Custom transparency

I’m trying to replicate in D5 something that I’m used to do in other renderer, but so far, I’m unable to do it…

The idea is to define the transparency level of a material by using an opacity map. It’s normally used to create complex skyscraper facade without modeling everything such as this one (photography…):

The thing is that in D5, if I’m adding a map in the transparency slot of a transparent material, I’m getting something quite bad, completly dark in the opaque areas. Is there a way to change that ? will it be added in the future ?

Example :
The map :

In Thea render (what I try to replicate) :

How it looks in D5 :