I failed the Ishihara test initially at school in the 1960's. When it came to my initial Class 3 (as it was in those days) I went to the CAA's medical centre at Heathrow (yes, they had one there in 1982!) Having safely passed the medical, though once again failing the Ishihara test, I enquired with the doctor in the white coat (yes, really!) about a future flying commercially. He kindly agreed to do the lantern test there and then, which I passed. However, I was -3.5 dioptre, which was within the Class 3 and Class 2 limits, but outside the Class 1 limits at the time. However, I pressed on and eventually obtained an unrestricted Basic Commercial Pilot Licence (BCPL), passing all the commercial exams and the CPL flight test. Fast forward 25 years and in my late 50's I decided to have a go at the Flying Instructor course. In those days, you needed a Class 1, so I presented myself at Gatwick Medical Dept. Since the 1980s, the visual acuity requirements had been sensibly relaxed and I was now comfortably inside. However, I again failed the Ishihara test. The doctor conducting the test got out a plastic flimsy and stuck it in a reader 'ah yes, you passed the lantern test at Heathrow in 1982. All good - pass!' Excellent record-keeping, thanks, CAA! Class 1 duly issued. 2,500 hours later, I'm still instructing, but sensibly on a Class 2 medical, once a year in stead of the every 6 months I had to endure until they changed the rules back.
Oh, and I converted the BCPL to a CPL on encouragement from the CAA - I had the hours by then and I think they were keen to get BCPLs off the books. One snag - I'd taken the CPL flight test in a Warrior, approved at the time but then they'd decided it needed to be a 'high performance complex type', so I had to do another check in a 201T Arrow...
TOO