsell D-15 test (D15). The first part of the test is for detecting a defect, and the second part is used to diagnose the type and severity of the defect. This study evaluates the CUT 3rd edition relative to the Ishihara and the D15 colour vision tests. METHODS: Fifty nine colour vision normal subjects and 60 subjects with a congenital red-green colour vision defect were recruited. Subjects were tested with the Ishihara and CUT tests. Subjects who failed the Ishihara also performed the D15 test. RESULTS: The agreement between the Ishihara and CUT screening plates was marginally higher when using the CUT failure criterion of >1 error compared with using >2 errors. If the diagnostic plates were included with the screening plates in determining the pass/fail outcomes, the agreement between the Ishihara and CUT was high, with a first-order agreement coefficient (AC1) of 0.90. The AC1 coefficient agreement between the D15 and CUT diagnostic plates in terms of pass/fail was 0.81 when using the D15 failure criteria of >1 or >0 crossing. CONCLUSION: The level of agreement between the 3rd edition of the CUT and D15 was lower than the 2nd edition of the CUT. The primary reason for the lower agreement of the 3rd edition of the CUT was that it had a lower specificity relative to the D15 compared to the 2nd edition. Although the CUT predictive value for failing the D15 is over 90%, the predictive value for passing shows that 19%-25% of patients who pass the 3rd edition of the CUT test will fail the D15. The 3rd edition tends to misclassify protans as deutans or cannot classify the type of defect relative to the D15 and Ishihara.

Visual processing and BDNF levels in first-episode schizophrenia.

Previous studies have shown that patients diagnosed with schizophrenia (SCZ) have deficits in early visual processing, namely contrast processing. The brain-derived neurotropic factor (BDNF) is an important measure to investigate neuroplasticity in some visual functions like visual perception. In this study, we investigated the relationship between visual processing and BDNF levels in first-episode SCZ patients. Thirty-nine healthy controls and 43 first-episode SCZ patients were enrolled. Contrast sensitivity measurements were conducted using low, mid- and high spatial frequencies. First-episode SCZ patients had higher contrast sensitivity than healthy controls for all frequencies, except for the middle spatial frequency. Negative correlations were found between BDNF, contrast sensitivity and clinical variables, mostly for middle and high spatial frequencies among females. Our results provide support for (i) the association of SCZ with alterations of magno- and parvocellular pathway functioning and (ii) decreased BDNF levels in first-episode SCZ patients. This study highlights the importance of using biomarkers along with other measures to investigate visual processing in SCZ and other disorders.

Pupil light reflex in Parkinson's disease patients with and without freezing of gait symptoms.

PURPOSE: Freezing of gait (FOG) is considered as a motor disorder that affects some Parkinson's disease (PD) patients; however, sensory systems may also be involved in FOG. The pupil light reflex (PLR) is a reliable measure of the autonomic nervous system. Different dilation and constriction pupil parameters may be used to investigate the integrity of the autonomic nervous system in PD patients with and without FOG symptoms. This study aimed to look at the integrity of autonomic nervous system and to investigate the nonmotor functions mediated by the cholinergic system in Parkinson's patients with and without FOG symptoms. METHODS: Constriction and dilation pupil light reflexes were measured by using a handheld pupillometer. Twenty-two patients with FOG symptoms, 25 patients without FOG symptoms, and 25 aged-matched healthy controls participated in this study. RESULTS: The results showed that most of the constriction parameters and dilation latency of both patient groups differed significantly from healthy controls. FOG patients showed larger pupil size under light condition and larger deficits in constriction latency than nonFOG patients. Both the groups of PD patients had longer dilation latencies than healthy controls. CONCLUSION: This study suggests that the cholinergic autonomic nervous system is affected in PD patients more than the adrenergic system. FOG patients had larger impairments in nondopaminergic mediated functions such as pupil light reflexes, which suggests that FOG patients have greater impairment in functions that involve cholinergic neurotransmitters.

