#### Blazer420

##### New Member

I have been putting some time into researching LED light options. They are not cheap, so I am taking my time. I found this piece that I think distills down some of the terminology and how to compare/contrast output.

LEDs are widely accepted as a low energy light source for illumination of living spaces and many questions have been raised by individuals attempting to understand how much light is produced by a High Brightness (HB) LED when compared to an incandescent light bulb. HB LEDs are characterized by luminous intensity, usually in millicandelas, candela, or Lumen (mcd, cd, lm) in an LED specification, while most incandescent bulbs are rated in watts and lumens. How can you compare them?

A 100 watt Bulb is rated at approximately 1700 lumens

A 60 watt incandescent bulb is rated at approximately 800 lumens

A 40 watt bulb is rated at approximately 400 lumens

A 25 watt bulb is rated at approximately 180 lumens

A 4 watt night light bulb is rated at approximately 20 lumens

LED ratings of 25,000 mcd become conceptually difficult to understand when we are accustomed to light being expressed in terms of lumens. Published lumens represent the sum of all the light emitted by the bulb in all directions.

Both candela and candlepower correspond to the amount of light (i.e. the quantity of photons) produced by a standard light source. Originally, the standard light source was a real candle so it is also called candlepower. But like most measurement standards, it's now a theoretical construct having a specific wavelength of visible light at a specific power level. The good news is that the intensity of the standard candela is nearly identical to the original standard candle. All other quantitative measurements related to light are derived from the candela.

Candelas are a measure of the amount of light generated at a source. Lumens quantify how much total light flows away from the source. Foot-candles and LUX indicate how well the surface area of an object, wall or floor is illuminated at some distance away from the source (intensity of the light reflecting off objects like the floor, wall, or work-surface).

So what does this mean? If you have a light source of 1 candela, it's also 1 candlepower in intensity. If you measured the light on a 1 square foor surface, 1 foot away, it will have 1 foot-candle of illuminance over it's surface. In metric terms, if you measured the amount of light on a square meter that was 1 meter away, it will have 1 LUX of illuminance.So you could say that the light source is generating 1 Lumen in the direction of the 1 square meter that is 1 meter away. If the light source is in the middle of a sphere and generating light in all directions, you will find that the 1 cendella light source is generating 12.57 lumens as explained below.

- 1 square meter (M2) = 10.7638 ft2

- 1 LUX is defined as 1 lumen/ M2 of a surface; thus 1 LUX= 0.0929 FC,

- 1 foot candle (FC) is 1 Lumen/ ft2 of a surface; thus 1 FC=10.7638 LUX

- There are 12.57 square meters on the surface of a 1 meter radius sphere.

- Full Daylight is about 10,000 LUX (imagine 10,000 lumens each and every square meter!)

- Cloudy day is about 1,000 LUX

- A lighted parking lot at night is about 10 LUX (average)

- A full moon is about 0.1 LUX

- Public spaces with dark surroundings.................................30 LUX

- Simple orientation for short temporary visits........................50 LUX

- Working spaces where visual tasks are only occasionally performed...100 LUX

- Performance of visual tasks of high contrast or large scale.........300 LUX

- Performance of visual tasks of medium contrast or small size........500 LUX

- Performance of visual tasks of low contrast or very small size.....1000 LUX

- lm is lumens - total luminous flux flowing from the LED. mlm = millilumens.

- cd is candelas - the luminous intensity at a specific forward current. mcd = millicandela.

Cd or mcd units are usually expressed as a value relative to the viewing angle. The larger the viewing angle, the more light flows given the same intensity. In other words, 1000 mcd over 130Â° viewing angle is a lot brighter (more lumens) than 1000 mcd over 20Â° viewing angle. Lumens (lm or mlm) usually refers to the total light output of the device at the rated current.

By definition, 1 lumen is the amount of light produced by a 1 candela source radiating out through 1 steradian (a specific cone shaped solid unit angle of 65.54Â°) within an imaginary sphere surrounding the light source. One candela illuminates the entire surface of a 1 meter radius sphere at an average 1 lumen for each sq. meter of surface in the 360Â° sphere. There are 4 π, or 12.57 steradians in a sphere. Thus, the standard candle at 1 candela intensity produces 12.57 lumens of total visible light radiated in all directions. Lumens are the total quanty of light flowing out in all directions.

a steradian An LED specification sheet shows the luminous intensity (mcd) of the LED for a specific viewing angle of the LED. The viewing angle is the angle of the beam of light produced by the LED and lens. The angle is bounded by the edges where the intensity falls to 50% of the max intensity usually found at the optical center of the beam. Thus, a 25,000 mcd LED with a viewing angle of 20Â° can provide 25 Candelas of light intensity within the 20Â° viewing angle. The question most often posed is how many lumens do you get from this type of LED light?

