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How is light measured? Units of measurement and luxmeter

The human eye is sensitive to a very wide range of light intensities, but at low levels of illumination, it can lose the ability to differentiate some details. 


For this reason, it is very important to be able to measure the light levels correctly according to the use that our space will have in order to have a correct illumination. From Faro Barcelona we want to give you some tips so that you can measure light correctly and be efficient in the expected result.


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What is light?


Before we look at how light is measured, let's look at a little more definition of this concept. Light is a form of electromagnetic radiation capable of exciting the retina of the human eye to produce a visual sensation.

Radiation energy in the form of waves is only perceptible when it interacts on a medium with a given direction. This transfer of energy from one body to another is what we call radiation.


Visible light



Visible light or the visible spectrum is the region of the electromagnetic spectrum that can be perceived by the human eye and corresponds to wavelengths between 380 and 750 nanometres.

At each end of the visible spectrum - and already imperceptible to the human eye - are the wavelengths corresponding to infrared light - 750 nm - and ultraviolet light - below 400 nm.

White light

In 1672, the scientist Isaac Newton demonstrated that sunlight, white light, was not pure as previously believed, but was composed of a mixture of different colours. He refracted white light through a prism and broke it down into the basic colours of the visible spectrum.


White light is therefore understood as light that contains all the colours of the visible spectrum, which can be classified as follows:

  • Red: 780-618 nanometres.
  • Orange: 618-581 nanometres.
  • Yellow: 581-570 nanometres.
  • Green: 570-497 nanometres.
  • Cyan: 497-476 nanometres.
  • Blue: 476-427 nanometres.
  • Violet: 427-380 nanometres.


How is light measured?


Light is the form of energy that illuminates things and makes them visible. It is an electromagnetic wave that can be perceived by the human eye and whose frequency determines its colour. 


In order to measure light, according to the International System of Measurement, we have to base ourselves on three specific aspects and on the distance of the set of luminous rays from the same origin.


The 3 units of measurement of illumination

  • Lumen (Lm): A lumen is the amount of visible energy that we can actually measure. It is the luminous flux of monochromatic radiation characterised by a frequency of 540x1012 hz and a radiant energy flux of 1/683 watts.
  • Lux (Lx): one lux is the equivalent of the energy produced by one lumen incident on a surface of 1 m2.
  • Candela (Cd): is the basic unit that measures luminous intensity. One candela is equivalent to 1 lumen per steradian (lm/sr). It is defined as the luminous intensity going in a specific direction, and is therefore related to the angle of aperture towards the light.


A luxmeter is used to measure luminance, but what exactly is this measuring instrument?



What is a luxmeter and what is it used for?


The luxmeter is a tool that helps to measure the lighting conditions of an interior space. As its name indicates, the luxmeter's unit of measurement is the lux.


In other words, it is a measuring device that allows us to know how much light or luminosity there is in an environment with which light is perceived by the human eye. 


How to measure lighting with a lux meter?


The operation is very simple, but before measuring the light it is advisable to take into account the preconditions. Thus, the distance and the angle between the luxmeter and the object to be measured should be fixed in order to obtain results with high reproducibility. 


Luxmeters can have different scales depending on the amount of light to be measured, in order to be able to have a more exact precision in case of stronger or weaker luminosity.


How to use a luxmeter?


There are a number of steps that must be followed to achieve optimum performance of this meter:

  1. Place the photoreceptor, which is responsible for absorbing the light, in front of the light source to be measured.
  2. On the luxmeter reader, we must place the appropriate scale, always bearing in mind how strong or weak the light we are going to measure is.
  3. We must press the power button on the reader and wait for the reading to appear on the screen, this step requires a few seconds of waiting. 
  4. Once the reading appears, the result must be multiplied by the number of the scale that was selected. For example, if a scale of 200 lux was chosen and the measurement was 30, the value is 200x30 = 6000.


Through the sensor inside the lux meter we can obtain, at a selected point, the value of the light intensity. In this way we can know if the light, artificial or natural, captured by the meter meets or does not meet the minimum values for its conversion to be adequate.  


What are the parts of a luxmeter?


The luxmeter consists of two parts:

  • A photoreceptor. In charge of perceiving the light intensity to be measured and transforming it into electrical energy, which is then transported to the reader. Part of a photoresistor.
  • A reader. In charge of receiving the electrical signal sent by the photoreceptor and transforming it into a measure of luminosity. This measurement is then displayed on the screen.


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