A Survey on Daily Activity Inclination and Health Complaints among Urban Youth in Malaysia.

Environmental influence is one of the attributing factors for health status. Chronic interaction with electronic display technology and lack of outdoor activities might lead to health issues. Given the concerns about the digital impact on lifestyle and health challenges, we aimed to investigate the daily activity inclination and health complaints among the Malaysian youth. A self-administered questionnaire covering lifestyle and health challenges was completed by 220 youths aged between 16 and 25. There were a total of 22 questions. Seven questions inspected the patterns of indoor and outdoor activities. Fifteen questions focused on the visual and musculoskeletal symptoms linked to both mental and physical health. The total time spent indoors (15.0 ± 5.4 hours/day) was significantly higher than that spent outdoors (2.5 ± 2.6 hours/day) (t = 39.01, p < 0.05). Total time engrossed in sedentary activities (13.0 ± 4.5 hours/day) was significantly higher than that in nonsedentary activities (4.5 ± 3.8 hours/day) comprised of indoor sports and any outdoor engagements (t = 27.10, p < 0.05). The total time spent on electronic related activities (9.5 ± 3.7 hours/day) was were higher than time spent on printed materials (3.4 ± 1.6 hours/day) (t = 26.01, p < 0.05). The association of sedentary activities was positive in relation to tired eyes (χ 2 = 17.58, p < 0.05), sensitivity to bright light (χ 2 = 12.10, p < 0.05), and neck pain (χ 2 = 17.27, p < 0.05) but negative in relation to lower back pain (χ 2 = 8.81, p < 0.05). Our youth spent more time in building and engaged in sedentary activities, predominantly electronic usage. The health-related symptoms, both visual and musculoskeletal symptoms, displayed a positive association with a sedentary lifestyle and a negative association with in-building time.

Effects of Nicotine Gum Administration on Vision (ENIGMA-Vis): Study Protocol of a Double-Blind, Randomized, and Controlled Clinical Trial.

Studies reported that tobacco addiction was related to visual impairments, but one unresolved issue is whether the impairments are related to the many compounds existing in the cigarettes or to the effects of nicotine. On the other hand, nicotine gum can be used as replacement therapy or as a neuroprotective agent for some diseases. The main purpose of this controlled trial is to investigate the effects of nicotine gum on vision. The ENIGMA-Vis trial aims to compare two dosages of nicotine gum (2 and 4 mg) and a placebo gum in a randomized, double-blind, placebo-controlled trial of 100 participants to be allocated into a single group assignment of repeated measures (two studies; N = 50 for each one). Eligibility criteria are healthy non-smokers not diagnosed with substance abuse and without an acute or chronic medical condition. Intervention will last three sessions for each participant with a window frame of 1 week per session. Study outcomes are (1) short-term effects of nicotine gum on contrast sensitivity; (2) short-term effects of nicotine gum on chromatic contrast discrimination; and (3) whether demographics, body mass index, or serum cotinine predicts response of visual processing. This study addresses an important gap in the effects of nicotine on vision. One of the main takeaways of this study is to understand the effects of nicotine on contrast sensitivity and chromatic contrast discrimination. This information will provide a further understanding of how nicotine interacts with early visual processes and help determine how the different components present during smoking can affect vision. Clinical Trial Registration Number: RBR-46tjy3.

Predicting the Farnsworth-Munsell D15 and Holmes-Wright-A lantern outcomes with computer-based color vision tests.

This study determined the AC1 agreement values between computer-based color vision tests and the Farnworth-Munsell D-15 (F-D15) and the Holmes-Wright Type A lantern (HWA). The computer-based tests were the United States Air Force Cone Contrast Test (OCCT), Cambridge Color Test, Innova Rabin Cone Contrast, Konan-Waggoner D15 (KWC-D15), and Color Assessment and Diagnosis (CAD). Sixty-eight color-vision-defective persons participated. The KWC-D15 had the highest AC1 with the F-D15 (${\rm AC1} = {0.88}$AC1=0.88). Both the CAD and OCCT had the highest values with the HWA (${\rm AC1} \gt {0.96}$AC1>0.96). The KWC-D15 would be the best substitute for the F-D15. Either the CAD or OCCT would be appropriate substitutes for the HWA.

