June 6

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Watts, lumens, kelvin … What’s what when buying lamps?

Buying lamps online means not seeing them in action beforehand. However, that’s no reason to spend hours scrambling in stores again because you’ll find all the important information in the product specs. This guide will show you the most important metrics, what they mean, and what to look for when buying lamps.

What do watts mean for lamps?

Not so long ago, there were only the classic incandescent lamps. Since then, we have internalised the wattage as a measure of brightness: 100-watt bulbs for large rooms, 40-watt bulbs for the bedside lamp, etc… Physically, however, this specification only tells us how much power is being absorbed. Modern light sources such as light emitting diodes (LEDs) have a much higher efficiency compared to the old incandescent bulb (see also energy efficiency class), which means that an LED bulb with an effective power of 8 watts produces the same amount of light as an old 60-watt bulb. So the good old wattage doesn’t say anything about brightness and primarily helps to compare with what we know.

What does lumen mean for lamps?

The measure of brightness is luminous flux, expressed in lumens (lm). After all, the amount of light emitted is ultimately what we want to know. The basic rule with watts is that the more, the brighter. You can use this table to obtain comparative values for incandescent bulbs.

Here’s how modern light sources compare to incandescent bulbs:

As we’ve already seen, more efficient light sources require less power input (read: lower power consumption) to produce the same brightness. While a conventional incandescent lamp has a luminous efficacy of 10 lumens per watt (lm/W) (95% of the absorbed energy is converted into heat!), LEDs generate between 60 and 170 lm/W. And as LED technology continues to evolve, LEDS may consume even less power to achieve the stated lumen output.

This is how modern light sources behave compared to the incandescent bulb

How many lumens do I need for my lighting?

There is no simple answer to this question; it always depends. For one thing, you don’t always want it to be equally bright – in the living room, a cozy dim mood is called for, while in the kitchen or bathroom, you need the best possible view. On the other hand, darker rooms need more light than bright ones, the room’s height and cut play another role, and ultimately everyone has their personal preferences. Also note the colour of light, which greatly impacts the atmosphere.

The measurement used here is lumens per square meter (lm/m²). The light output of individual luminaires can be added together. This table gives approximate guidelines for basic lighting that you can use as a guide.

  • Living room: 100 lm/m²
  • Hallway: 140 lm/m²
  • Bedroom: 100 lm/m²
  • children’s room: 100-140 lm/m²
  • Study: 200-300 lm/m²
  • Kitchen: 300 lm/m²
  • Bathroom: 300 lm/m²

An example: Your living room has an area of 25 m². You want to illuminate it with LED recessed luminaires, each emitting 350 lm. To reach the minimum luminous intensity of 2,500 lm (25 * 100 lm) for this room, you need 2,500 lm / 350 lm = 7.14, i.e. seven spots (you would probably arrange six or eight spots here, depending on the floor plan). You can supplement the basic lighting with floor, table or wall lamps.

Tip: If you use dimmable lamps, you can regulate the light intensity depending on the situation.

Energy efficiency class

You’re probably familiar with the chart on the right because electrical appliances and lamps have to be classified on this scale. This gives consumers information at a glance about how economical an appliance is. The extent to which technology has advanced since its introduction in 1998 is already evident from the fact that the scale has been expanded to include the grades A+, A++ and A+++, and F and G are no longer printed on the scales. From 2020, the classification will be redefined in the new EU Energy Label Regulation, and we will return to grades A to G (analogous to the English school grading system). With LEDs as illuminants, you are definitely on the safe side regarding the economy.

What does the Kelvin rating mean for light?

Not all light is the same. The color temperature indicates whether the light is “warm” or “cold”. This colloquial distinction can also be represented scientifically in a scale: the Kelvin scale (K). A low Kelvin value indicates many red components (“warm” light), and a higher one indicates bluish (“cold”) light.

The color of light is important not only for its decorative effect but also for our biorhythm. After all, it changes during the day: In the morning, the sun rises red, and our body switches to activity. Daylight reaches its highest Kelvin value around midday. In the evening, more red components are mixed back into the spectrum – a sign for the body to come to rest.

In the days of the incandescent lamp, people only knew the typical warm light. On the other hand, modern LEDs can display any color temperature. To distinguish between them, the following terms have come into use to express the quality of the light:

Warm white (1,500-3,300 K): light with more red components has a cozy effect on us and helps us relax. The traditional light bulb burns at a color temperature of 2,700, a candle flame at 1,500 Kelvin. This type of bulb is properly used for a pleasant ambience in which you want to come to rest. Take them for basic lighting in the living room, bedroom or study.

Neutral white (3,300-5,300 K): Daylight in the morning or afternoon is in this range. Halogen lamps often emit light in this spectrum. It’s great as a work light in the kitchen or bathroom and is also recommended if you want to perceive colors accurately (for example, when painting).

Daylight white (5,300-8,000 K): Although this light has the same spectrum as the warm midday sun, white daylight lamps appear surprisingly bluish to us because we are used to artificial lighting in the warm white spectrum. Daylight white bulbs switch our bodies to activity, promote concentration, and suppress fatigue. When visual demands are high (for example, when you want to perceive fine details or color nuances), your eyes need light as close to daylight. That’s why daylight white light sources are used in task lights, factory lighting, for illuminating photos and videos, or some desk lights.

Incidentally, as the Kelvin number increases, the light appears to us bluer and brighter. Thus, a daylight white lamp appears brighter than a warm white lamp at the same light output.

In addition, the lamp itself also influences the quality of light. For example, lampshades made of fabric or diffusers of ceiling lamps usually color the light additionally and give it a warmer appearance.

Tip: It’s the mix that counts. Ensure a mix of light sources depending on the application area: warm white ceiling lights for atmospheric ambient lighting, neutral white kitchen counter lighting, and daylight white desk lamps for working.


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