Online Lighting Calculators

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Lighting Calculators

1. Candela to 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.

So why all of the different ratings? 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.

For uniform, isotropic light source, the luminous flux Φ_v in lumens (lm) is equal to the luminous intensity I_v in candela (cd),

times the solid angle Ω in steradians (sr):

Φ_v(lm) = I_v(cd) × Ω_(sr)

The solid angle Ω in steradians (sr) is equal to 2 times pi times 1 minus cosine of half the cone apex angle θ in degrees (°):

Ω_(sr) = 2π(1 - cos(θ/2))

The luminous flux Φ_vin lumens (lm) is equal to the luminous intensity I_v in candela (cd),

times 2 times pi times 1 minus cosine of half the apex angle θ in degrees (°):

Φ_v(lm) = I_v(cd) × ( 2π(1 - cos(θ/2)) )

lumens = candela × ( 2π(1 - cos(degrees/2)) )

lm = cd × ( 2π(1 - cos(°/2)) )

Lumens and candela are both lighting measurement units, but they measure different aspects of a lighting device. Knowing about the difference between lumens and candelas will help you choose lighting devices that are ideal for your workplace. More importantly, this knowledge will ensure that your lighting supplier does not cheat you. Some may try to convince you about the merits of a lighting device by quoting impressive lumens or candela numbers. However, they may not be relevant for your needs.

Defining Lumens and Candela

Lumens refers to the total amount of light a lighting apparatus emits. We represent lumens as L. The higher the lumens value of a lighting device, the greater the area it illuminates. On the other hand, candela refers to the amount of light emitted by a lighting device in a particular direction. We represent candela with CD.

The lumens unit does not consider the spread or divergence of the light being emitted by the device. But the candela value of a lighting device is concerned primarily with a focused beam of light. For instance, a standard fluorescent lighting device that emits a wide-spread beam can have a rating of 1,700 lumens and 135 candelas. But if the light emitted from this apparatus narrows and must shine within a 20-degree beam, then its candela value will increase to 18,000.

Candela is another word for candlepower. Candlepower measurement is from times when the candle was the primary source of illumination. The word candlepower was substituted with the term candela in 1948. One candela is approximately equal to the light that a candle with specific dimensions, produces. In this context it is worth mentioning that the term Maximum Beam Candlepower (MBCP) is often used to refer to the maximum intensity of the focused beam of light emitted usually from the center of the lighting device, which is usually the source, and projected in a single direction.

So what is the difference?

The difference between lumens and candela can be better understood in the context of a comparison between a pencil torch and an incandescent or a fluorescent bulb. The torch has a high candela value because its light concentrates into a single point. It projects in a single direction as a high-intensity beam. While buying a torch, don't concern yourself with its lumens value because you will only use this device to illuminate a very small area. On the other hand, an incandescent light or a fluorescent light bulb has a low candela value but a high lumens value. This is because it emits a greater amount of light spread over a larger area than the pencil torch.

2. Candela to lux

It happens far too often. You walk into a store, pick up light bulb or a handheld spotlight and look at the specifications- only to find that one says lumens, another says lux, and the last reads something about candela. Inside, you question why no one uses the same system for measuring these things, and take your best guess based on the other items on the shelf. The short answer is that they’re all separate standards for measuring different kinds of lighting effects. Let’s break it down into the long answer to dispel a few more shadows.

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.

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.

Candela to lux calculation with distance in feet

The illuminance E_v in lux (lx) is equal to 10.76391 times the luminous intensity I_v in candela (cd),

divided by the square distance from the light source d² in square feet (ft²):

E_v(lx) = 10.76391 × I_v(cd) / (d_(ft))²

Candela to lux calculation with distance in meters

The illuminance E_v in lux (lx) is equal to the luminous intensity I_v in candela (cd),

divided by the square distance from the light source d² in square meters (m²):

E_v(lx) = I_v(cd) / (d_(m))²

3. Footcandles to luxFootcandles to lux

Take the same beam of light and measure it from one metre away from the source and that will give you the lux measurement. One footcandle equals 10.764 lux. One lux equals 0.09 ftc.

