When buying a light fixture, the lumen rating is what you see first. But do you know there are other equally important light measuring units like candela and lux?
If you are buying spotlights or laser light, consider candela for focused lighting. And for general lighting fixtures like LED strips, tube lights, bulbs, etc., check out the lumen rating. However, for lighting efficiency, the lux value gives you more accurate information.
Let’s explore candela, lux, and lumen and learn how to use these units to choose the best light fixture for any application:
What is Candela?
Candela (cd) is the unit of luminous intensity. This term originated from the Latin word ‘Candela’ and is related to the intensity of a candle. For instance, one candela is approximately equal to the brightness of a candle.
However, with the advancement of technology, the definition of candela has changed over generations. In 1979, General Conference on Weights and Measures (CGPM) defined candela as:
“The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.”
However, the latest definition, as modified on May 20, 2019, is far more complicated and mainly targeted at technical experts. This goes as:
“…… the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012 Hz, Kcd, to be 683 when expressed in the unit lm⋅W−1, which is equal to cd⋅sr⋅W−1, or cd⋅sr⋅kg−1⋅m−2⋅s3, where the kilogram, meter and second are=-0987654321`defined in terms of h, c and ΔνCs.”
To simplify, candela measures the intensity of light in a particular direction or angle. For instance, think of a spotlight that directs the light in a specific direction and of a bulb that spreads the light in all directions. Which will have a higher candela value?
The spotlight has a narrow beam angle and gives focused lighting. Thus, its light intensity in that particular direction will be high. In contrast, the lighting of the bulb will be diffused in all directions, resulting in a low candela value.
What is Lux?
Lux (lx) is the measuring unit for illuminance or luminous flux per unit area. It measures the amount of light per square meter of area.
1 lux equals the total lighting amount of a 1 square meter surface lit by a light source of 1 lumen located 1 meter apart. To simplify, 1 lux is equal to 1 lumen per square meter.
To simplify, 1 lux is equal to 1 lumen per square meter.
So, for a lux rating, you need to consider the room’s area or the light source’s distance from a particular point. In this case, the value of lux is inversely proportional to the square to the distance.
For example, a light with 500 lumens lits a one square meter surface. So its lux rating will be (500 lm ፥ 1 m2) = 500 lx. Now, install the same light of 500 lumens directed toward a five-square-meter area. The lux value here will be (500 lm ፥ 5 m2) = 100 lx, decreasing by 5 times.
That is, as the distance between the surface and the light source increases, the value of lux decreases. Thus, the light that seems bright in a small room will not look as bright when put in a large room.
What is Lumen?
Lumen measures the value of luminous flux. It indicates the overall brightness of a light source from all directions.
Lumen is defined as the amount of visible light a light source of 1 candela emits into 1 unit of solid angle (1/4π of the area of a sphere). Unlike candela (cd), lumen does not calculate the illumination of a particular direction. Instead, it considers the total light output of the source from all directions.
For instance, a bulb with 3 candela ratings has a lumen rating of 38 lm (approx). Now if you obscure half of the bulb and let the light illuminate through a single hemisphere, the candela value will still remain the same. This is because, from a particular distance, the light intensity of the obscured bulb is still the same. However, the lumen value will be reduced to half and be 19 lm as you have blocked half of the light source.
Relationships Between Candela, Lux, and Lumens
The most common relation among these three terms is that they all are the measuring unit of light. To clarify the concept, let’s get to the root of these terms-
a. Lumens and Candela: Relationship
Lumens is a derivative form of candela. Both terms measure luminous; that is, they state the brightness of a light source. The difference is that lumens take account of light beams from all directions. In contrast, Candela considers that for a particular direction. A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of:
| 1 cd × 4πsr = 4π cd · sr ≈12.57 lm 1 candela = 12.57 lumens |
Note: Since candela and lumen are two different units, you can interchange them.
b. Lumens and Lux: Relationship
Lux is derived from the lumen, so the lumen value has a greater impact on lux. In mathematical terms, 1 lx = 1lm/m². This equation provides a more accurate representation of light’s brightness than lumens.
c. Lux and Candela: Relationship
Lux is the metric version of foot candles (an old term referring to candela). Thus, candela, lux, and lumens are interrelated with each other. They state three separate identities, but they are connected and convertible.
| Derived from Derived from Lux ———————→ Lumens ——————–→ Candela 1 lx = 1 lm/m2 = 1 cd·sr/m2 |
Candela vs Lux vs Lumens – What Are The Differences?
Candela vs Lux
Candela defines the brightness of the light at a specific region. In contrast, lux indicates the amount of light reaching a definite surface area. That is, lux is related to the area of the illuminated space, but candela doesn’t deal with surface area.
However, the value of lux depends on the surface area and distance between the light source and the surface. Besides, it also depends on the candela value. But candela is independent of these factors.
| Criteria | Candela (cd) | Lux (lx) |
| Measures | Light’s intensity at a specific direction | Amount of light in a specific surface area |
| Distance of Light Source | Doesn’t affect | Affects; inversely proportional to distance. |
| Area of Illuminated Surface | Doesn’t affect | Affects; inversely proportional to distance. |
Lux vs Lumens
Lumen describes the amount of light illuminated by a light source. In contrast, lux defines the amount of light falling into a certain area of the surface.
