Capturing Radiance is a new Wish Mechanic introduced in Genshin's Version 5.0 update. It increases the chances of obtaining a Limited 5-Star Character in their corresponding banner.
First of all, the guarantee is still in place. When you lose your 50/50, the next character will be the limited event character. This means the number of wishes you need to guarantee a character from 0 pity is still 180. On that note, pity goes up to 90 for 5 stars, not 100.
The new mechanic kicks in after you’ve lost your 50/50 twice in a row, meaning you’ve pulled a total of four 5 stars. Two of these were the guaranteed limited character you were pulling for, and the other two were standard banner 5 stars that you obtained from losing the 50/50.
On this third 50/50, Radiance will kick in and essentially change it to a 75/25, meaning you have a 75% chance of pulling the limited 5 star and winning what would normally be a 50/50. Losing this means going through one more cycle of pulling for your guarantee, but after that, Radiance changes the next 50/50 to a 100/0, meaning you are guaranteed to pull the next limited character you pull for.
Finally, once you win your 50/50 at any point in this process, you get reset to the beginning, meaning your next two 50/50s will be the same as usual.
In essence, Radiance acts like a pity system for 50/50s. If you lose a couple, then the chance of you winning goes up, and you’re eventually guaranteed a win after enough losses.