That's a great link, thanks. I was actually looking for an image of testcard 61 (at the bottom of that page), a copy of which I saw being used only a few years

That's a great link, thanks. I was actually looking for an image of testcard 61 (at the bottom of that page), a copy of which I saw being used only a few years ago! 57 is interesting too; the black square in the centre is actually a hole leading into a box lined with black felt, to provide a black reference. The generation and use of test cards and patterns is a surprisingly complicated subject; even in the digital age.