There are three ways to brighten up a dark loft when you want to do it using natural light; it has to be done with windows.
There are three different ways to install windows in a loft to brighten it up;
When you place skylights in the room you allow light into the top of the building envelope, this light filters down from the top of the cathedral ceilings that are prevalent in loft style second floors.
The skylights should be placed facing south if it’s possible, this will maximize the amount of light that can come into the loft.
Skylights should be spaced out in the roof to allow for the proper amount of waterproofing around the skylights.
Enlarge windows in the gable ends
Most loft style windows have there windows in the gable end walls. Usually they are the only windows in the loft. If you were to add more windows in the gable ends or increase the size of the existing windows then more light will be able to filter in.
Depending on the pitch of the roof this will limit your ability to install larger windows. Gable end walls are usually not load bearing walls, this allows you to place larger windows in the gable as they do not have to worry about load bearing points requiring smaller headers.
Velux the skylight manufacturer makes a product called roof windows. They are literally windows that are installed in your roof. They even open like a real window. This gives you light and also ventilation that helps cool down hot lofts.
This is the biggest expense and also the most work but it also is the best way to allow more light into the loft. One of the great things about a dormer is that it also adds livable space in a loft and they are a great place to install window seats or reading corners.
Dormers can be built as high as the peak of the roof will allow you too and you can go as wide as the trusses will allow as well.
Whatever you do you can never go wrong adding more windows to a dark loft area.