Lamp shades and diffusion

Everywhere else on this site we insist on published lumens, because a number you can check beats a marketing adjective. This page is the honest footnote to that insistence.

A lumen figure describes what leaves the bulb. What reaches your book is what leaves the bulb minus everything the shade absorbs, and shades vary enormously in how much that is. A pale linen drum passes a great deal of light through its sides; a lined opaque shade passes essentially none, so the room only receives what escapes the top and bottom openings.

This is why two lamps with identical bulbs can feel completely different, and why we never quote a lamp's room-level performance from a bulb spec alone. The figure is real and useful. It is just measured before the shade.

Two shades can look the same from the outside and differ by a wide margin in useful output, because of what is on the inside.

A white or silver lining reflects light that would otherwise be absorbed by the shade material back down and out of the openings. A dark or unlined interior soaks it up and turns it into a very small amount of heat. If you are choosing between two similar shades, look inside them - it is a quicker test than any specification.

The same principle explains why the corner rule works for uplights, and it is worth connecting the two. Light that hits a pale surface mostly comes back; light that hits a dark one mostly does not. That is true of a shade lining, a wall, and a ceiling alike - which is why the placement advice in where to put a floor lamp keeps coming back to pale surfaces near the source.

The practical sequence is backwards from how people usually do it. Decide what the lamp is for, pick a shade shape that sends light there, and only then choose the bulb - because the shade determines how much output you need and how visible the source will be.

If the shade is pale and open, the bulb is doing ambient work, so color quality matters more than raw output. A CRI 95 bulb at 2700K makes everything in the room look right, and 800 lumens through a pale shade goes a long way.

If the shade is dark or opaque, you need more lumens to get the same result, and you need a lamp that lets you supply them. This is the strongest practical case for a lamp with a standard socket like the Brightech Eden over an integrated-LED design: light quality and quantity stay your decision rather than the manufacturer's, and a failed bulb costs a few dollars instead of a new lamp.

If the fitting shows the bulb - an open cage, a bare socket, an arc lamp head at eye level - the bulb is now part of the object. That is the one place a 2200K filament bulb earns its keep, with the output and color caveats set out in vintage Edison LED bulbs. Dim it, or it is simply a small bright object in your eyeline.

Multi-socket lamps like the Trilage multiply all of this by three, which is either an opportunity or three times the mistake. Fit all three with the same bulb - mixed color temperatures in one fitting look like a fault rather than a choice.