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Several Units About the Brightness of the Torches:  Lumen, Candela and Lux

Several Units About the Brightness of the Torches: Lumen, Candela and Lux

10 Jun, 2022

In everyday life, is the brightness of a torch expressed in the same units as other lamps?


When you and your friend are walking under a street light at night and he asks you, "How bright is this light?" What would you answer?


Do you know how bright your torch is when you need to use it when you go out at night?

What do you think is brightness?


To answer this question you need to know something about the physics of luminance units.


The commonly used units related to brightness in physics are:

1. "Candela" :which represents luminous intensity

2. "Lumen": which means luminous flux

3. "lux" :which means illuminance


What do these 3 units of luminance represent? Are there any differences between them?


When you need to buy a torch, you will want to know these things because you need to know which one has the right brightness for you. Similarly, when you buy a mobile power bank to charge your phone, you all need to know how many milliamps it has and how many times a rechargeable battery can supply your phone's battery for every full charge so you can plan your time to use your phone during the journey.


So, this is the knowledge we need to know to meet the demands of our daily lives.


Is watt a unit of brightness? The higher the wattage, the brighter the light will be?


Most people have helped their parents buy light bulbs when they were children. When I was a child my father always told me: "Go to the store and buy a 10W bulb. You need a screw port, not a bayonet. The rest of the money is for you to buy your candy", I always didn't understand, but did as I was told.


The light bulbs in the house are usually marked with the number of watts, where "W" is a unit of physics, the full name being "watt", a unit of power measurement. A 50 watt incandescent light bulb is no brighter than a 10 watt LED torch, so watt is not a unit of brightness.


Well, let me explain to you about "light brightness".

To describe brightness, let's first understand what brightness is. The eyes can see something glowing because they are actually picking up the electromagnetic energy emitted by that thing in the visible wavelength range, and the more energy your eyes pick up, the brighter they feels.

Brightness is related to energy, but the power of the bulb does not fully represent brightness.

1. Luminous intensity——Candela

The most basic of these units is the luminous intensity, Candela, the symbol is cd, which is used to indicate the intensity of the luminous intensity of the light source in one direction. Candela, like many other units of physics, has an incomprehensible definition: at a standard atmospheric pressure of 101325 Newtons per square metre, an absolute “black body” with an area equal to 1/60 square centimeter (that is, it can absorb all external light without Ideal reflection object), at the solidification temperature of pure platinum (Pt) (about 2042K or 1769°C), the luminous intensity along the vertical direction is 1 Candela (1cd).

If you can't understand it after reading it five times, try to understand it in another way: Candela is the Latin term for candle, so the brightness of a candle is 1 Candela, and a 100 cd LED-Spot is as bright as 100 candles.


If you want to know how bright a 1 Candela is, buy a candle and light it.

2. Luminous flux——Lumen

With this basic unit of brightness, Candela, we can define other units.

Torch enthusiasts often ask how many lumens a torch is. The "lumen" here is a unit to measure the total luminous brightness of the light source, and its definition is: the luminous intensity is the amount of light passed by a 1Candela luminous body in a spherical area of one square degree.


  • Candela & Lumen


The luminous flux of torches used in everyday life is a few dozen to a couple of hundred lumens, while some high-intensity torches can have a luminous flux of several thousand or even tens of thousands of lumens.

3. Illuminance-Lux

When light shines on an object, we say that the surface of the object is illuminated. But to what extent is the surface of a light-receiving object illuminated by light, and how much light is  exactly shining on that object? This needs to be measured by illuminance: the quotient of the luminous flux irradiated on a certain surface unit divided by the area of  this surface unit. (Lumen Flux: Symbol: Φ, Unit: Lumen)


We all know from experience when using a torch that if the spot of light is concentrated very small, it looks very bright, whereas if the spot is enlarged, it is not so bright. This brings us to another concept related to luminous flux or brightness.

The unit of illuminance is Lux. The definition of Lux is the luminous flux per unit area. When the luminous flux of 1lm is uniformly irradiated on the area of 1㎡, the illuminance on this area is equal to 1lx.

(The luminous flux of 1 square meter area is 1 lumen, which is 1lux, and the formula is 1lux=1lm/)


  • The relationship between Lux and Lumen

To understand this, an analogy can be drawn with the units of pressure. The same force of 10 Newtons is applied to your hand through a tile and you won't feel much; but if the same force is applied to your hand through a pin, it will be deeply embedded in your flesh.


Similarly, a 100 lumen torch used to illuminate an entire basketball court would barely be able to see the words in a book; but if it were converged into a laser beam, it would be much brighter!


l Luminance - Candela per square metre (in cd/) and Illuminance - Lumen per square metre (in lm/)


Luminance refers to the intensity of the light that is reflected by the illuminated object and enters the human eye. The unit is cd/=1 nt; and the illuminance refers to how much light actually shines on the illuminated object, and the unit is ( lm/㎡=lux);

Luminance is directly related to the reflectivity of the illuminated object. Objects with smooth surfaces have high reflectivity. Under the same illuminance, we feel different luminance when illuminating objects with different reflectivity, but the illuminance is relatively constant.



1. Lumens are how much light is given off

2. Lux is how bright the surface of the object is

3. Candela measures the visibility of the human eye from a light source.

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