Colour is used extensively in the railway industry to enhance conspicuity, code information and group objects of interest together. An acceptable level of colour discrimination is therefore required to enhance visibility even when colour is used redundantly, thus justifying the need to adequately screen all employed personnel. Rail Safety and Standards Board (2002, 2007) state that colour vision must be normal, as assessed by the Ishihara Plates Test or City University test. However, current conventional screening tests are limited in design and have high variability. As a result, a study was commissioned by Transport for London (TfL) to derive empirically, new limits of colour vision loss that could be classed as safe for train operators, within the London Underground (LU) environment. An initial visual task analysis was conducted to indentify the most safety-critical, colour-related task that LU train drivers encountered. This was determined to be the 'within tunnel' signal lights. Unique to this study was the original and faithful laboratory construction and reproduction of the within-tunnel signal light task which made it possible to establish experimentally new colour limits. A total of 100 subjects were recruited for the study, 40 normal trichromats and 60 congenital colour deficient'. They were assessed on conventional screening tests, the Colour Assessment and Diagnosis (CAD) test and compared against the simulated Trains Light (TL) test. Safe colour vision limits were established for two approach distances of 220 m and 110 m. Analysis indicated that for an approach distance of 110 m, deuteranomalous subjects with RG CAD thresholds < 7 and protanomalous subjects with RG CAD thresholds < 10 performed the TL test as well as normal trichromats and were therefore classed as safe to operate a train. An extended analysis of the proposed limits which included previous study data and LU data was conducted. A total of 606 subjects' data was reviewed (normal n = 205, deutan n = 269, protan n = 132) to provide accurate predictive outcomes. Based on the proposed 110 m limits, - 40% of colour deficient applicants would be classed as safe to drive a train. If the approach distance of 220 m were adopted, - 11 % of colour deficient applicants would be classed as safe to conduct a train based on stricter RG CAD thresholds limits of < 2.5 for deuteranomalous subjects and < 9 for protanomalous subjects. TfL have now accepted the CAD limits proposed for an approach distance of 110 m, classifying - 40% of the colour deficient population as safe. The CAD system now replaces the Ishihara test in the LU occupational health centres, and is used during initial and renewal certification of medical fitness assessments. The second study focused on acquired visual function loss associated with ageing and retinal disease. Studies had suggested that visual acuity measurements were not a good predictor of early retinal disease where a significant loss in VA was perceptible only if the majority of the photoreceptors were dysfunctional (Sunness et al., 1997; Gellar et al., 1992). Coupled with the high inter-session measurement variablitiyfor visual acuity (Patel et al., 2008), the aim of the study was to identify the most appropriate and sensitive clinical test capable of detecting early morphological changes associated with early Age-related Macular Degeneration (AMD). A total of 71 subjects were recruited for the study, a normal control group (n = 45) and an active AMD group (n = 26) of varying disease severity. Subjects underwent extensive clinical screening to ensure they adhered to strict,inclusion criteria. Advanced psychophysical assessment of visual function for visual acuity, contrast sensitivity, mesopic and photopic temporal and chromatic sensitivity were conducted. Exclusive to this study was the refinement of a new temporal resolution test. Significant clinical predictors for early-AMD were determined based on bivariate regression analysis and entered stepwise into a multiple linear regression model, controlling for eye and age. Yellow-blue (YB) chromatic discrimination was identified as the best clinical predictor, explaining an additional 9 %-12 % of the variance for early-AMD detection and - 22 %-25 % for AMD disease. Receiver operating characteristic (ROC) and area under the curve (AUC) analysis confirmed YB chromatic sensitivity to be 13.5 % more sensitive and 16.7 % more specific than the standard clinical Log MAR acuity test for detecting early-AMD retinal changes. A review of AMD and diabetic case studies also indicated that accurate Psychophysical methods of assessing chromatic sensitivity enhanced our clinical ability to detect, quantify and monitor changes associated with retinal disease and / or treatment outcomes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:654960 |
| Date | January 2014 |
| Creators | Biba, Matilda |
| Publisher | City University London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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