Lighting for color vision examination in the era of LEDs: the FM100Hue Test.

Lighting conditions nominated for color vision testing are many and varied. The recommendation of CIE color rendering index (CIE CRI) ≥90 and correlated color temperature of close to 6500 K is widely made for color vision testing generally. With the demise of incandescent and fluorescent lighting and their replacement by light-emitting diodes (LEDs), this is an opportune time to revisit the recommendation. In this paper, we consider the current sources, acceptable and unacceptable, and improvements to the recommendation as it applies to the Farnsworth-Munsell 100 Hue Test (FM100Hue Test). We conclude that there is no need to treat LEDs as a special case but propose a modified CRI measure.

Predicting the CN Lantern Test for Railways with Clinical Color-vision Tests.

SIGNIFICANCE: This research will help clinicians in advising their color-vision-defective patients regarding their career options. PURPOSE: In Canadian railways, individuals with a color-vision-defect (CVD) may qualify for positions at shorter sighting distance from signal lights. The railway companies' medical units use the CN Lantern (CNLan) test, and there is little information available as to whether clinical color-vision tests (CCVTs) can predict the CNLan results. This study determines the ability of some CCVTs to predict the CNLan performance to assist clinicians in advising their CVD patients regarding career options. METHODS: The CNLan viewing distance was varied between 4.6 and 0.57 m using a geometric progression. The CCVTs were the Hardy, Rand, and Rittler; Ishihara; ColorDx pseudoisochromatic plate (PIP); the Rabin Cone Contrast Test; Color Assessment and Diagnosis; Cambridge Color Vision Test; U.S. Air Force Operational Based Vision Assessment Cone Contrast Test; Farnsworth Munsell D15; and ColorDx D15. Fifty-six normal-color-vision and 63 CVD subjects participated in this study. RESULTS: Failure of either the Farnsworth Munsell D15 or ColorDx D15 essentially guarantees failure on the CNLan at the 4.6-m distance. The agreement values decreased as the viewing distance decreased. CONCLUSIONS: To counsel patients regarding a career as a locomotive engineer, clinicians should have either the Hardy, Rand, and Rittler or ColorDx PIP and a D15 test. For patients applying for a position in the yard, a mild-to-moderate classification CVD on HRR or ColorDx PIP indicates a high probability of passing CNLan.

The CN Lantern Test and Different Viewing Distances.

SIGNIFICANCE: This research shows that some color-vision-defective patients could identify railway signal lights correctly if they are working in the yard where sighting distances for signal lights are shorter. PURPOSE: When interpreting railway signal lights, sighting distance can vary depending on the employee's location and job requirements. Individuals with a color-vision-defect may pass railroad employment color vision testing for positions with shorter sighting distances, despite failing to qualify for positions with longer sighting distances. The CN Lantern (CNLan) simulates railway signal lights. We evaluated performance and repeatability on CNLan at different viewing distances in color-normal and color-deficient individuals. METHODS: Fifty-six subjects with normal color vision and 63 subjects with a red-green color-vision-defect participated. The CNLan test was performed at 4.6-, 2.3-, 1.15-, and 0.57-m viewing distance. The test was repeated after 10 days. RESULTS: All individuals with normal color vision passed the CNLan at all distances at both visits without errors. For the group with a color-vision-defect, the pass rate increased from 12% at 4.6 m to 62% at 0.57 m. The repeatability of the CNLan between visits for the color-vision-defective group was very good with AC1 agreement values greater than 0.85. CONCLUSIONS: An increase in retinal illumination was likely responsible for the improved performance as the test distance was decreased. Typical sighting distances in railway yards correspond to the 0.57-m test distance in our study. The results of this study suggest that 62% of the individuals with a red-green color-vision-defect may correctly identify colored signal lights in a railway yard where sighting distances are less than 100 m.

