Light box: part of a plan to revive the urban realm and create a sense of neighbourhood identity, this new mediatheque combines formal rigour with material invention.

Apparently just a gleaming metal box, Dominique Perrault's new mediatheque contains some intriguing surprises. Located in Venissieux, a southern suburb of Lyons, the project forms a key part of the local mayor's urban revitalization scheme. The suburb is currently dominated by the Minguettes tower-block housing estate on a hilltop to the west of the mediatheque site. To the north is Venissieux's Hotel de Ville, which dates from the mid 1970s.

One of four designs presented to a jury in 1997, Perrault's building makes no obvious reference to its surroundings. External walls are double-glazed, with folded and perforated aluminium panels set between the glass, facing both outwards and inwards. From a distance, the inward-turning panels resemble depressed keys on a piano keyboard. The facade is divided horizontally into three layers, each one distinguishable as a row (or three rows, in the case of the top tier) of 1 .8m wide aluminium-framed modules. On an overcast day, the bluish-green external skin reveals only the ghostly outlines of furniture and people within.

This particular use of perforated panels has its origins in another of Perrault's buildings, a factory built for the fabric manufacturer Aplix, near Nantes. During a site inspection Perrault noticed some unusual lighting effects generated by acoustic panelling- a serendipitous observation from which the Venissieux project has benefited. The building's massive, metallic appearance also conveys a necessary robustness, as it is near the tough Minguettes area. Windows on the north and south-facing sides of the library's three upper floors are screened by Venetian blinds, giving librarians greater control over the amount of light entering their offices.

The entrance is marked by the word 'mediatheque' printed in playful outsized letters on the sliding doors. From inside, the Venissieux townscape becomes a Pointillist backdrop - Paul Signac rather than Georges Seurat. Yet when viewed from deeper inside the building, a very different impression is created, like looking through a layer of ice on a window pane. Tiny holes equipped with filters set along the base of the walls help to minimize any distortion of the glass that could result from abrupt changes in air temperature.

The internal organization emphasizes the interaction of light and materials. A 3m wide space, delimited by the facade and shelving units in oukame (Gabon mahogany), runs around the edge of the ground floor. In plan it resembles the margin around the text on a page. This internal street allows people to enter the central maze of shelves, racks and study areas from several points. It also parallels the 3.9m wide footpath that wraps around the outside of the building.

In keeping with Perrault's rigorous rules of alignment, no piece of furniture is higher than 2.25m, which corresponds to the height of the lowest set of modules on the facade. Everything is equally aligned with the 16 slab-like concrete columns that support the 6.2m high ceiling. Furniture in shades of warm brown contrasts with the ubiquitous cool greys of the aluminium panels, computer terminals, rubber flooring, concrete columns, trusses and exposed ceiling ducts and wiring. Perforated and galvanized steel panels, confined to one part of the ceiling between east and west entrances, play a double role: the bundles of wire-and-tube spaghetti below the three-storey administration and storage block were considered so dense and unruly that Perrault had them hidden. The panelling also signposts the function of the long east-west oriented space below, which contains the loans and returns counters and lavatories.

Unlike the other shortlisted designs, which segregated the different readerships - adult, adolescent and child - by placing them on different floors, Perrault's proposal keeps them on one level. The two older age groups share the south wing, with children's spaces occupying the north wing. Separating these two volumes are a 50-seat auditorium (only 2.1m wide), a security surveillance office and a lift to the three floors above and basement loading bay below.

Offset from the centre of the building is a three-storey structure that deliberately breaks the formal symmetry to give the mediatheque its distinctive inverted-T shape. The angled north frontage further increases the asymmetry. Behind this skewed facade is a triangular space for children's activities--playing with toys, chatting with friends and taking part in group storytelling, all conducted under the watchful gaze of their parents. Cars flash past outside, pedestrians walk by, oblivious to the animation inside. At dusk, as the setting sun strikes the north end, the aluminium panels assume yet another unexpected appearance.

Inside, they gleam, as if a thousand spiders had spun their threads into a radiant golden fabric. After dark, the one-way mirror effect is reversed, and the mediatheque becomes a glowing lantern in the spring night.