The diagonal light rule

Light that falls directly above an object (pure overhead) flattens its volumes. All surfaces are illuminated the same way, shadows disappear or concentrate at the foot of the object.

Light that strikes the same object diagonally, that is to say at an angle of between 30° and 60° from the vertical, reveals its volumes. Part of the object is lit, the other part is in shadow, and it is this contrast that gives relief.

This is exactly the principle used by photographers since the beginning. The "Rembrandt light" in portraiture is a 45° diagonal that illuminates one side of the face and plunges the other into soft shadow. In interior design, the same logic is applied to furniture, objects and textured walls.

To enhance a sofa. Place an articulated floor lamp 1 metre from the sofa, to the side, at shoulder height or slightly higher. The light comes diagonally, illuminates the sofa at an angle, and makes it appear "sculpted". Reference brands: Anglepoise, Tala Voronoi, Vibia Pin.

To enhance an isolated armchair. Same logic: floor lamp or table lamp 60-80 cm to one side of the armchair. The face of whoever sits in it will also be flattered by the diagonal (Rembrandt portrait effect).

To reveal a textured wall. Place the sconce at an angle (not facing the wall, but to one side), so the light sweeps it diagonally. This is what distinguishes a flat wall from a "sculpted" one (see also rule 02.3 on grazing light).

To enhance a framed artwork. An adjustable spot on the ceiling, placed at 30-45° to the artwork, gives the best rendering. Directly overhead, the painting appears flat. At horizontal angle, reflections interfere. At 45°, it is right.

The rule that unites all of these: avoid pure overhead light whenever you want to reveal a volume. Reserve overhead for purely functional uses (kitchen, bathroom, hallway).

Professional photographers and cinematographers define their lights by angle. Frontal light (1990s studio portrait style, flat). Side light (Rembrandt, dramatic). Back light (contre-jour, silhouette). Three-quarter light (45°, versatile, the most used). In interior design, the three-quarter is the best compromise.

Pierre Yovanovitch often integrates articulated floor lamps (Anglepoise, Tala) in his projects, which allow the user to orient the light diagonally themselves. The diagonal is never fixed permanently, it adjusts according to the moment.

A sophisticated technique: doubling the diagonal. Place two floor lamps with opposing diagonals (one on the left, one on the right) to sculpt a central sofa with two crossed flows. Very architectural presence effect, used in hotel lobbies.