You may use the Flash-based Online Foot-candles/Lux Converter to convert either a reading you have taken in Lux or Foot-candles using a light meter or the sensitivity factor from a light detector's calibration certificate in Lux or Foot-candles, to the opposite unit in Lux or foot-candles. Please note: The sensitivity factor on your calibration certificate is expressed in (Amps)(m²)(lm^-1) for Lux or (Amps)(ft²)(lm^-1) for foot-candles, but you may simply choose Lux or Foot-candles in the converter.

You may enter the number to be converted either as a decimal (i.e. "0.000341") or as a number in scientific notation (i.e. "3.41e-04"). When using scientific notation, make sure you type the "e" before the exponent. You do not have to type the units of this number as you will select those in step 2.

Lux to footcandles conversion formula

The illuminance E_v in lux (lx) is equal to the illuminance E_v in footcandles (fc) divided by 0.09290304:

E_v(lx) = E_v(fc) / 0.09290304 = E_v(fc) × 10.76391

4. Lumens to candelaLumens to candela

The Lux or Candela lumens calculator below will help you choose the lighting solution that best meets the specific requirements of your installation. Airfal offers a wide range of high quality and energy efficient LED lights for a wide range of industrial applications and to meet multiple technical requirements.

With this calculator, you can convert a lumens value of a product into a lux value if you know the beam angle and measurement distance. Enter the distance and angle of the beam, then enter a lumens value and click on’Convert to Lux’ or a Lux value and click on’Convert to Lumens’. You can also convert to Candelas.

Iluminance (lux): the iluminance refers to the incidence of the light flux on a surface, per unit of surface
The unit of illumination is the lumen per square metre: lux (lux). A single lux is equal to one lumen per square meter.

Luminous flux (Lm): Total amount of light emitted in all directions by the light source. The lumen (unit lm) gives the total luminous flux of a light source by multiplying the intensity (in candela) by the angular span over which the light is emitted. .

Here are some examples:

Candle 12 lm
Incandescent lamp 60 W 750 lm
LED 1 W 130 lm 130 lm

For example, a 1 candela light source will produce 1 lumen per square meter at a distance of 1 meter.
A lux is equal to one lumen per square meter (lux = lumens/m2).

Lumens to candela calculation

For uniform, isotropic light source, the luminous intensity I_v in candela (cd) is equal to the luminous flux Φ_vin lumens (lm),

divided by the solid angle Ω in steradians (sr):

I_v(cd) = Φ_v(lm) / Ω_(sr)

The solid angle Ω in steradians (sr) is equal to 2 times pi times 1 minus cosine of half the cone apex angle θ in degrees (º):

Ω_(sr) = 2π(1 - cos(θ/2))

The luminous intensity I_v in candela (cd) is equal to the luminous flux Φ_vin lumens (lm),

divided by 2 times pi times 1 minus cosine of half the apex angle θ in degrees (º):

I_v(cd) = Φ_v(lm) / ( 2π(1 - cos(θ/2)) )

candela = lumens / ( 2π(1 - cos(degrees/2)) )

cd = lm / ( 2π(1 - cos(º/2)) )

5. Lumens to lux

When purchasing a lamp, it is nice to know how much light the lamp is giving out exactly. Sometimes this is indicated in Lumen and sometimes in Lux. What do these values mean and what's the difference?

Lumen is the unit of the light current. This indicates the total amount of light emitted by the light source, no matter what direction it is. In Lumen, efficiency is especially important because it indicates how efficient the light source converts power into light. This is called the Lumen / Watts ratio (Lm / W). With LED lights this value is much higher because they need less power to generate a particular Lumen current.

Lux is used to measure the amount of light output in a given area - one lux is equal to one lumen per square meter. It enables us to measure the total "amount" of visible light present and the intensity of the illumination on a surface. Thus, it is important in what direction the light source shines. This value is irrespective of the number of light sources it uses, the colour or the structure where the light shines. If the amount of Lux is indicated by a lamp, it is usually in the centre of the beam where the light intensity is highest.

The difference

The difference between lumen and lux is that the lux takes into account the area over which the luminous flux (lumens) 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.

For example:

A 500 Lumen light source illuminates 1 square meter with 500 Lux. The same light source that needs to illuminate 10 square meters, which illuminates this area with only 50 Lux.

