How to Calculate Lux: Step-by-Step Guide for Students
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.
FAQs on Lux in Physics: Meaning, Calculation & Applications
1. What is lux and what does it measure?
Lux (lx) is the standard SI unit for illuminance, which is the measure of how much luminous flux, or visible light, is spread over a given area. In simple terms, it quantifies the brightness or intensity of light that falls upon a surface. One lux is defined as one lumen per square metre (1 lx = 1 lm/m²).
2. What is the relationship between lux and lumen?
Lumen and lux are related but measure different aspects of light. A lumen (lm) measures the total amount of visible light emitted by a source in all directions. In contrast, lux (lx) measures the concentration of that light on a particular surface. For example, a 1000-lumen light bulb will produce an illuminance of 1000 lux if its light is focused onto a one-square-metre surface, but only 100 lux if that same light is spread over ten square metres.
3. Is lux a unit of energy?
No, lux is not a unit of energy. This is a common misconception. Lux is a photometric unit, which means it measures light in terms of its perceived brightness to the human eye. The corresponding unit for energy in physics is the joule (J), and for power, it is the watt (W). Watts measure the rate of energy consumption or transfer, whereas lux measures the level of illumination on a surface.
4. How does a lux meter work to measure illuminance?
A lux meter operates using a light-sensitive sensor, typically a photodiode or photocell. When light strikes this sensor, it generates a small electrical current that is directly proportional to the light's intensity. The internal circuitry of the meter then processes this current, converts it into a digital reading, and displays the illuminance value in lux (lx).
5. What are some examples of typical lux levels in everyday environments?
The required illuminance varies greatly depending on the environment and task. Here are some typical examples:
- Direct Sunlight: 32,000 to 100,000 lux
- Overcast Daylight: Approximately 1,000 lux
- Bright Office or Classroom: 400 to 500 lux
- Home Living Room (Ambient): Around 150 lux
- Moonlight (Full Moon): Approximately 0.1 lux
6. Why can't you directly convert the watts of a light bulb to a lux value?
You cannot directly convert watts to lux because they measure entirely different quantities. Watts (W) measure the electrical power consumed by a light bulb, while lux (lx) measures the brightness of light on a surface. The lux value depends on three key factors:
- The bulb's efficiency (how many lumens of light it produces per watt).
- The distance from the light source to the surface.
- The angle and spread of the light beam.
7. What is the difference between lux, lumen, and candela?
These three units describe different aspects of light measurement:
- Lumen (lm): Measures the total amount of visible light emitted from a source, regardless of direction. It quantifies the source's total light output.
- Lux (lx): Measures the amount of light that actually lands on a specific surface area (illuminance). It is a measure of brightness at a location.
- Candela (cd): Measures the luminous intensity, which is the amount of light emitted by a source in a single, specific direction.
8. How does the inverse square law affect the measurement of lux from a light source?
The inverse square law is fundamental to understanding lux. It states that the illuminance (lux) from a point source of light is inversely proportional to the square of the distance from the source. This means if you double the distance from a light, the lux reading on a surface will decrease to one-fourth (1/2²) of its original value. This is why a surface close to a lamp is brightly lit, while the light appears much dimmer farther away.
9. Why is it important to have specific lux levels for different tasks, like reading or working on a computer?
Having appropriate lux levels is crucial for visual performance, comfort, and safety. Different tasks require different levels of illumination to prevent eye strain and ensure accuracy. For example:
- Detailed tasks like reading or drafting require higher illuminance (e.g., 500 lux) for clarity.
- General office work on a computer needs moderate light (e.g., 300-400 lux) to avoid glare on the screen.
- Ambient lighting in hallways or corridors can be much lower (e.g., 100 lux) as it's only needed for safe navigation.
10. What is the full form of 'lux' in the context of lighting?
In the context of lighting and physics, 'lux' is not an acronym and does not have a full form. It is derived directly from the Latin word 'lux', which means "light". It is the internationally recognised SI unit for measuring illuminance.
