I understand there are various grades of Colour Deficiency, what test is used to determine this? Also where can you get a Lantern Test done?
The most common test used in the High Street is the Ishihara pseudoisochromatic plates where you have to read a number in a circle of dots. This grades colour deficiency only into moderate or strong. There are other tests used and the employer often states which test they require. However, if you pass the Ishihara test then you will not have a colour vision defect and will (or should) pass any other tests. The lantern test was originally designed to test the colour vision of railway workers and just tests a few primary colours. You can still be colour deficient and pass it. It is very difficult to find a practice with a lantern test as they don't seem to have been manufactured for many years.