Lighting terms in one place: lumen, lux, candela, kelvin, CRI

Lumen, lux, candela, kelvin, CRI: a plain guide to the terms you meet on lamp packaging, and how they relate to one another, all in one place.

Pick up a lamp box and you are faced with a string of symbols: lm, K, Ra, lm/W, 36°, IP. The shop assistant talks about "lux", and articles mention "candela". Most of these terms are connected, and once you see how they fit together, every later choice becomes easier. Below are the most common ones, grouped in a logical order.

Quantity of light: lumen, candela, lux

Lumen (lm) is the total amount of light a lamp emits in all directions, known as luminous flux. It answers the question "how much light does this bulb give?" Watts, by contrast, only tell you the electrical power used.

Candela (cd) is the intensity of light in a particular direction. Of two lamps with the same lumens, the one that concentrates its light into a narrow cone has a higher candela value in that direction. Spot lamp packaging often states centre beam intensity in this unit. Historically, one candela is roughly the intensity of a single candle flame.

Lux (lx) is the amount of light falling on a surface. One lux equals one lumen spread over one square metre. Lux tells you how well lit a table, a floor or a workstation is. For example, if 1,000 lumens fell evenly and without loss on 10 square metres, the average illuminance would be 100 lux. In a real room, losses and reflections change the sum.

An easy way to remember the trio: lumens are the light leaving the source, candelas are the light going in one direction, lux is the light arriving on a surface. If you compare it with water, lumens are the total flow from the tap, candelas the force of the jet, and lux how densely the drops land on the ground.

What the eye sees: luminance

Luminance (cd/m²) describes how bright a surface looks to the eye. A white wall and a dark wall receiving the same lux look different, because the white one returns more light to the eye. Screen brightness and the glare from a fitting are also assessed in these terms.

Colour and quality of light

Kelvin (K) is colour temperature. It describes whether light looks warm (yellowish) or cool (bluish). Low kelvin figures mean warm light, high figures mean cool light: roughly 2700–3000K is warm white, 4000K neutral, and 5000–6500K cool white.

CRI or Ra is the colour rendering index, on a scale up to 100. It expresses how naturally light shows the colours of objects. The general Ra is the average result for eight standard pastel test colours.

R9 is a separate score for a saturated red test colour. It is not part of the general Ra, but it matters for making skin, meat, fruit and red fabrics look lively.

SDCM shows how much lamps from the same batch may differ in colour tint. The smaller the number, the more alike they look side by side.

Efficiency and distribution

Luminous efficacy (lm/W) is the number of lumens per watt of electrical power. It shows how well a lamp turns energy into light and is the fairest figure for comparing technologies.

Beam angle (°) is the opening of the light cone of a directional lamp. A narrow angle concentrates light; a wide one spreads it.

UGR is an index used in offices and public spaces to rate how much discomfort glare the fittings cause. A lower number means less disturbing glare.

Electrical and protection markings

Flicker is a rapid variation in light output, visible or not. A good driver keeps it to a minimum; it is most noticeable in video recording and during long reading sessions.

IP rating describes a fitting's protection against dust and water. The first digit covers solid particles, the second covers water. It matters for bathrooms and outdoor use.

Power factor (PF) indicates how efficiently a device draws power from the grid. It is barely noticeable at home but is taken into account in buildings with many lamps.

How the terms come together in practice

When choosing lighting, the questions line up like this: how much light the room needs (lumens and, as a result, lux), in what tone (kelvin), how accurately colours should appear (Ra, R9), how the light should be spread (beam angle), and how much energy it takes to get there (lm/W).

The Zaferoğlu Elektrik website has tools that let you see some of these ideas in action: the colour temperature simulator shows what a change in kelvin looks like, and the savings calculator works out how wattage and daily hours affect yearly consumption.