Lumens, kelvin and CRI: the three numbers that decide a bulb

The options around bulb specifications are set out side by side below, with the conditions that genuinely favour one over the other.

Lumens are the brightness figure

A lumen is a measure of visible light output, and it is the number to compare when replacing a bulb. As a rough anchor, an old sixty watt incandescent produced somewhere around seven to eight hundred lumens, and a hundred watt roughly fifteen hundred. LED efficiency varies, so two LEDs of the same wattage can differ noticeably in output, which makes wattage useless as a comparison.

Beam angle then determines whether those lumens land where you want them, which is why a spotlight and a globe of equal output light a room very differently.

Kelvin is colour, not brightness

Colour temperature in kelvin runs from warm orange-white at around two thousand seven hundred, through neutral around four thousand, to cool blue-white above five thousand. Domestic living spaces generally sit between two thousand seven hundred and three thousand, which approximates the incandescent light people are used to. Cooler light suits task areas, garages and mirrors where you want to see detail, and reads as clinical in a living room.

Live with it a while and mixing colour temperatures in one room is the most visible lighting error there is, and it happens whenever bulbs are replaced piecemeal.

CRI is the number nobody checks

The colour rendering index compares how a source shows colours against a reference, with a hundred being perfect. Below about eighty, reds and skin tones look grey and food looks unappetising, which people notice without being able to name. Above ninety is worth seeking wherever you eat, cook, dress, apply makeup or look at art and people.

Room to room, the extra cost of high-CRI lamps is modest and the difference in a kitchen or bathroom is immediately visible.

Dimming is a compatibility problem

A bulb must be marked dimmable, and even then it has to be compatible with the specific dimmer, because LED drivers and older dimmers frequently disagree. The symptoms of a mismatch are flicker, buzzing, a limited dimming range and dropping out suddenly at the low end. Trailing-edge dimmers generally suit LEDs better than older leading-edge types designed for incandescent loads.

Most dimmers also have a minimum load, which is why a single low-wattage LED on an old dimmer often behaves badly.

Flicker and the things not on the box

All mains lighting fluctuates, and cheap drivers produce a deep flicker that some people perceive as headache or eye strain even when they cannot see it. Waving a pen under the light, or looking at it through a phone camera, reveals bad flicker surprisingly well. Lifetime figures on packaging are estimates under test conditions, and heat is what actually kills LEDs, so enclosed fittings shorten life considerably.

Underneath it, this is why a bulb in a sealed glass shade fails years before the same bulb in an open one.

Budgets and buildings differ, and old buildings differ most.

Getting it right across a house

Choosing one colour temperature as the house standard, and buying spares in it, prevents the mismatched-bulb problem entirely. Warm-dim lamps shift warmer as they are dimmed, reproducing the behaviour of incandescent light and making evenings feel right. Keeping a note of which fitting takes which cap, wattage and beam angle saves repeated trips and wrong purchases.

Room to room, it is worth buying one of a new bulb and living with it for a week before buying twelve.

Side by side

The takeaway

Match lumens to the old bulb, pick one kelvin figure for the house, and pay the small extra for CRI above ninety.

The best rooms look like a decision, not a purchase.