Lux - Unit of Energy Measurement

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Introduction to Lux

Lux is a unit of illumination which is in the International System of Units that is SI. One lux Latin for “light” is said to be the amount of illumination that is provided when one lumen is evenly distributed over an area of one square metre. This is also said to be equivalent to the illumination that generally would exist on a surface all points of which are one metre from a point source of one international candle or candela. One lux is said to be equal to 0.0929 foot-candle. In this article we are going to discuss a few more things related to this topic lux.


Lux Definition

The lux which has the symbol: lx is the SI unit that is derived from illuminance measuring the flux luminous per unit area. The lux is said to be equal to one lumen per square metre. In photometry this is used as an intensity measure. This is so because it is perceived by the human eye of light that hits or passes through a surface. It is analogous to the unit that is radiometric watt per square metre.

But with the power at each of the weighted wavelengths according to the luminosity function there is a standardized human model visual brightness perception. 

In English,  the word -"lux" is used as both the singular and plural form.

The word is said to be derived from the Latin word that is for "light",- lux.

The power per unit area on a surface that is  illuminated and sometimes known as areance is distinguished from the similar quantity for the source. 

In radiometry the areas of the surface may be known as irradiance and luminous areance may be known as illuminance. This is the importance in the practical quantity  in judging whether an area is lighted well enough for reading or other activities. The illuminance is said to be measured in lux. 

But the unit that is the older one is the footcandle which is still encountered.

The lux is defined as a lumen per square meter and lux is said to be a unit of illuminance. A term which is more equivalent is luminous density of flux.

As such, we can say that it measures the amount of visible light striking a surface. The standard symbol for it is Ev. We must take into account the sensitivity of the eye for the wavelengths of light which is involved in this. But that is taken care of in the establishing the lumens number.


Lux Light Measurement

We are going to understand this by performing an experiment. For performing this experiment we require:

  • Any Smartphone or tablet with internet access to it and permission to download and install an app as well.

  • A surveillance of adults that is to help verify and download the app.

  • There should be different light sources that are the flashlight, lamp, ceiling light, etc.

  • It should be a different location that is a dark closet, room with windows, outdoors, etc.

  • The ruler that is optional.


Preparation of the Experiment

  • We can ask an adult to help us search for a "lux meter" or we can say a"light meter" app on a smartphone or tablet. There are many free options also which are available please note that some apps which might have ads or in-app purchases enabled.

  • Then we get to know our lux meter app.


Procedure to Follow

  • We will test how lux readings change with distance from a fixed source of light. For example we can directly stand under a ceiling light. And we can hold our phone with the screen facing up and move the phone up and down for a time being. 

  • Now we need to alternatively hold the phone and sideways and aim it toward a floor lamp as you walk closer to and farther away from the lamp. How do the readings change as the distance changes?

  • Now we need to compare different lights which are the artificial sources of light at the same distance. We  can even use a ruler for this or any convenient object or a body part such as our forearm as a spacer.

  • The exact distance doesn't matter until  we keep it constant. Now we can ask a few questions that are how does a flashlight compare with a light bulb? What about the light which comes from a TV or computer screen? What light source in our house is the brightest? 

  • Finally now we can measure ambient light levels in different locations. We need to turn off all sources of artificial light and measure the same way which is mentioned above. 


The Observations and Results

We  probably will notice how dramatically lux changes with distance from a light source. We  might only read a few hundreds or tens of lux when we are across the room from a light bulb. But originally if we hold our phone right up to the bulb, then we can see that the reading could be in the thousands or even tens of thousands. This is because of a mathematical relationship known as the inverse square law. 

As the light expands outward from the main source the amount of light which is hitting each area drops off very rapidly. The sun is so far away we might find it surprising that the reading of lux in direct sunlight is so high that it goes in the tens of thousands of lux. This gives us a sense of just how very bright the sun itself is and this way we can measure light also.


FAQ (Frequently Asked Questions)

1. Explain how is lux level measured?

The lux is simply the unit of measure that is used to describe the number of lumens which are falling on a square foot or the footcandles or square meter of a surface. So If all of those lumens are around 1,000 lumens which are spread over a surface area of 1 square meter, then we would have an illuminance of 1,000 lux – that is  the brightness of an overcast day.

2. Explain how many Lux is a 100 watt light bulb?

1600 lumens can replace a 100W incandescent bulb. If we want something dimmer so we need to go for less lumens and if we prefer brighter light look for more lumens.

3. Explain how lux meter work?

We know that a lux meter works by using a photocell to capture light. The meter then generally converts this light to an electrical current. That is the measuring of this current allows the device to calculate the lux value of the light it captured. The measure of the light of the lux meter varies depending on the light's intensity and distance.