By lumen, you can identify how bright a light source is. A higher lumen rating means brighter light output. Meanwhile, a high lux rating means the light ray falling in a specific area shines brightly. So, the difference here is that the value of lux depends on the distance between the light source and the lighting surface. But lumen is independent of distance.
Though lumen is the most used rating for measuring light, lux is a more important value to measure light’s effectiveness. This is why you should consider the lux value for well-lit task lighting. Higher lux value ensures appropriate brightness in your workstation, allowing you to work with ease.
| Criteria | Lux | Lumen |
| Determines | The amount of light that reaches a surface | The total light output of a light source |
| Dependence | Value depends on the distance and lumen rating | Value depends on the capability of the light source |
| Distance of Light Source | Matters; as the surface area increases, the lux value decreases | Doesn’t matter |
| Surface area | Matters; as surface area increases, lux value decreases | Doesn’t matter |
| Typical Usage | Task lighting, focused lighting, for example: street lights, spotlights, etc. | General lighting, such as LED strip lights, floodlights, etc. |
Lumens vs Candela
The main difference between lumen and candela is that lumen considers the lighting output from a light source. In contrast, candela measures the intensity of the light in a specific direction.
By using the lumen rating, you can get an idea of the brightness of the light source. But candela lets you know how concentrated a light will appear in a particular spot. So, a light fixture with a high lumen value can light up a large space. Meanwhile, a high candela light means the fixture directs a focused beam, resulting in more concentrated light.
Therefore, if you are looking for general lighting, consider lights with a high-lumen rating. And if you are into accent or task lighting and need focused illumination, consider high candela-rated bulbs.
| Criteria | Lumen (lm) | Candela (cd) |
| Consideration of light direction | All direction (360° sphere) | Specific direction (at a particular angle) |
| Beam Angle | Wide | Narrow |
| Dependence | Value depends on the total light output of a source | Value depends on beam concentration in a specific direction. |
| Typical Usage | General lighting; for example, tube light, bulb, floodlight, etc. | Focused lighting, such as spotlights, laser lights, track lights, etc. |
Overall Differentiating Chart: Candela vs Lux vs Lumens
| Specifications | Candela | Lux | Lumens |
| Symbol (SI) | cd | lx | lm |
| Definition | Candela is the brightness level of a light source at a particular angle and direction. | The amount of illumination that strikes per square meter area is Lux. | Lumen indicates the total output of light in all directions. |
| Measures | Luminous Intensity | Illuminance | Luminous Flux |
| Dependence on Distance | No | Yes | No |
| Dependence on Surface Area | No | Yes | No |
How to Calculate Candela, Lux, and Lumens?
Calculating Candela
1 candela is equal to 12.57 lumens. However, if you see the lumen rating of the bulb as 1000lm, it doesn’t mean the luminous intensity (cd) of the bulb is also 1000cd. You need to consider the solid angle of the illuminated light to find the lumen. The formula goes as:
| Candela = ( Lumen/Solid Angle) cd = (lm/sr) |
That is, the bulb with a brightness of 1000lm, distributing its illuminance over a solid angle of 2π steradians, has a luminous intensity (cd) of (1000lm/2π sr) = (1000/6.2832) ≈159.15 cd.
(Note: Vale of π (pie) ≈ 3.14159)
Calculating Lux
Lux is the measurement of the amount of light emitted by a 1-lumen bulb on one square meter surface. So, the formula goes as follows:
| 1 Lux = 1 Lumen per square meter Or, 1 lx = 1 lm/m² |
That is, if you install a 1000 lumen light in a 50 square meter room, the illuminance on the surface will be: (1000 lm/50m²) = 20lx.
Calculating Lumen
You can calculate lumen considering both candela and lux. Considering the value of candela, the formula for lumen goes as follows:
| Lumen = Candela x Solid Angle 1 lm = 1 cd·sr |
For example, if you have a bulb with a luminous intensity of 100 cd and a solid angle of 2π sr, its lumen value is 100 cd×2π sr =100×6.2832 =628.32 lm.
However, to find the lumen value using lux, you need to consider the area of the illuminated surface. The formula goes as:
| Lumen = Lux x Area lm = lx × m² |
Suppose, a floor with 5 square meter area receives an illuminance of 200 lx. The lumen value of the bulb will be: 200lx x 5m² = 1000lm.
Factors Affecting the Values of Candela, Lux, and Lumens
1. Distance
The value of lux depends on the distance of the illuminated surface/object and the light source. Lux is inversely proportional to the square of the distance. For example, if the distance increases by double, the lux value will reduce to 1/4th of the initial values.
Candela and lumen value doesn’t change with distance. For instance, a 1000-lumen bulb will emit 1000-lumen even if you stand 1m away or 10 m away from it. However, the light will seem to be less bright as you go far from the light source.
2. Angle Of Radiance
A small angle produces more concentrated light, increasing the value of candela and lux. Again, for a wider angle, the area of illuminated space increases, and it decreases the candela and lux values. However, the lumen value remains unaffected by the angle of radiance.
