Lantern testing
Colour vision lanterns are vocational tests which employ colour naming. Many countries have developed lanterns to fulfill national colour vision requirements for transport services and the armed forces. Some lanterns display single colours and others show colours in pairs. The aim is to determine if signal lights can be identified correctly, but only the Holmes Wright lantern contains actual signal colours. The Holmes Wright lanterns, the Farnsworth lantern and the Beyne lanterns, are currently approved by the CIE.
Moreover, the Holmes Wright (H/W), the Beynes lantern and the Spectrolux lantern are approved by the JAR regulations.
The Farnsworth lantern (Falant)
The Holmes Wright lantern (UK)
The Holmes Wright lanterns (Holmes and Wright, 1982) replaced the Board of Trade lantern (1913) and the Martin lantern (1939). The Holmes Wright lantern type A show pairs of colour separated vertically, while the Type B lantern shows colour pairs separated horizontally. Both lantern have two reds, two greens and white, which have x, y chromaticity coordinates within the internationally agreed specifications for signal lights. Colours are shown in nine pairs representing all the possible colour combinations. The lanterns are viewed at 6 meters (20ft). Detailed operation instructions are provided. The lanterns are no longer manufactured but are robust and should remain in service for the foreseeable future.
The Holmes Wright Type A lantern is used by the UK armed services and by the British Civil Aviation Authority. There are three luminance settings and the test can be given in normal room illumination (approx. 200lux) or in the dark following dark adaptation. The examination in normal room illumination is with the colours at high luminance (200µcd). The 9 pairs of colours are shown three times making 27 presentations in all. Each colour pair is shown for about 5 seconds and the subject names the top colour first. The test is the repeated in the dark. If any error is made in the initial run after the lights are switched off, the subject is dark adapted for 15 minutes and three sequences of the nine colour pairs are shown. A single error results in failure of the test. The colours are either at high or at low luminance (20µcd) in the dark, according to individual requirements. In some examination procedures, the dark-adapted examination is made first.
Birch (1999) found that an examination at high luminance in room illumination has a screening efficiency of 97 per cent compared with the Ishihara plates. However, neither the number of errors nor the number of qualitative error categories identified different types, or severities, of colour deficiency. Misnaming green as “white” and white as “green” were the most frequent qualitative errors in all types of colour deficiency. Red/green naming errors very only made by 40 per cent of dichromate and 25 per cent of anomalous trichromats. An examination at high luminance following dark adaptation allows a small number of colour deficient people to pass (Cole and Vingrys, 1983). All colour deficient people male more mistakes with the low luminance setting in the dark. An examination at low luminance after dark adaptation is a very exacting test and some normals make mistakes either due to night myopia or because green/white discrimination is very close to the visual threshold (Birch and Rodem, 1993). In the UK armed services, both the Ishihara plates and the Holmes Wright Type A lantern are used to categorize colour perception.
The Holmes Wright Type B lantern is used by the British Marine and Coastguard Agency (previously the Board of Trade) to select personnel for the merchant marine service. The standard test uses the Ishihara plates, and the Holmes Wright Type B lantern is only given on appeal. The test is performed in complete darkness following a minimum of 8 minutes of dark adaptation. The colours are shown singly at a large aperture, as an introduction, and the test is discontinued if these are named incorrectly. The paired colours represent ship’s navigation light at 2 miles when viewed at 6 meters. For some branches of the service the test distance is reduced to 3 meters to represent ship’s light at 1 mile. Three runs of nine colour pairs are shown. A single error results in failure of the test. Standards for some branches of the merchant service are based on the Ishihara plates and the Farnsworth D15 test.
JAA Manual of Aviation medicine reports that Holmes Wright lantern has an aperture size of 1-6mm, corresponding to a visual angle of 0-9 minutes of arc.
The Farnsworth lantern (Falant) (USA)
The Farnsworth lantern (Falant) is the standard test used in the US armed services and the US coastguard, and by the Federal Aviation Administration (FAA) medical examiners. Although the examination procedure is similar to that of the Holmes Wright lantern, the test parameters differ in several respects. Most importantly, the lantern displays selected isochromatic colours, not signal lights, and the angular subtends of the colours are much larger. The test is performed in a normally lit room at a test distance of 8 feet. The pass/fail is also determined by an error score.
FAA error score standards are displayed on the internet page: http://www.leftseat.com/baggish.htm.
A single showing of the nine colour pairs is recommended for initial assessment, and the sequence in only repeated a second and third time if mistakes are made. The error score is the average number of mistakes on the second and third runs. A score of 1.5 (three errors on two runs) or more results in a failure of the test. An error naming one of both colour in the pair is counted as a single error. Birch and Dain (1999) showed that all dichromats and 75 per cent of anomalous trichromats failed the Falant. Error scores were continuous and there was no clear demarcation between pass and fail. Neither the error score nor the number of qualitative error categories distinguished the type and severity of red-green colour deficiency. It is impossible to identify anomalous trichromats likely to pass the Falant from anomaloscope of Farnsworth D15 results.
A new “Farnsworth” lantern, the Optec 900, has recently been manufactured by the Stereo Optical Company.
The Beyne lantern (lanterne chromoptométrique de Beyne) was developed in France before 1951 and modern versions are available from Luneau Ophtalmologie.
There are two lanterns: Type 1 is for marine and rail transport and Type 2 is for aviation. The colours are slightly different in the two models. Both lanterns show single and paired colours derived from narrow wavelength bands in the blue, green, yellow, and red parts of the spectrum. These are not signal colours.
There are six aperture sizes but the 1 minute of arc aperture is recommended. The test distance is 5 meters and the colour exposure time is very brief (1 second). The JAA Manual of Aviation Medicine reports that the 3 minutes of arc aperture should be used, and that no errors are accepted.
