The Wallace Expo/Disc is everything that Lee L mentions and more.
I have had my Expo/Discs, 52mm and 72mm for well over 20 years, they have done a lot of work in the field and for in the darkroom.
Combine the grey negative one gets from exposing a frame with the Wallace Expo/Disc, with the Bob Mitchell COLORBRATOR, and you have about the fastest and most accurate way of colour negative printing I know of.
Colour to die for, plus density that is so correct (generally) itís painful. Great colour prints first up from the get go, superb prints from the second one onwards.
Bob Mitchell sent me, ďone of his own thin plastic diffusers, with which I could cut to size and install on my formerly red filter holder. It transmits considerably more light than the Expo/Disc does, I am sureĒ Unquote.
It does transmit more light and is slightly better that way, but it isnít on the money like the Wallace Expo/Disc.
White balance is nothing I have ever done, grey balance is what I have always done in a darkroom. Getting white on a colour paper is haphazard at best, mainly due to the different white colour base of each paper.
I have a few friends who have the new(ish) Expo/Disc and they are slightly different to my old style, plus they do not come with the National Bureau of Standards step wedge check, that was written in hand on each individual Wallace Expo/Disc transmission and colour fidelity certification slip.