• Science Of Gadgets


LED bulb and lumens, kelvins and watts

Today LED bulbs are very common all over the world, more than 50 % of the world's light sources are now LEDs. The main reason is that LED bulbs are almost 75 % more energy-efficient than incandescent bulbs and about 25 % more efficient than fluorescent bulbs. Led bulbs also have a much longer lifespan than other bulbs, some of them can last up to 20 years. Although it largely depends on your geographical location, because geographical location determines how much artificial light we use during the year. The days near the equator are longer and there is no need to use so much artificial light as in the far north or south.

LED bulbs are currently available in a huge variety, they may vary in size and shape. When choosing a LED bulb, it is also important to know what kind of base this bulb has. The LED bulb could have a screw, bayonet, bi-pin, or wedge base. The most common bulb base is the well-known screw base, which is known as the Edison screw. In this post, however, we will explore in detail what the lumens, lux, kelvins, and watts mean, when we talk about LED bulbs. These characteristics are always specified for each LED bulb and help us determine if this bulb is right for us or not.

Let's start with the lumens (lm). To put it simply, lumens show how much visible light is emitted from a light source in one unit of time. The higher the number of lumens, the brighter the light source appears to us. Another important unit of lightning is the candela (cd). Candela shows how much light is emitted in a certain direction. If we have a light bulb, marked with a certain number of lumens, then this number is a constant quantity and shows how much visible light this lightbulb emits per unit of time. However, the value of candela may vary, depending on how much light is concentrated in a certain direction.

candelas vs lumens

The image above has two different light sources. They both emit the same amount of light - 1000 lumens, but the light from the flashlight is concentrated in a narrow beam and has a much higher light intensity, thus the value of candelas is large. The light bulb on the left however emits all that light over a very large area and thus has a lower candela value.

But now let's define what lumen exactly is and how it relates to candela. The definition reads as follows: lumen is equal to luminous flux, emitted by a light source with an intensity of one candela uniformly across a solid angle of one steradian. Ok, I know this may sound a little confusing, but let's illustrate this with a picture and explain everything.

Defenition of lumen

Imagine that we have a sphere with a radius of 1metre and in the center of the sphere is a light source with an intensity of 1 candela (it is about as bright as a candle flame). The angle of one steradian is like a cone of light of a certain volume inside that sphere, with a bottom area of 1m2. Now imagine, that this light source with the intensity of 1 candela emits a certain amount of light rays, uniformly in each direction over the entire surface of the sphere - we call it luminous flux. The number of light rays (or luminous flux) that passes through that 1m2 rea of the sphere defines the lumen. It's easy to calculate that this sphere has a surface area of 12,57 m2, which means that a light source with the intensity of 1 candela produces 12,57 lumens of luminous flux in that sphere (1 candela = 12,57 lumens).

how candela, luumen and lux are related

As we can see from the picture, we get 1 lumen of luminous flux per square meter - this is called an illuminance and its unit is lux (lx). Obviously, the farther we move away from the light source, the less luminous flux we get per square meter and the less surface is illuminated.


If we have a LED bulb with a light intensity of 1 cd, which is 12,57 lm, then at a distance of 1 meter away from the light bulb the illuminance is 1 lx, at a distance of 2 meters 1/4 lx, at a distance of 3 meters 1/9 lx and so on. This means that the illuminance is inversely proportional to the square of the distance from the light bulb.

There are many sources on the internet, that specify how much lux is needed to ensure good illumination. The rule of thumb is that if you multiply the area of your room by the required lux value, you will know how many lumens are needed. About 120 - 150 lx is recommended for the living room, which means that if the floor area is, for example, 20 m2, a minimum of 20x120 = 2400 lumens is needed. About 150 - 200 lx is needed in the kitchen and 400 - 500 lx in the office.

Kelvin (K) represents the color temperature of the LED light and is related to the black body radiation and Kelvin temperature scale. In physics a black body is a body, that absorbs all the light that falls on it, this means that no light is reflected from this body. Regardless of its chemical composition, such a body always emits light of a certain color at a certain temperature. This temperature is measured in Kelvins, which is a base unit of temperature measurement. Although the ideal black body does not exist in nature, we can still compare many objects to black bodies. One example would be stars. Depending on their temperature, they emit light of a certain color. The colder stars, with a surface temperature of about 3000 Kelvins are reddish. As the temperature rises, the light turns yellow, which is about 5000 Kelvins, then white (about 7000 Kelvins), and finally bluish, (about 9000 Kelvins).

LED bulb color temperature scale

Note that the color temperature of the LED bulb does not mean that the bulb will ever be as hot as the stars, but only show the color output of the LED bulb. So you can be absolutely sure that you will not burn your finger with it :) The picture above illustrates a nice, continuous temperature color spectrum of the LED bulbs from red to blue, or from warmer light to cooler light.

Watts (W) is related to the energy that a LED bulb needs to produce a certain amount of light. The higher the wattage, the more electricity bulb consumes, and the more light it emits. In the case of old incandescent bulbs, only watts are usually given instead of lumens, which gives an idea of how much light that bulb can produce. Since incandescent bulbs consume most of the energy to generate heat, but not light, they are very inefficient light producers. The light output of a LED bulb is usually compared to the wattage of an incandescent bulb with a similar light output. Then it is written on the packaging of the LED bulb, for example, that 40 - watt equivalent or 75 - watt equivalent, which indicates how high the wattage of the corresponding incandescent bulb must be, in order to produce as much light as the corresponding LED bulb.

The energy consumption of the LED bulbs is very low. LED bulbs with 4 - 6 watt energy consumption can already produce 400 - 500 lumens, which is equivalent to a 40 - watt incandescent bulb. 25 - 28 watt LED bulbs can produce nearly 3000 lumens, which is equivalent to a 150 - watt incandescent bulb.

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