Buying Lights Without the Jargon: Lumens, Kelvin, Ra and “Dimmable”

Watts measure power consumption, not brightness – with LEDs, the old watt reflexes are therefore worthless. The EU has regulated this consistently: on the packaging, the luminous flux in lumens must appear in type at least twice as large as the wattage. And so that nobody has to convert, the energy labelling regulation defines official equivalence values: 249 lumens correspond to the old 25-watt bulb, 470 lumens to the 40-watt, 806 lumens to the 60-watt, 1055 lumens to the 75-watt and 1521 lumens to the 100-watt incandescent bulb. These figures are the most reliable anchor when buying light bulbs. Since September 2021, the new energy label from A to G (without A+ and A++) has also applied – what it rates is efficiency in lumens per watt. Good to know: by design, decorative filament LEDs often end up in the lower classes, yet they are still many times more economical than the incandescent bulbs whose look they quote.

The second mandatory detail is the colour temperature in kelvin. The established classification – a lighting-standards convention – has three groups: warm white below 3300 K, neutral white from 3300 to 5300 K, daylight white above that. For living, dining and sleeping areas, warm white has proven itself – that is convention, not law, but it aligns with circadian research, which recommends warm, dimmed light in the evening. The most important practical mistake is quickly explained: different kelvin values in the same room zone. So when you buy replacement bulbs, note down the kelvin figure of the existing ones – it is stated on every package.

The colour rendering index (CRI, stated as Ra) measures, according to the definition of the International Commission on Illumination CIE, how faithfully a light source renders colours compared with a reference light source of the same colour temperature – as an average across eight standardised test colours. By law, LED light bulbs must achieve at least Ra 80. For rooms where materials are meant to work their effect – wood grain, fabrics, art, mouth-blown glass – Ra 90 and above has established itself as the quality threshold; that is market convention above the legal requirement, but the difference is visible at the dining table and in front of the wardrobe. In our lighting consultations we prefer to show the effect directly: the same fabric sample under Ra 80 and Ra 90+ is the most convincing argument.

Two details that hardly anyone reads, but that reveal a lot about quality: first, flicker – the EU limits it measurably. Mains-operated LED light bulbs must comply with the short-term flicker metric PstLM of at most 1.0, and for the stroboscopic effect the tightened limit SVM ≤ 0.4 has applied since September 2024. Cheap products used to stand out unpleasantly here; today's limits have raised the level noticeably. Second, lifetime: it is defined as L70B50 – the point in time at which half of a population of light bulbs still delivers at least 70 percent of its initial luminous flux. So as a rule an LED does not “die” suddenly; it slowly grows dimmer over the years. 25,000 hours L70B50 therefore does not mean “25,000 hours, then darkness” – it describes a statistical brightness curve.

Here too, the law helps: if a light bulb cannot be dimmed at all, or only with certain dimmers, this must be stated on the packaging, and the manufacturer must provide a list of compatible dimmers online. The rest is practical knowledge: old dimmers and LEDs often get along badly – if you experience flickering or humming, check the compatibility list first instead of swapping the bulb. The short checklist for buying: