Light read: One of the world's greatest academic libraries has been radically transformed from a dull badly-converted commercial building by giving it a new focus full of light and air. (interior design).

The London School of Economics and Political Science is an august institution founded in 1895 by Beatrice and Sidney Webb. Committed to Fabianism and social reform, they envisaged a school devoted to the teaching of and research into the social sciences. Today, LSE has more than 7000 undergraduate and graduate students from all over the world, 18 departments and more than 30 research centres. Its library is considered the largest and most important social sciences library in the world.

The campus is a collection of disparate edifices acquired over the years by the school and clustered around the main nineteenth-century building on Houghton Street, a pedestrian alleyway running north from the Aldwych. Almost since its inception, LSE has suffered from congestion -- though less so now than formerly, for gradual acquisition has relieved it and plans for improving the various parts are under way.

One of the earlier acquisitions was the Lionel Robbins building (previously the headquarters of W. H. Smith & Sons) which, built in 1916, is on the north side of the campus. Taken over in 1973, it was converted for use by the library then uncomfortably housed in the main building. Now, almost thirty years later, Foster and Partners has renovated and enlarged the four-storey building, and transformed its interior.

Fosters' scheme retains the integrity of the brick and stone facades, and the basic fabric of the awkwardly shaped building (on plan made up of two dissimilar triangles joined together); only perimeter windows in a poor state of repair have been replaced. But the interior has been transformed into an airy library full of muted light and movement, and pale colour. At a stroke, the scheme has increased space and, with imagination, taken care of circulation and servicing.

Such transformation has been achieved by converting the old lightwell around which the library once revolved into an atrium, a great cylinder driven down to the basement and bringing daylight into the heart of the building. Filling out the bottom four layers of the cylinder, is a stepped, helical ramp spiralling around a pair of glass lifts. The whole confection in this central part of the library -- the rhythms of thin balustrading around ramp and surrounding galleries, the vertical embrace around the ramp of slender columns, the constant movement of people ascending and descending and hum of voices -- has a cinematic dream-like quality.

Capping the atrium is a dome, with a glazed section cut at an angle to admit north light and eliminate glare and solar gain. Design assists natural ventilation, for air drawn in through perimeter windows rises as it warms and escapes through vents in the dome's glazing.

On each floor, bookshelves leading away from the atrium define passageways to quiet study areas around the perimeter. These are separated by blocks of bookstacks. In the basement, a light-filled, double-height study was created by removing part of the ground floor slab. A new fifth and existing fourth floors accommodate a secluded research centre, which has its own identity: a separate entrance and lift, and distinct signage.

Enveloped in north light, and uniformly painted white, the library does provide quiet studious workplaces, though the basement study, overlooked by the open gallery at the entrance, must suffer from noise. But evidently the lesson of the Cambridge University Law Faculty (AR March 1996), where openness and hard surfaces combined with disastrously resounding effect, has been learned. Perimetor study areas are protected by noiseabsorbent shelves full of books and by the floor covering of soft grey carpet; levels become quieter as you move up the building away from the entrance and there are silent retreats enclosed by glass walls.