Glare and visual comfort

Most of this site is about quantity: work out a target, count what you have, close the gap. That method is sound and it is incomplete, because a room can hit its number exactly and still be somewhere nobody wants to sit.

The missing variable is contrast. Your eye does not measure absolute brightness - it adapts to the average of what it can see, and then judges everything against that average. A source far brighter than its surroundings forces constant readaptation, and that is experienced as harshness, squinting and fatigue.

Which means glare is not caused by too much light. It is caused by light arriving from too few places, or from the wrong place.

This is the single most repeated recommendation on this site, and glare is the reason for it.

Take a room needing 3,000 lumens. Deliver it from one ceiling fixture and you have one very bright object with a large dark area around it - maximum contrast, hard shadows from everything, and a source you cannot look up at. Deliver the same 3,000 lumens from four sources at 750 lumens each, spread around the room, and the average brightness is identical while the peak is a quarter of what it was.

Bouncing goes further still. A 2,190 lumen torchiere aimed at a pale ceiling converts a small bright object into a very large soft one - the ceiling itself becomes the source, at a brightness per square foot low enough that you can look straight at it. That is the whole mechanism behind the corner rule in where to put a floor lamp.

Concealment is the third route. A COB strip tucked behind a lip or under a cabinet delivers its full output to the room with the emitter itself invisible - which is why under-cabinet lighting is one of the most comfortable light sources in a house despite sitting at eye level. The COB construction matters here too: a continuous emitting line has no visible dots, where a discrete-LED strip does.

Free: move your seat or the lamp. Sit where you normally sit and look where you normally look. Anything bright in that cone is a glare source, and most of the time it can simply be moved a foot.

Free: check the reflections. Turn everything on at night and look at your screen, your worktop, your television and your windows. A dark window at night is a mirror, and a lamp reflected in it is doing the same damage as one in the room. This is the specific problem behind home office lighting complaints.

Cheap: diffuse the source. A paler or frosted shade converts a point into an area at the cost of a few lumens. The trade is quantified in lamp shades and diffusion.

Cheap: add a source rather than brightening one. A second lamp at a modest output does more for comfort than doubling the output of the first, because it raises the background your eye is adapting to.

Cheap: get task light close and aimed. An under-cabinet bar at 1,245 lumens twelve inches above a counter delivers more usable light there than a far brighter ceiling fixture, with no bright object anywhere in your view.

Worth paying for: dimming. Not because dimming removes glare - it does not, since the ratio stays similar - but because it lets a room that is correctly sized for the afternoon also be correct at 10pm. See plug-in dimmers.