Lighting Glossary: Lumens, Kelvin, IP Ratings & More

Lighting terminology can make a straightforward decision sound unnecessarily technical. This glossary gives short, plain-English definitions of the terms we are asked about most often in the showroom. Where a subject needs more explanation, follow the link to the full Illuminations guide rather than relying on a one-line rule.

What is colour temperature or Kelvin (K)?

Colour temperature describes how warm or cool white light appears and is measured in kelvin (K). Lower values such as 2700K look warmer; higher values such as 4000K look cooler and crisper. The best choice depends on the room, finishes and atmosphere you want. Read our 2700K, 3000K or 4000K guide.

What are lumens?

Lumens measure the amount of visible light a lamp or fitting produces. They are a much better brightness comparison than watts for modern LED lighting, but the same lumen figure can feel different depending on shades, glass, beam direction, ceiling height and the surfaces in the room. Read our Lumens & Brightness guide.

What is the difference between lumens and watts?

Watts measure electrical power used; lumens measure light output. Two LED lamps can use similar wattage and still produce different amounts of light, so compare lumens when brightness is the main question. See the full lumens explanation.

What is lux?

Lux describes how much light reaches a surface: one lux is one lumen per square metre. Lumens tell you what a light source emits; lux helps describe the illumination arriving on a worktop, desk, floor or other surface. In a home, the usefulness of either figure depends on what the room is used for and how the light is distributed.

What does an IP rating mean?

An IP rating describes a product's protection against ingress. The two digits refer to protection against solid objects and water respectively. A higher number is not automatically required everywhere: the correct rating depends on the location and the applicable installation requirements. Bathrooms are divided into zones, so avoid choosing a fitting from the IP number alone. Read our Bathroom Lighting & IP Ratings guide.

What is CRI?

CRI stands for Colour Rendering Index. It is a widely used measure of how closely colours under a light source appear compared with a reference source. A higher CRI can be useful when accurate colour appearance matters, but CRI is not a complete description of light quality on its own.

What does dimmable mean?

Dimmable means a lamp, fitting or LED driver is designed to operate with a compatible dimming system. It does not mean every dimmable LED will work perfectly with every dimmer. Compatibility, load and the manufacturer's guidance still matter. Read our LED Dimming & Flicker guide.

What is integrated LED?

An integrated LED fitting has its LED light source built into the product rather than using a conventional replaceable household lamp. Whether the LED module or driver can later be serviced or replaced depends on the design and manufacturer, so check the product specification rather than assuming the whole fitting must always be replaced.

What are E27, E14, B22 and GU10?

These describe common lamp caps. E27 is the larger Edison screw; E14 is the smaller Edison screw; B22 is the familiar bayonet cap; and GU10 is the twist-lock cap commonly used on mains-voltage spot lamps. Always check the fitting before buying replacement lamps.

What is a flush ceiling light?

A flush light sits close to the ceiling with little or no visible drop. It is useful where headroom is limited or where you want a visually quiet ceiling fitting. See our guide to lighting for low ceilings.

What is a semi-flush ceiling light?

A semi-flush fitting hangs a short distance below the ceiling. It gives more visual depth than a true flush light while still keeping the overall drop compact. The actual minimum drop matters more than the marketing label, so check the dimensions before buying. Read our low-ceiling lighting guide.

What do minimum drop and maximum drop mean?

The minimum drop is the shortest overall hanging height a pendant or chandelier can normally be installed at; the maximum drop is the longest. These measurements are especially important for low ceilings, stairwells and fittings over tables or islands. Always check whether the suspension is adjustable and whether shortening requires an installer.

What are ambient, task and accent lighting?

Ambient light provides general illumination, task light helps you perform a specific activity, and accent light highlights features or creates emphasis. Good rooms usually combine more than one layer rather than asking a single ceiling fitting to do everything. Read our Layered Lighting guide.

Why can the same light look different in a photograph and in a room?

Photographs can show shape and finish, but they do not fully communicate scale, sparkle, reflections, glare, shadow or the way light spreads around a real room. Glass, crystal, metallic finishes and shades can change dramatically once illuminated. Read why seeing lighting switched on changes the buying decision.

Still unsure what a specification means?

Bring the product details, room measurements or a photograph into Illuminations of Camberley and we can explain the specification in practical terms before you buy. 66–68 High Street, Camberley, Surrey GU15 3RS. Open Monday to Saturday, 9am–5:30pm. No appointment needed.

Review and primary sources

Reviewed by Illuminations of Camberley on 21 August 2026. This guide combines practical experience from displaying and advising on decorative lighting with the primary guidance below. Product specifications and electrical requirements should always be checked for the individual fitting and installation.

• European Product Registry for Energy Labelling (EPREL) — official light-source product data

• HSE — lighting, visual comfort and task-lighting principles