Lux and Lumens

The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux.

What is lux?
The Las Vegas hotel right? Wrong, that’s the Luxor. Lux (lx) measures illuminance, which is the amount of light on a surface per unit area. A single lux is equal to one lumen per square meter. If the lamp displays its brightness as a measurement of lux, it usually lists a distance from the bulb since any change in distance or bulb type changes the lux level. As an example, if you place a 100 lumen bulb in a flood light that shines on only one square meter of surface, that surface will be lit at 100 lx. However, if you back the flood light away to shine on four square meters, the surface is now lit with 25 lx.

What are lumens?
Lumens are now the most common measurement for a light bulb. The lumen (lm) is a measurement of luminous flux, or the total amount of visible light. To put it simply, the lumen rating is how much total visible light is produced by a light source.

To show the difference between lumens and candela, let’s go back to the example previously used for candela with the partially obscured light bulb.

For a bulb emitting 1 cd, that bulb would also have a luminous intensity of 12.57 lm. Obscuring half the bulb (making it a hemisphere instead of a full sphere), a 1 cd bulb will emit only 6.28 lm. This is because lumens measure the total amount of visible light from a light source.

What is candela?
We’ll start with candela (cd) – also commonly called candlepower. Candela is the base measurement for describing luminous intensity. It tells you how bright the light source is which shows how far away from an object you can be and while still being able to see it.

Any light source eventually becomes too dim to see the further away you are. This is different from lumens (a total light output) because it’s the value of light intensity from any point in a single direction from the light source. Laser pointers or spot lights have the highest candela rating since the majority of their light is focused in a single direction.

A simple comparison is that 1 candela is roughly equivalent to the light from a single candle. If you have a light bulb generating 1 cd and block part of the light, every direction not obscured still produces 1 cd. This is because the same intensity of light can be seen from any non-obscured direction at the same distance. Again, candela is the luminous intensity which describes how bright a light source is.

Since candelas, lux, and lumens are all measuring something different, you can gain insight into how a lamp is useful. A laser pointer will have an extremely low lumen value but a very high candela rating, since a laser pointer doesn’t give off very much light but is visible from great distances.

Light bulbs are usually listed in lumens to show how much illumination the uncovered bulb produces. And lamps will often display a lux value for a set distance to give you an idea of how bright your surface will be for task lighting.

Lux measurement chart

Recommended Light Levels (Illuminance)for Outdoor and Indoor Venues

This is an instructor resource with information to be provided to students as the instructor sees fit.Light Level or Illuminance, is the amount of light measured in a plane surface (or the total luminous
flux incident on a surface, per unit area). The work plane is where the most important tasks in the room or space are performed.
Measuring Units of Light Level – Illuminance
Illuminance is measured in foot candles (ftcd, fc, fcd) or lux (in the metric SI system). A foot candle is actually one lumen of light density per square foot; one lux is one lumen per square meter.
• 1 lux = 1 lumen / sq meter = 0.0001 phot = 0.0929 foot candle (ftcd, fcd)
• 1 phot = 1 lumen / sq centimeter = 10000 lumens / sq meter = 10000 lux
• 1 foot candle (ftcd, fcd) = 1 lumen / sq ft = 10.752 lux

How to convert lux to lumens

How to convert illuminance in lux (lx) to luminous flux in lumens (lm).

You can calculate lumens from lux and surface area.  Lux and lumen units represent different quantities, so you can’t convert lux to lumens.

Lux to lumens calculation formula

Lux to lumens calculation with area in square feet

The luminous flux ΦV in lumens (lm) is equal to 0.09290304 times the illuminance Ev in lux (lx) times the surface area A in square feet (ft2):

ΦV(lm) = 0.09290304× Ev(lx)× A(ft2)

For a spherical light source, the area A is equal to 4 times pi times the squared sphere radius:

A = 4⋅π⋅2

So the luminous flux ΦV in lumens (lm) is equal to 0.09290304 times the illuminance Ev in lux (lx) times 4 times pi times the squared sphere radius r in feet (ft):

ΦV(lm) = 0.09290304× Ev(lx)×4⋅π⋅r(ft)2

So

lumens = 0.09290304×lux × (square feet)

or

lm = 0.09290304×lx × ft2

Lux to lumens calculation with area in square meters

The luminous flux ΦV in lumens (lm) is equal to the illuminance Ev in lux (lx) times the surface area A in square meters (m2):

ΦV(lm) = Ev(lx)× A(m2)

For a spherical light source, the area A is equal to 4 times pi times the squared sphere radius:

A = 4⋅π⋅2

So the luminous flux ΦV in lumens (lm) is equal to the illuminance Ev in lux (lx) times 4 times pi times the squared sphere radius r in meters (m):

ΦV(lm) = Ev(lx)× 4⋅π⋅2

So

lumens = lux × (square meters)

or

lm = lx × m2

Example

What is the luminous flux on a surface of 4 square meters and illuminance of 500 lux?

ΦV(lm) = 500 lux × 4 m2 = 2000 lm

LUX to candela

A lux is equal to 1 lumen per square meter. Calculate a measurement in lumen to a measurement in candela (cd). The candela takes the radiation angle into account, which is measured in steradians (sr). The steradian is the SI unit for a solid angle and is equal to 1/4 pi of the entire sphere.

How to Convert Lux to Candela?

The lux is the International System of Units unit for luminous emittance. It’s used to measure the apparent intensity of light as it strikes a surface. The candela is the International System of Units (SI) unit for luminous intensity. It measures the apparent intensity of a light source in a specific direction.

Convert a measurement in lux (lx) to a measurement in lumen (lm). The lumen is the SI unit of luminous flux, which measures the perceived power of light.

The difference between lux and lumen is that lux takes the area of luminous flux into account, whereas the lumen does not. The lux may therefore be expressed in terms of lumen per unit area. A lux is equal to 1 lumen per square meter.

Calculate a measurement in lumen to a measurement in candela (cd). The candela takes the radiation angle into account, which is measured in steradians (sr). The steradian is the SI unit for a solid angle and is equal to 1/4 pi of the entire sphere. A lumen is equal to 1 candela x steradian.

Express the lux in terms of the candela. Step 1 shows that 1 lx = 1 lm / m ^2. Step 2 shows that 1 lm = 1 cd x sr. This shows that 1 lx = 1 lm / m^2 = 1 cd x sr / m^2, so 1 lx = 1 cd x sr / square meter.

Convert lux to candela. The equation 1 lx = 1 cd x sr / square meter is equivalent to 1 cd = 1 lm x m^2 / sr. A candela is therefore equal to 1 lumen x square meter per steradian.

Resources:

https://sciencing.com/how-to-convert-lux-to-candela-12751786.html

https://www.rapidtables.com/calc/light/how-lux-to-lumen.html

https://blog.1000bulbs.com/home/whats-the-difference-between-candela-lux-and-lumens