Lumen (unit)

A lumen tells us how much light we see.

It measures the light that comes from a source. It is different from heat. Our eyes see some light better than others. A lumen counts the light our eyes can feel. Many lamps use lumens on their labels. This helps us know how bright they are. You can use them to pick a good bulb. It is a great way to measure light.

A lumen tells us how much visible light a source sends out. We use the symbol lm for this unit. It measures luminous flux, which is the power of light we can see. This is different from total power. Total power includes light we cannot see, like heat.

Our eyes see some colors better than others. The lumen counts light based on how our eyes feel it. This is done using a special model. This model helps us weigh different colors.

You might also see the word lux. A lux is one lumen spread over one square metre. If you have 1,000 lumens in a small space, it is very bright. If you spread those same lumens over a large area, it looks dimmer.

Many light bulbs use lumens on their labels. This helps you know how much light they make. For example, an LED lamp might give 470 lumens. An old bulb might need more power to make the same light.

Projectors also use lumens to show brightness. Some use a special way called ANSI lumens. This method is very accurate for comparing projectors.

A lumen is a unit that measures how much visible light a source sends out. We use the symbol lm for this unit. It measures something called luminous flux. This is the perceived power of light that our eyes can actually see. Luminous flux is different from total power, which is called radiant flux. Radiant flux includes all waves, even ones we cannot see like heat. The lumen only focuses on the light that matters to us.

How does a lumen work? It uses a special model called a luminosity function. This model weights light based on how the human eye feels different colors. Our eyes are most sensitive to certain wavelengths. For example, a source using one watt of green light at 555 nm has 683 lumens. This means a lumen is at least 1/683 of a watt of visible light. If a light shines in all directions, we call it isotropic. An isotropic source with one candela of intensity has a total flux of 4π lumens.

People have been studying these measurements for a long time. André Blondel first proposed the lumen in 1894. He created it along with other units for photometry. These units were based on the metre and the Violle candle. Today, groups like the CIE and ISO set the standards for these weights. This ensures that light measurements are the same everywhere in the world. It helps scientists and engineers talk about light in the same way.

You might see lumens on the labels of light bulbs. Many places now require this by law. For instance, the European Union passed a law on 1 September 2010. This law says bulbs must be labeled by lumens instead of electric power. An LED lamp might use only 6 watts to give 90 lumens. An old incandescent bulb might need 15 watts for that same light. A 23 W spiral lamp can emit between 1,400 and 1,600 lumens.

It is easy to confuse lumens with a unit called lux. The difference is how the light is spread out. A lux is one lumen per square metre. If you have 1,000 lumens in one square metre, you have 1,000 lux. If you spread those same 1,000 lumens over ten square metres, you only have 100 lux. Projectors also use lumens to show how bright they are. They often use a more accurate method called ANSI lumens. This helps people compare different projectors more easily.

The lumen is the SI unit used to measure luminous flux. Luminous flux is a way to quantify the perceived power of visible light from a source. This measurement is different from radiant flux, which is the total power of all electromagnetic waves. Radiant flux includes non-visible waves like infrared thermal radiation. The lumen focuses only on the light that the human eye can actually perceive. This makes it a practical unit for understanding how bright a light appears to us.

To calculate lumens, scientists use a model called a luminosity function. This function weights light based on how sensitive the human eye is to different wavelengths. The CIE and ISO organizations standardize this weighting process. For example, the human eye is most efficient at a wavelength of 555 nanometers in the green region. A light source radiating one watt of power at this specific wavelength produces 683 lumens. Therefore, a single lumen represents at least 1/683 watts of visible light power.

Mathematically, a lumen is defined as being equivalent to one candela-steradian. A steradian is a unit of a solid angle. An isotropic light source is one that radiates light uniformly in all directions. A full sphere contains a solid angle of 4π steradians, which is approximately 12.56637 sr. An isotropic source with a luminous intensity of one candela has a total flux of 4π lumens. If you cover half of that source with an ideal absorbing hemisphere, it would radiate 2π lumens. The intensity would remain one candela in the directions that are not covered.

It is important to distinguish lumens from lux. While lumens measure the total light emitted, lux measures illuminance. One lux is defined as one lumen per square metre. You can see this difference by how light is spread across a surface. If you concentrate 1,000 lumens into one square metre, the illuminance is 1,000 lux. If you spread those same 1,000 lumens over ten square metres, the brightness drops to 100 lux. The lux measurement accounts for the area over which the flux is distributed.

Lighting technology has changed how we use these measurements. In the past, people often looked at electrical power in watts to judge brightness. However, European Union legislation changed this on 1 September 2010. Under the Eco-design Directive for Energy-using Products, equipment must be labeled primarily by luminous flux. For instance, an energy-efficient bulb claiming to replace a 60 W tungsten bulb must output 700 to 810 lumens. This shift helps consumers choose bulbs based on actual light output rather than just power consumption.

Modern light sources show great differences in efficacy. An LED lamp producing 470 lumens might use only about one sixth the power of an incandescent bulb. A 23 W spiral compact fluorescent lamp can emit between 1,400 and 1,600 lumens. Typical fluorescent systems have a luminous efficacy of 50 to 100 lumens per watt. Looking at common bulbs, an incandescent bulb needs 60 watts to produce 800 lumens. In contrast, an LED can produce those same 800 lumens using only 13 to 15 watts.

Video projectors also use lumens to describe brightness. Many manufacturers use a specific standard called ANSI lumens. This method was defined in the IT7.215 document created in 1992. To measure this, a projector displays a full white field in a room at 25°C. Technicians measure the brightness at nine specific locations around the screen and average them. This average is multiplied by the screen area to find the ANSI lumens. This is more accurate than "peak lumens," which only measures light in a small center area. For example, a CRT projector might claim 1,200 peak lumens but only produce 200 ANSI lumens.

The history of these measurements began with André Blondel. In 1894, he proposed the lumen for use in photometry. He developed it alongside other units based on the metre and the Violle candle. Today, these units allow us to connect the physics of light to human biology. By using the lumen, we can accurately describe the relationship between energy, wavelength, and human sight.