Specifications

Colour temperature,
explained for shoppers.

Kelvin, lumens, CRI and beam angle — and how to write each one so a buyer can decide.

6 min read Updated 22 September 2026

Four numbers decide whether someone is happy with a lamp, and most product pages list all four in a way that helps nobody. Here is what each one means and the sentence to put next to it.

Kelvin: how warm the light is

Colour temperature is measured in kelvin, and the scale is backwards from what people expect: lower numbers are warmer, higher numbers are cooler.

• 2200–2400 K — candlelight, amber. Atmosphere, not tasks. Bars, bedside, decorative bulbs.
• 2700 K — the classic warm bulb. Living rooms, bedrooms, anywhere the point is to feel comfortable. The safe default for homes.
• 3000 K — warm but cleaner. Kitchens and bathrooms where you want comfort and still need to see.
• 4000 K — neutral. Workshops, offices, garages. In a living room it reads as an institution.
• 5000–6500 K — daylight, blue-white. Task and inspection work. Almost never right in a home.

On the page: «2700 K — warm, the colour of a traditional bulb. Right for living rooms and bedrooms.» The number stays for the people who want it; the sentence does the selling.

Lumens: how much light, and why watts stopped meaning anything

Watts measure what a lamp consumes, not what it produces. That was a usable shorthand when every bulb was incandescent and it stopped being one when LEDs arrived — a 9 W LED and a 60 W incandescent can give the same light.

Lumens measure the output. Most people have no intuition for them, so translate:

• ~250 lm — a bedside lamp. Reading for a while, not lighting a room.
• ~450 lm — old 40 W. Accent light, a corner.
• ~800 lm — old 60 W. The workhorse of a living room.
• ~1500 lm — old 100 W. A kitchen ceiling, a work surface.

On the page: «806 lm — about as bright as an old 60 W bulb.»

CRI: the number nobody lists and everybody notices

The colour rendering index says how truthfully a light shows colour, against daylight at 100. Under a low-CRI lamp, wood goes grey, skin goes ill and a red sofa turns brown.

Below 80 is visibly poor. 80–89 is the ordinary range. 90 and above is where materials look like themselves, and it matters most where people look at people and at food.

On the page: «CRI 90+ — colours stay true. Wood looks like wood and skin looks healthy.» If your lamps are CRI 90 and your competitors do not publish the number at all, say it loudly: it is a real advantage that costs you a line.

Beam angle: where the light actually goes

The most ignored of the four, and the one that causes the most disappointment. A narrow beam makes a pool; a wide one fills a room. A pendant that throws light straight down is right over a dining table and useless as the only light in a living room.

On the page: «Throws down, not sideways — good over a table, not enough on its own for a whole room.» Telling someone where the product is wrong is what makes them believe you about where it is right.

Where the numbers stop

All four together still do not show what the light looks like. A shopper reading «2700 K, 806 lm, CRI 92, 120°» has to assemble a mental image out of four abstractions, and most people will not — they will look at the photograph instead.

Which is why the photograph should show the lamp lit. The specifications confirm a decision; the image makes it. See how to photograph a lamp that is on.