A question we are asked frequently is "how will it look once our leaded lights have been encapsulated into double glazed units?".
The best way to answer this is with examples so you can see for yourself.
Below you will see an image of before (right) and after (left) encapsulation.
As you can see, it's very difficult to see the difference.
Below you will see an image of the same window but viewed from the outside.
From the outside is where one would notice the difference. This is because the light reflects from the outer pane of the double glazed unit. Also you can see the lead is not as prominent due to being behind the 4mm float.
There are many benefits to having double glazed leaded lights. They insulate better, they are preserved and protected from weather, they're easier to clean, they're more secure and safe etc. But the one downside is they do loose some of the charm from the outside. That said, it's hardly an eyesore. And if we are to preserve the charm of the property while bringing it up to modern day living, then there has to be a little compromise somewhere.