Lumens to lux calculation formula

Calculation with area in square feet -

The illuminance E_v in lux (lx) is equal to 10.76391 times the luminous flux Φ_V in lumens (lm) divided by the surface area A in square feet (ft²):

E_v(lx) = 10.76391 × Φ_V(lm) / A_(ft2₎

The illuminance E_v in lux (lx) is equal to 10.76391 times the luminous flux Φ_V in lumens (lm) divided by 4 times pi times the squared sphere radius r in feet (ft):

E_v(lx) = 10.76391 × Φ_V(lm) / (4⋅π⋅r_(ft)²)

Calculation with area in square meters

The illuminance E_v in lux (lx) is equal to the luminous flux Φ_V in lumens (lm) divided by the surface area A in square meters (m²):

E_v(lx) = Φ_V(lm) / A_(m2₎

The illuminance E_v in lux (lx) is equal to the luminous flux Φ_V in lumens (lm) divided by 4 times pi times the squared sphere radius r in meters (m):

E_v(lx) = Φ_V(lm) / (4⋅π⋅r ²_(m2₎)

6. Lumens to millicandela

It is a conversion calculator that is used to convert the luminous flux in lumens (lm) to the luminous intensity in Millicandela (mcd). It uses a simple program in executing the calculations making it an effective online calculator. The initial step is to enter the value of the luminous flux in lumens. Proceed to the next text field where you enter the apex angle in degrees. Afterwards, click on the ‘Calculate’ button to execute the conversion. To perform new calculations, you will use the ‘Reset’ button to clear the previous calculations from the text fields. The luminous intensity result in Millicandela will be displayed in the bottom platform of the calculator below the two controls.

For example;

Determine the luminous intensity in Millicandela if the luminous flux in lumens is 324 (lm) and the apex angle in degrees is 65°.

First, you need to enter 324 in the luminous flux in lumens text field and 65 as the apex angle in degrees respectively. Afterwards, click on the ‘Calculate’ button. The luminous intensity result in Millicandela will be displayed as; 329268.10307 (mcd).

This calculator uses particular formulas in executing the calculations

For uniform, isotropic light source, the luminous intensity I_v in millicandela (mcd) is equal to 1000 times the luminous flux Φ_vin lumens (lm),

divided by the solid angle Ω in steradians (sr):

I_v(mcd) = 1000 × Φ_v(lm) / Ω_(sr)

The solid angle Ω in steradians (sr) is equal to 2 times pi times 1 minus cosine of half the apex angle θ in degrees (°):

Ω_(sr) = 2π(1 - cos(θ/2))

The luminous intensity I_v in millicandela (mcd) is equal to 1000 times the luminous flux Φ_vin lumens (lm),

divided by 2 times pi times 1 minus cosine of half the apex angle θ in degrees (°):

I_v(mcd) = 1000 × Φ_v(lm) / ( 2π(1 - cos(θ/2)) )

millicandela = 1000 × lumens / ( 2π(1 - cos(degrees/2)) )

mcd = 1000 × lm / ( 2π(1 - cos(°/2)) )

7. Lumens to watts

Old light sources such as incandescent bulbs or halogen lamps were easy to choose. Here the wattage was the most important indicator for the brightness. If a 60W bulb fails, it could be simply replaced by a new 60W bulb. The replaced lamp was just as bright as before and there was no need to worry about the correct brightness.

For LED lamps, the power consumption is also indicated in watts. However, the wattage plays a minor role now and allows only a very rough conclusion on the actual brightness. Different components in various LED lamps lead to varying brightness with the same power consumption.

For example, a 5W LED illuminant of type A can light up much brighter than a second 5W LED lamp of type B. This is possible because lamp A could achieve a better efficiency through higher quality components and therefore emits more light. For the comparability of LED lamps the brightness is indicated in lumens.

Lumens to watts calculation formula

Energy saving lamps have high luminous efficacy (more lumens per watt).

The power P in watts (W) is equal to the luminous flux Φ_V in lumens (lm), divided by the luminous efficacy η in lumens per watt (lm/W):

P_(W) = Φ_V(lm) / η_(lm/W)

Lumens to watts table

Lumens	Incandescent light bulb (watts)	Fluorescent / LED (watts)
375 lm	25 W	6.23 W
600 lm	40 W	10 W
900 lm	60 W	15 W
1125 lm	75 W	18.75 W
1500 lm	100 W	25 W
2250 lm	150 W	37.5 W
3000 lm	200 W	50 W

8. 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.