Visual processing speed in freezing and non-freezing Parkinson's disease patients.

INTRODUCTION: Parkinson's disease patients are usually characterized by body motor dysfunction due to dopaminergic reduction in the central nervous system. Freezing of gait is a motor disorder that affects certain Parkinson's disease patients. However, it is hypothesized that non-motor functions mediated by the cholinergic system are also involved in developing freezing of gait. Visual information processing speed, or inspection time is independent of the motor response, and can be used a reliable measure of the cholinergic system integrity. OBJECTIVE: Inspection time can be used to investigate whether Parkinson's disease patients with freezing of gait symptoms have a larger impairment in cholinergic mediated functions than those patients who have no freezing of gait symptoms and healthy controls. METHODS: The inspection time was determined by a simple length discrimination task. Twenty-two Parkinson's disease patients with freezing of gait, 25 Parkinson's disease patients without freezing of gait, and 25 aged matched healthy controls participated in the study. RESULTS: Based on the log values of IT score, Parkinson's disease patients with freezing of gait symptoms had statistically significant slower inspection times (mean of 1.793 ms) than Parkinson's disease patients without freezing of gait (mean of 1.655 ms) and healthy controls (mean of 1.523 ms). Inspection times for the Parkinson's disease patients without FOG symptoms were also significantly slower than healthy controls. CONCLUSION: The results of this study support the hypothesis that the cholinergic system integrity is affected more in Parkinson's disease patients with freezing of gait symptoms.

Stereopsis and ocular alignment in Parkinson's disease patients with and without freezing of gait symptoms.

BACKGROUND: Parkinson's disease patients are classically described by having motor disorder symptoms. Freezing of gait is one of these motor symptoms that presents in some of these patients. Even though freezing of gait is classically considered as motor dysfunction, it is now widely accepted that deficits in other sensory systems, for example visual system, may lead or contribute to freezing of gait. The purpose of this study is to characterise some of the binocular vision functions in freezing of gait patients, non-freezing of gait Parkinson's disease patients and age-matched healthy controls. METHODS: Binocular vision measurements included local and global stereopsis using different clinical stereo tests, along with fixation disparity and fixation disparity curves. The fixation disparity measures were primarily an assessment of the vergence motor system, and the clinical stereo tests assess the combined effects of motor and sensory aspects of binocular vision. Twenty-two freezing of gait patients, 25 non-freezing of gait patients, and 25 aged-matched healthy controls completed all of the measurements in this study. RESULTS: The freezing of gait group had worse stereopsis than the non-freezing of gait group, and the non-freezing of gait group had worse stereopsis than the healthy controls. The impairment of global stereopsis was more common than local stereopsis in Parkinson's disease patient groups. The reduction in stereopsis among Parkinson's disease patients was not associated with fixation disparity. CONCLUSIONS: Results from this study clearly indicate that the freezing of gait patients group has a greater loss in stereopsis than the other two groups, especially for the global tests. Whether these impairments are contributing to the freezing of gait or just associated with freezing of gait is uncertain.

Limitations and Precautions in the Use of the Farnsworth-Munsell Dichotomous D-15 Test.

