How the PH Three Shade System Works and What the Numbers Mean

Every PH lamp carries a number, and those numbers are not model codes. They describe a geometry Poul Henningsen worked out in the mid 1920s and kept refining for the rest of his career: three shades, all drawn from one curve, arranged so that the bulb stays out of sight. This is how that system works and how to read the names.

Paris, 1925: the problem Henningsen set himself

Henningsen belonged to the first generation of Danish designers to live with electric light at home, and he disliked what he saw. The incandescent bulb was a small, hard point of light that threw sharp shadows and glared back from every polished surface, where the oil lamps it replaced had spread their light through a diffusing glass. He wanted the comfort of the older light without giving up the convenience of the new one.

His answer took shape in the lighting he showed at the Paris exposition of decorative and industrial arts in 1925, where it was awarded a gold medal. The lamp shown there is still known as the Paris lamp, and the three-shade principle behind it went into development with Louis Poulsen through 1925 and 1926. That partnership ran for the rest of Henningsen's life, and the shades in production today descend directly from those first drawings.

One curve, used three times

A PH shade is a section of a logarithmic spiral. The useful property of that curve is that a ray leaving its centre meets the surface at the same angle wherever it lands. Put the bulb at that centre and reflection becomes predictable across the whole shade: light striking close to the socket and light striking near the rim leaves at the same relative angle, so the shade throws an even pool instead of bright bands and dark gaps.

Henningsen drew that curve at several sizes and stacked three of them. The top shade takes the largest share of the output and turns it down and outward. The middle shade catches what escapes the first and redirects it. The lower shade takes the remainder and finishes the beam. Light is caught, softened and turned downwards three times before it reaches a table.

The undersides do as much work as the curves. On the glass pendants the shades are mouth-blown white opal, glossy outside and sandblasted matt underneath, so the surface facing you scatters light rather than returning a highlight. The PH 2/1 Pendant uses exactly that treatment at 200 x 140 x 200 mm and 1.6 kg, the smallest set in the family.

Reading the numbers

The first figure is the size of the top shade, close to its width in decimetres: a size 2 shade measures 200 mm across, a size 3 measures 285 mm, a size 5 measures 500 mm. The second figure gives the size of the two lower shades. A PH 3/2 Pendant therefore sets a size 3 top shade above size 2 lower shades, while the PH 3/3 Pendant keeps all three shades in one size, the group Louis Poulsen calls the whole numbers.

The difference reads from across a room. A fraction gives a stepped, tapering silhouette that stays visually light; a whole number gives the fuller, lantern-like form of the earliest models, its middle and lower shades held in the 3:2:1 proportion of the original. Weight follows the material rather than the size: the PH 3/3 runs from 2.8 kg in soft-touch black painted aluminium to 4.1 kg in three-layer opal glass, with the mixed metal and glass version between them at 3.5 kg.

Why the bulb never meets your eye

Stacking the shades does something a single shade cannot: it closes the sightlines. Seen in section, the three shades overlap, so a line from your eye to the socket ends on a painted or sandblasted surface from any normal position, standing in the room or seated at the table. The bulb is hidden not by being buried deep in a shade but by being screened at every angle a room is actually used from, which is also why a PH pendant can hang low without becoming uncomfortable.

Two details hold the calculation in place. The shades are separated by spacers, rolled aluminium on the PH 5, so the gaps stay as drawn. And each pendant hangs from its own suspension and canopy on a fabric cord, 3 m on most of the family and 4 m black textile on the PH 3/3, so the stack sits true rather than tilting.

Because the geometry does the work, glare control does not depend on the bulb. Henningsen designed the PH 5 in 1958 so that it would give soft, comfortable light with any bulb, at a moment when bulb shapes were changing quickly. An E27 LED behaves the same way inside the same shades, a century after the first drawings.

The PH 5 and where each size belongs

The PH 5 Pendant Lamp is the version most people meet first, and it restates the whole system in one piece: a 500 mm main shade, 267 mm high, 4.6 kg, in matt lacquered aluminium with white inner sides, with an anti-glare shade and small red and blue inner cones that refine the way colours read underneath it. Named after that 500 mm shade, it has stayed in production at Louis Poulsen since 1958 and now runs through monochrome, pastel, hues and metal finishes, including copper and brass.

Placement follows the numbers. Hang the PH 5 around 60–70 cm above a dining table or kitchen island, where its width gives an intimate island of light. The PH 3/2 works at roughly 60 cm over a two-person table, the end of an island or a reading corner. The PH 2/1 suits a small table, a counter or a bedside, and a pair or trio at staggered heights turns quietly sculptural. The PH 3/3 asks for height instead: because it glows evenly from every angle, it holds a stairwell, a hallway or a double-height living room where a wider pendant would sit heavily.

One practical note carries across all four. The glass versions ask to be dusted like any mouth-blown piece, inside and out, since the sandblasted undersides are what keep the light even. If you are weighing up the sizes in more detail, our PH 5 pendant lamp buying guide sets the options side by side, and you can follow the three-shade idea through the rest of the catalogue in the Louis Poulsen collection.