The first duty of a window is to let in light. This is what is said in countless books and articles on stained glass. I would agree (and wonder why some of the Victorian stained glass makers apparently didn't). But light is also what makes stained glass work. It is as much a part of the artist's repertoire as the glass itself, the paints, the enamels, the stain, and the black lines of the lead cames. Without light coming through, stained glass is nothing, as you can see when you look at the exterior side of a window which is matt, dull and opaque. It took me a while to understand that the great skill - and the genius of the very best makers - lies in the manipulation of light through the pieces of coloured, painted, stained glass, and through plain or 'white' glass as well.
Sometimes it's good to simply stand and appreciate the power of light (and stained glass windows are nothing if not celebrations of light itself), especially on a overcast or rainy day. In fact these are often excellent conditions for viewing, as strong, direct light can be harsh and cause halation - it's not a coincidence that most of the world's best stained glass is in northern countries (France, Belgium, Germany) which have grey weather. Even on the dullest of days, it's possible to find windows positively glowing and playing with light.
These photos were taken in Chichester Cathedral when it was cold and wet outside.
These windows are by Christopher Webb who left a lot of glass untouched and used large areas of delicate grisaille (as above) to create pale windows which work well with the English weather. He also knew how to use pure, bright colour to counterbalance the coolness.
Windows need to be backlit. This is why photos of stained glass look so good on computer screens, and why they have a cinematic, dramatic, theatrical quality.
But nothing beats seeing the light for yourself.