3. Luminous Efficiency
Due to increasing power input, the lumen value of the light also increases. However, to ensure luminous efficiency, you must consider the wattage of the power source. The light that offers brighter illumination with low wattage rates has high luminous efficiency.
Practical Application of Candela, Lux, and Lumen Ratings In Selecting The Right Fixture
Candela, lux, and lumen together guide you in picking the ideal light for any kind of application. This is how you should use these units while light designing:
| General Lighting | Home Lighting Design | • Candela guides you to find more concentrated illumination for task lighting • Lux helps to pick the appropriate illuminance levels for different rooms • Lumen rating gives you an idea of the overall brightness of your space. |
| Outdoor Street Lighting | • Candela lets you pick appropriate street lights, ensuring proper visibility. • Lux helps you determine the illuminance in the roadways. • Lumens rating guides you on whether a light is bright enough to illuminate a large open space. | |
| Theatrical Stage Lighting | • High candela value ensures the spotlight on the theatrical stage produces focused beams on the performers. • Considering the lux value, you can achieve the preferred brightness on the stage. • Lumen ratings let you set the overall ambience of the stage. For instance, a low lumen will make the stage dim, and for a high lumen, your stage will look bright. | |
| Professional Lightings | Architectural Lighting Designers | • Candela help you in highlighting special elements with a focused beam angle • Lux rating help you to bring optimal brightness level in different zones • Lumens let you pick the right brightness level to illuminate your buildings. |
| Photographers and Cinematographers | • Candela guides the creation of unique shadows and highlights for photography needs. • By considering lux value, you can get the desired exposure for detailed shots. • Lumens ensures the object you click and its background is properly visible. | |
| Industrial and Workplace Lighting | • Candela guides to bright a focused lighting on workstation • Lux maintains adequate illuminance in the working space • Lumens provide general lighting for the safety and proper visibility of the employee. | |
| Urban Planners and City Designers | • Candela, lux, and lumen guide the designer in installing the right fixture on the street, roads, and public places. | |
| Retail and Visual Merchandisers | • Candela aids you in highlighting unique items • Lux ensures that your product showcasing area is well-lit • Lumen ensures a comfortable ambience for the customer to enhance their shopping experience. |
Common Misconceptions About Candela, Lux, and Lumens
Equating Watts with Brightness
The most common misconception about light’s brightness is measuring it by wattage. Remember, watts define the power consumption of the bulb, not its brightness. For instance, a 1000-lumen incandescent bulb has a 65-wattage rating, but an LED light can give the same brightness with just 10-15 watts.
This means LED light can bring the same brightness as the incandescent bulb, burning far less energy. This makes LEDs more energy efficient.
Ignoring Angle and Directionality
Another common mistake is not considering evaluating the angle and direction of the light source while considering candela value. Candela measures the light intensity in a specific direction. So, you must consider the lighting angle and direction to get the desired focused lighting.
Focusing Too Much on Lumens Alone
While purchasing any light fixture, the lumen rating is the only matrix that 98% of general people look for. But it’s a wrong practice, and you can’t get the desired lighting effect considering only the lumen value.
For instance, if you are buying spotlights to highlight your artwork, consider candela value. This way, to get the optimal light output, take all three matrices into consideration.
Challenges in Measuring and Interpreting Candela, Lux, and Lumens
- Variation in light source and environmental factors affects the measurement of the light matrix. Thus, the standardized measurement procedure for candela, lux, and lumen sometimes leads to inaccuracy.
- Professionals often deal with errors in interpreting the angle and directionality of the light. This changes the value of candela.
- With the update of lighting technology, there is a greater need to improve the methodology and machinery for measuring light properties.
Which is The Standard Light Measuring Unit For LED Strips?
As LED strip lights are mostly designed for general lighting, its standard light measuring unit is lumens. But, we often make mistakes measuring the brightness of LED stripes with wattage. That is entirely an unfair practice as different lights have different luminous efficiency.
However, the value of lumens varies with the color of the LED stripe. For instance, white LED stripes have a higher lumen value than colorful LED stripes.
Besides, depending on the application, you need to consider other units like- candela and lux too.
How Do Lumens & Wattage Relate?
Lumens indicate the total output a light source produces, whereas wattage is the energy used to run the light. Still, to judge the light’s performance, you must consider the light’s efficiency by knowing the lumen and Wattage value.
In this case, energy-saving bulbs are the most efficient. Because they produce a higher lumen rate utilizing the least energy (wattage). That is why they are called energy-saving bulbs.
The efficiency of light is calculated as lumen per watt. That is:
| Light Efficiency, E = lm/W |
So, the light’s performance increases when the efficiency improves. At the same time, high-efficient lights minimize your electric bill.
FAQs
Wrapping Up
Though candela, lux, and lumen measure light, all three are different units. So, you should never mix or interchange them. For the best light setting, consider all three units side by side while light designing.
However, to ensure the rating accuracy mentioned in the light packaging, buy fixtures from reliable brands. Also, check their certification for better assurance.
