The PH 3½-2½ glass floor is a member of the PH 3-shade family, and the principle behind the PH 3-shade fixture was made in the winter of 1925-26 for a large exhibition hall in Copenhagen, Denmark called "Forum".

This is what PH said about his new fixture in 1926: "The real innovation in the PH-fixture is that it produces lighting which is both glare free and economical. It is easy enough to create glare-free light, indirect and strongly shaded, if a portion of the light is allowed to be wasted.

It is equally easy to make fixtures which are highly efficient if you ignore the fact that they severely irritate the eye, but it is an art to make light both economical and glare free." The first fixtures were all made with metal shades, but PH also wanted to develop a variant of the fixture, which would show a light distribution curve with less downward lighting.

The result was shades made in opal glass with a sandblasted underside. The glass shades allowed 12% of the light to penetrate and contribute to the overall lighting of the room while most of the light was still reflected outward and downward from the matte undersides of the shades.

The PH3-shade system started out as a pendant but very quickly it also developed into other kinds of fixtures for tables, floors, walls and a great number of chandeliers. As a result of using different sizes, materials, surfaces, colours and light sources, more than a thousand variants were developed over the years.