The Holmes Wright lantertest is a color vision perception test.
It is approved by JAR regulations as ishiara plates, nagel anomaloscope, beynes lantern, spectrolux lantern.
It is taken when somebody fails ishiara test plates.
It consists in a black box which has two littly tiny holes on it, that lit in three different colours: red, green, white. One light is vertically above the other one.
They lit in certain configurations commanded by the examiner.
You sit in a chair, 6/7 meters far away from the lantern, and you have to guess what the colors displayed are.
There are two types of H/W lanterns, the A type which is used in Gatwick and the B type which it is said to be thougher but I've never tried that type out.
Hope I answered your questions.