What is a Farnsworth Lantern?
The Farnsworth Lantern is a very old piece of equipment used to detect colourblindness. As most colourblindness is red-green, it is able to detect moderate to severe red-green colour blindness. Historically it was developed by Commander Farnsworth in the US Navy during World War II and adopted by the US Navy in 1954 for colour vision testing for sailors. It was subsequently adopted in flying.
It is a colour vision test of only 3 colours (red, green and white) which you may be required if you fail the Ishihara colour test during your medical (all Classes). The Ishihara colour test is best for detecting any type of colourblindness. The Farnsworth Munsell 100 Hue test is a more comprehensive colour test of colourblindness.
What does the test involve?
It consists of 9 pairs of coloured lights in different combinations shown for 2 seconds each. If colour deficiency is mild, some patients can still correctly identify the colours. The white light (which looks yellowish) has a 50% neutral grey filter to reduce differentiation based on luminance. The pairs of lights are presented randomly so that patients cannot memorize them.
The lights can be presented as a round of 9 pairs or a round of 18 pairs (each pair being shown twice). Correct identification of the lights in either round will constitute a Pass. To pass the test, no errors are permitted in a round of 9 pairs, or a maximum of 2 errors in the round of 18 pairs.
What do the pairs of lights in a Farnsworth Lantern look like?
Click here to watch a Youtube video of the test