SIGNIFICANCE: Clinicians who administer the Farnsworth-Munsell D-15 test need to pay attention to the quality and quantity of lighting and the time that they allow for completion of the test, and all repeat attempts need to be included in reports on compliance with color vision standards. PURPOSE: The validity of the Farnsworth-Munsell D-15 has been questioned because practice may allow significantly color vision-deficient subjects to pass. In this article, we review the influence of practice and other factors that may affect the performance. These relate to both the design and the administration of the test. METHODS: We review the literature and present some calculations on limitations in the colorimetric design of the test, quantity and quality of lighting, time taken, and repeat attempts. RESULTS: In addition to the review of the literature, color differences and luminance differences under selected sources are calculated, and the increases in luminance clues under some sources and for protanopes are illustrated. CONCLUSIONS: All these factors affect the outcome of the test and need specification and implementation if the test is to be applied consistently and equitably. We recommend the following: practitioners should never rely on a single color vision test regardless of the color vision standard; lighting should be Tcp '' 6500 K and Ra > 90; illuminance levels should be between 200 and 300 lux if detection of color vision deficiency is a priority or between 300 and 1000 lux if the need is to test at the level where illuminance has minimal influence on performance; illuminance should be reported; time limits should be set between 1 and 2 minutes; repeat testing (beyond the specified test and one retest) should be carried out only with authorization; and initial and repeated results should be reported. A set of test instructions to assist in the consistent application of the test is provided in the Appendix.

Military Research ColorDx and Printed Color Vision Tests.

PURPOSE: To determine the equivalence of the ColorDx Military Research version (mColorDx) test and three printed pseudoisochromatic tests (HRR, Ishihara, and PIPIC) for color vision testing. METHODS: Participating in the study were 75 color-normals and 47 subjects with red-green color vision defects. Color vision was classified by an anomaloscope. The HRR (4(th) edition), Ishihara 38-plate edition, and PIPIC tests are printed color vision tests, whereas mColorDx test figures were displayed on a calibrated computer desktop monitor. All tests were repeated in about 1 wk. RESULTS: The kappa level of agreement (κ) values with the anomaloscope for screening for each test was 0.96 or greater. The values were statistically identical. Specificity for each test was at least 0.99 and sensitivity was at least 0.95. The repeatability of the screening sections for all tests was very good with κ values greater than 0.95. Deutans tended to miss the tritan screening plates on the HRR and mColorDx tests. The Spearman rank correlation coefficients between the severity of the defect and anomaloscope range was moderate with r = 0.45 for the mColorDx and r = 0.6 for the HRR. Both the mColorDx and HRR had perfect agreement with the anomaloscope in classifying the defects as either protan or deutan. CONCLUSION: The validity of the four tests for color vision screening was statistically identical; however, the HRR may be preferred because it had the highest sensitivity of 0.99, a specificity of 1.0, and a reasonable correlation between the severity rating of the defect and the anomaloscope range.

Repeatability of Associated Phoria Tests.

PURPOSE: H.J. Haase developed a set of tests for measuring associated phoria and stereopsis using a variety of different targets for each. This testing method is known as the MKH-Haase method and it is used commonly in Germany. The aim of this study was to investigate the test-retest repeatability of the distance and near associated phoria tests for the MKH-Haase charts and other clinical tests. METHODS: Horizontal and vertical associated phorias were measured at distance and near for 34 symptomatic and 40 asymptomatic participants with different tests. The tests are Cross, Pointer, Double Pointer, and Rectangle tests of MKH-Haase charts at distance and near. The other common tests include the Mallett Test and the American Optical (AO) Slide at distance. At near, there are the Mallett Test, the AO Card, the Saladin Card, the Wesson Card, and the Sheedy Disparometer. RESULTS: The 95% limits of agreement for all of the distance horizontal values for the symptomatic group were within ±1.25, except for the AO Slide limits, which were larger by 0.43. The limits of agreement for the asymptomatic group were within ±0.875. At near, the 95% limits of agreement for most of the horizontal associated phoria tests were ±2.00. The exception was the symptomatic group's Sheedy Disparometer limits, which were -4.25 to 5.75. Except for the Disparometer values for the asymptomatic group, the mean between-session differences were not statistically significant different from zero based on the 95% confidence interval. The asymptomatic group's mean Disparometer value was less eso at the second session. The 95% limits of agreement for all of the vertical values at distance and near for both groups were very narrow (i.e., within ±0.375). CONCLUSIONS: Most of the tests showed good repeatability for both subject groups at distance and near, except for the Sheedy Disparometer. The reason for the lower repeatability could be the design of the Disparometer.

Protan response times to red lights in a mildly hypoxic environment.