Lux to candela calculation with distance in feet

The luminous intensity I_v in candela (cd) is equal to 0.09290304 times the illuminance E_v in lux (lx),

times the square distance from the light source d² in square feet (ft²):

I_v(cd) = 0.09290304 × E_v(lx) × (d_(ft))²

Lux to candela calculation with distance in meters

The luminous intensity I_v in candela (cd) is equal to the illuminance E_v in lux (lx),

times the square distance from the light source d² in square meters (m²):

I_v(cd) = E_v(lx) × (d_(m))²

9. Lux to footcandles

Illuminance in lux (lx) to footcandles (fc) conversion calculator and how to convert.

Enter the illuminance in lux (lx) and press the Calculate button to get the illuminance in footcandles (fc):

Lux to footcandles conversion formula

The illuminance E_v in footcandles (fc) is equal to the illuminance E_v in lux (lx) times 0.09290304:

E_v(fc) = E_v(lx) × 0.09290304 = E_v(lx) / 10.76391

10. Lux to lumens

The lux to lumens calculator below will help you choose the illumination solution that best meets the specific requirements of your application. Banner Engineering offers a number of high-quality, energy-efficient LED lighting products that can be used to solve a diverse range of industrial applications.

Lux: The amount of light that is cast on a surface is called illuminance, which is measured in lux. This can be thought of as light intensity within a specific area.

Lumens: The total output of visible light from a light source is measured in lumens. Typically, the more lumens a light fixture provides, the brighter it is.

One lux is equal to one lumen per square meter (lux = lumens/m²). Essentially, as light travels from the emitter, it will disperse throughout an area. The further the light has to travel the more it will be dispersed. Therefore, the amount of lux in an area or on a surface can vary depending upon the distance the light travels and the angle at which it is dispersed.

Where the lumens provided by a given emitter are the same, light that travels less distance and/or is more narrowly focused will appear brighter because lux are concentrated in a smaller area than light that has traveled a greater distance and has been dispersed over a wider area.

Illuminance in lux (lx) to luminous flux in lumens (lm) calculator and how to calculate.

Enter the illuminance in lux (lx), select area unit type, enter radius in meters for spherical light source or surface area

Lux to lumens calculation formula

Calculation with area in square feet

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

Φ_V(lm) = 0.09290304 × E_v(lx) × A_(ft2₎

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

Φ_V(lm) = 0.09290304 × E_v(lx) × 4⋅π⋅r_(ft) ²

Calculation with area in square meters

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

Φ_V(lm) = E_v(lx) × A_(m2₎

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

Φ_V(lm) = E_v(lx) × 4⋅π⋅r ²_(m2₎

11. Lux to watts

It is a conversion calculator used to convert the illuminance in lux (lx) to the electric power in watts (W). It has four text fields and two controls that perform different functions of the calculator. The ‘Calculate’ button is used to execute the conversions from lux to watts while the ‘Reset’ button is used to erase all data of the previous calculations from the text fields.

The initial step is to enter the value of illuminance in lux. Select the light source in the options provided by the calculator. You can also fill in the luminous efficacy in lumens per watt in the next text field. Afterwards, enter the surface area or the spherical radius and then click the ‘Calculate’ button. The power result in watts will be displayed in the bottom platform below the two controls.

For example;

Determine the power consumption if the illuminance in lux is 450 (lx) with a luminous efficacy of 17 lumens per watt and a surface area of 100 square feet.

When using the calculator, you will first enter 450 in the first text field, and then 17 as the luminous efficacy in lumens per watt. Proceed to the next text field and enter 100 as the surface area and then click the ‘Calculate’ button.

The spherical radius will be calculated automatically as; 2.8209479177 ft and the power result in watts as; 245.91981176 watts (W).

