How colour blind are you?
Visually demanding professions, such as the fire service and the maritime and aviation industries, specify 20/20 vision as a prerequisite for entry. Others also require applicants to have normal colour vision, preventing those with even very mild colour vision loss from entering the profession.
Accurately assessing the severity of colour vision loss is important because many colour-deficient people have been shown to have sufficient residual 'chromatic sensitivity' to be able to perform visually demanding, colour-related tasks as well as those with normal colour vision.
Professor John Barbur and Dr Marisa Rodriguez-Carmona from the Division of Optometry and Visual Science in the School of Health Sciences, co-authored research into assessing the severity of colour vision lost, published in Aviation, Space and Environmental Medicine.
The problem with conventional colour assessment tests such as the Ishihara Test (IT) is that they produce inconsistent results and do not quantify accurately an individual's severity of colour vision loss. Since these tests were developed to screen for normal colour vision, not the severity of colour vision loss, there are no objective pass/fail limits for deciding when an applicant can no longer be considered safe.
The research examines how well the number of failed IT plates captures a subject's loss of colour vision and whether a level of importance can be attached to each IT plate to improve the test's predicted severity of loss.
The research investigated 742 subjects using a 25-plate IT test with a Colour Assessment and Diagnosis (CAD) of chromatic sensitivity, which quantifies the signals needed to see just red, green, yellow and blue colour differences.
The subjects' IT error scores provided plate-specific weights used to calculate a Severity Index (SI) of colour vision loss, measured and compared in three subject groups.
The researchers found that colour thresholds provide a good measure of the severity of colour vision loss since they relate linearly to "red", "green" and "blue" signals but neither the number of plates failed nor the SI value computed in this way can be used to determine reliably the severity of colour vision loss.
Professor Barbur said:
The findings from this study prove convincingly that the number of plates failed does not provide a good measure of the severity of colour vision loss. New approaches are needed to establish safe but fair pass/fail limits that relate directly to the applicant's ability to carry out the colour-related tasks within visually-demanding occupational environments with the same accuracy one would expect of those with normal colour vision.