The Holmes Wright test is basically being able to tell the difference between the two red, green, or white dots that the lantern produces.
Both HW tests that i did were done via a mirror to give the correct distance that it is ment to be done from (aaaaaand i dont know that distance, sorry!). Two dots will appear, one above the other, in combinations between of the three possibilities, (ie red-green, white-red, green-green.. etc) .. and you just call em as you see them. Here in NZ you can have two mistake before you fail the test.
The first test i did, the HW-B was horrible.. it was anyones guess what the lights were up there coz they were so damn small!.. and im not kidding, i could tell the red fine, but it was harder for the white and green. I dont know the exact size of the light, or distance, but you could compare it to looking at something smaller than a LED from 5-6m away... it looked like a pin hole.
That is the main difference between the two tests, and is why the Holmes Wright A is said to be easier, is that he lights that you view are larger with the HW-A than with the B. It was much easier to distinguish between the whites and greens, altough still hard, it was a breeze compared to just guessing those of the B.
Hope this helps!
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laters!