PURPOSE: This study was conducted to determine whether protans have slower reaction times to red lights than individuals with normal color vision and to identify whether protan reaction times increase differentially in a mildly hypoxic environment. METHODS: Simple reaction times (SRT) to a red light-emitting diode (LED) display were measured using the Psychomotor Vigilance Task (PVT) at ground (1293 ft/394 m), simulated 12,400-ft (3780-m) altitude, and 20 min after returning to ground. Subjects were 13 individuals with normal color vision (NCV), 12 with a deutan color vision defect, and 4 with a protan color vision defect. RESULTS: The mean reaction times increased by 8% with altitude and decreased after returning to ground for all groups. However, the reaction times of the protans were often faster than the NCV mean and never below the NCV 10(th) percentile. The only significant difference between color vision groups was the slowest mean reaction time of the NCV group was slower than both the pooled dichromats and pooled anomalous trichromats across all conditions by 23%. The number of lapses did not vary with altitude, but the dichromatic subjects had significantly fewer lapses than the trichromatic subjects across all conditions. CONCLUSION: Although protans may be slower to respond to some red warning lights, this decrement in performance could not be demonstrated under the conditions of our experiment. Furthermore, the protan group's simple reaction times were not differentially affected by mild hypoxia. These results suggest that the red LEDs were sufficiently bright for these protan observers.

Forward light scatter and visual acuity through scratched protective eyewear.

BACKGROUND: Scratches and other surface abrasions on protective eyewear can shorten their life cycle by scattering light from the image, which will compromise visual performance. Although scratch and abrasion resistance testing is well developed, there is little information available regarding visual acuity through lenses scratched in an actual work environment. METHODS: We collected protective eyewear from mine and smelting operations that had been returned because vision through the lenses was unacceptable due to scratches and abrasions. Forward scattered light was measured on both cleaned and partially cleaned lenses using both haze and light diffusion measurements. Visual acuity through a selection of lenses was determined for both high and low contrast letters under photopic light levels. RESULTS: Lenses with scratches in the central region had haze values ranging from one to 30 per cent. The highest haze values were measured on lenses that were both scratched and covered with dirt, whereas haze values less than 10 per cent were measured on lenses that had the dirt removed and just had scratches in the central region. Light diffusion values were highly correlated with haze. Visual acuity for high and low contrast letters became worse as the percentage haze increased but the linear correlation was only moderate. CONCLUSION: The appearance of the scratches on the lens surfaces strongly suggests that they were a result of improper lens cleaning. The findings also suggest that haze values less than two per cent or a luminance reduction factor less than 3.0 cd/(m(2) lx) are unlikely to affect high and low contrast acuity.

Trichromatic and dichromatic relative sensitivity to green light in a mild hypoxic environment.

INTRODUCTION: Several studies have reported that individuals with normal color vision have a relative decrease in sensitivity to green light in hypoxic environments approximating altitudes above 4000 m. Because there is little available information describing the effects of mild hypoxic environments (less than 4000 m) in subjects with deficient color-vision, we examined the effect of mild hypoxia on the relative sensitivity to green light for color-normal and color-deficient subjects. METHODS: Relative sensitivity to the green light was measured using the Medmont C-100 at ground and 3780 m in an altitude chamber. There were 30 subjects, 13 with normal color vision and 17 with a congenital red-green defect, who participated in the study. The relative sensitivity to the green light was determined from the average of four settings measured during the 4.5-h trial. RESULTS: Color-normals and anomalous trichromats showed a small decrease in the relative sensitivity to the green light at 3780 m compared to ground. In contrast to the trichromatic results, the relative sensitivity of the dichromats to the green light did not differ between 3780 m and ground. DISCUSSION: Our results show that a decrease in the relative sensitivity to green light can occur in hypoxic environments that are equivalent to altitudes greater than 3700 m in individuals with trichromatic but not dichromatic color vision. Although the change in sensitivity was significant, it was small and unlikely to have any operational impact.