Lux to watts calculation formula

Lux to watts calculation with area in square feet

The power P in watts (W) is equal to the 0.09290304 times the illuminance E_v in lux (lx) times the surface area A in square feet (ft²), divided by the luminous efficacy η in lumens per watt (lm/W):

P_(W) = 0.09290304 × E_v(lx) × A_(ft2₎ / η_(lm/W)

Lux to watts calculation with area in square meters

The power P in watts (W) is equal to the illuminance E_v in lux (lx) times the surface area A in square meters (m²), divided by the luminous efficacy η in lumens per watt (lm/W):

P_(W) = E_v(lx) × A_(m2₎ / η_(lm/W)

12. Millicandela to lumens

It is a conversion calculator used to convert the luminous intensity in Millicandela to luminous flux in lumens. It is comprised of two text fields and two controls each performing an independent function of the calculator. Since Millicandela and lumens are of different quantities, the apex angle in degrees is required for the conversion to be successful. The initial step is to enter the value of luminous intensity in Millicandela. Proceed to the next text field and enter the apex angle in degrees then click the ‘Calculate’ button. If you want to perform new conversions you will use the ‘Reset’ button to clear the previous calculations from the text fields. The luminous flux result in lumens will be displayed below the two controls in the bottom platform of the calculator.

For example;

Determine the luminous flux in lumens if the luminous intensity in Millicandela is 156 (mcd) with an apx angle of 64°.

When using the calculator, you will first enter 156 as the value in Millicandela and 64 as the apex angle in degrees respectively. Afterwards, click the ‘Calculate’ button to initiate the conversion.

The luminous flux result in lumens will be; 0.14893974726 lm.

For uniform, isotropic light source, the luminous flux Φ_v in lumens (lm) is equal to the luminous intensity I_v in millicandela (mcd),

times the solid angle Ω in steradians (sr) divided by 1000:

Φ_v(lm) = I_v(mcd) × Ω_(sr) / 1000

The solid angle Ω in steradians (sr) is equal to 2 times pi times 1 minus cosine of half the apex angle θ in degrees (°):

Ω_(sr) = 2π(1 - cos(θ/2))

The luminous flux Φ_v in lumens (lm) is equal to the luminous intensity I_v in millicandela (mcd),

times 2 times pi times 1 minus cosine of half the apex angle θ in degrees (°) divided by 1000:

Φ_v(lm) = I_v(mcd) × ( 2π(1 - cos(θ/2)) ) / 1000

lumens = millicandela × ( 2π(1 - cos(degrees/2)) ) / 1000

lm = mcd × ( 2π(1 - cos(°/2)) ) / 1000

13. Watts to lumens

What the watt?When buying an incandescent bulb, the watts rating gives consumers a good indication of how bright a bulb is. The more watts, the brighter the bulb.

That rule doesn't apply to LED bulbs, though. An LED that uses 60 watts is in no way comparable to an incandescent bulb that uses 60 watts. In fact, a 60-watt LED just may blind you. LEDs are designed to use less energy and naturally have a lower watt rating. This means it's useless to use watts to determine brightness.

To fix the problem, bulb companies have started using lumens to rate bulbs. This gives you a more accurate indication of how much light to expect from an LED.

Why lumens?Measuring in lumens isn't a new concept designed just for LEDs. It's a rating that's been used for decades as a measurement of how much light a bulb (or anything else) emits.

watts (W) to luminous flux in lumens (lm) calculator and how to calculate.

Energy saving lamps have high luminous efficacy (more lumens per watt).

The luminous flux Φ_V in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W):

Φ_V(lm) = P_(W) × η_(lm/W)

14. Watts to lux

It is used to convert the electric power in watt (W) to illuminance in lux (lx). It has four text fields and options of executing the conversion. The watts to lux calculator also has two controls, ‘Calculate’ and ‘Reset’ buttons which operate independently performing different functions of the calculator. The first step is to enter the value of power in watts.

Select the light source in the options provided by the calculator. If you do not find the particular appliance, you will manually enter the value of luminous efficacy in lumens per watt. The next step is to enter the surface are or the spherical radius and then click the ‘Calculate’ button to initiate the process of conversion. Your results are displayed within a single click in the bottom platform of the calculator below the two controls.

For example;

Determine the illuminance in lux of a LED lamp having 240 watts and a surface area of 1000 square feet.

Watts to lux calculation formula

Watts to lux calculation with area in square feet

The illuminance E_v in lux (lx) is equal to 10.76391 times the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W) divided by the surface area A in square feet (ft²):

E_v(lx) = 10.76391 × P_(W) × η_(lm/W) / A_(ft2₎

Watts to lux calculation with area in square meters

The illuminance E_v in lux (lx) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W) divided by the surface area A in square meters (m²):

E_v(lx) = P_(W) × η_(lm/W) / A_(m2₎