We can find this out by understanding the relationship of candelas, steradians and spheres using the diagram below. Note that the sphere segment has a 1 meter radius.

A steradian is a solid cone having 1 square meter of surface area on a sphere having a 1 meter radius. There are 4π (12.57) steradians in a sphere. The light grey area is known as a spherical cap that we will illuminate with a point light source.

The surface area of a spherical cap is calculated using the formula S=2πRh where h, the height of the cap, is completely dependent on the viewing angle (apex of the cone). A smaller angle, results in a smaller area on the surface and a smaller height of the center of the cap.

LED Viewing Angle Calculation If you remember your trigonometry, we can calculate the height of the cap h, in a 20Â° cone (the viewing angle) by finding the length of a 10Â° right triangle. A 1 Meter radius simplifies the calculation and the height is found using the formula 1-Cos(10Â°). Notice we use half of the viewing angle for the calculation. LED specs will show either viewing angle or half angle so it is necessary to distinguish between the two when reading the spec. Viewing angle is denoted by "2∅" in most LED specs.

At 1 meter, a 25,000 mcd LED with a 20Â° viewing angle covers a spherical cap area of 2*π*1*0.015 = 0.095 square meters with 25 candelas of intensity. In order to determine total lumens flowing, we must determine how bright the intensity would be if the same amount of light flow were covering an entire 1 square meter area on the surface of that 1 meter radius sphere. We know that the 20Â° cone covers 0.095 M2 and multiplying that small area times the candela value tells us how many lumens the device delivers into a 1 M2 area. This half angle cosine formula 2 π*(1-Cos(∅))*25 Candelas yields [6.283*(1-cos(10Â°))*25] = 2.39 lumens.

Now that we've standardized our light output in lumens per square meter, the same LED die, when used with a wider beam lens (i.e. 40Â°), will have a lower candela value. Using the above calculation, a 25,000 mcd 20Â° viewing angle LED has the same luminous flux as a 6,250 mcd 40Â° viewing angle LED. This is the same LED die with a different lens. Recent LED specs have begun to include lumen values to help in this determination of total light output.

This mathematical method is a close approximation of lumens using the candela value and the viewing angle of the LED. Other factors in the construction of the LED can change the luminous flux relationship with the luminous intensity of the die. For example, some LEDs will have less light lost to the sides and rear of the die and reflect more of the intensity out through the lens.

A radiation diagram for a specific LED shows how the viewing angle is distinguished from all the other light emitted by the LED. The red arrows indicate the 60Â° half angle or 120Â° viewing angle where the light drops to 50% of max intensity. This is the forward radiation. Note that the most light is usually on the 0Â° X axis and this represents the maximum candela output. Note also that as the light spreads out across the viewing angle, the relative candela output drops to zero (in this particular case) at 90Â°.

Any light that is absorbed into the substrate of the LED base is not seen or measured. You can see why altering the construction of the LED to reduce light lost in other directions would cause a change in the lumen output.

When an LED spec sheet specifies the lumen output rather than candela or millicandela, the Total Luminous Flux (lm) number represents all the light that is emitted from the device as measured by an integration sphere. The radiation diagram is used to see what the relative level of light is as it radiates out in any given direction.

Let's compare a standard 4-watt night light bulb with a high brightness white LED. The night light bulb is rated at about 20 total lumens. The difference between an LED light source and a free standing bulb is that the 20 lumens are radiated out in a 360Â° 3D space. We would need 10 LEDs rated at 2 lumens each to obtain the equivalent total luminous flux. An object in the room will receive only a portion of this flux. When the 4-watt, 20 lumen bulb is placed in the center of a 1 meter radius 360Â° sphere, the surface area receives 20 / 12.57 = 1.6 lumens per M2. At 1 meter, that is 1.6 lux.

An LED having a 20Â° viewing angle illuminates 0.095 M2 at a distance of 1 meter. The LED is required to have a luminous intensity of 1.6 lumens / 0.095 or 16.8 candelas to deliver 1.6 lumens over the entire square meter to equal the 4-watt bulb's light output. With luminous intensities ranging from 20 up to 40 candela, today's hi brightness LEDs can easily produce this light output.

When a night light is plugged into a wall receptacle, typically half is directed into the room while the other half is reflected off the wall. Any reflection off the wall increases this with indirect lighting. A mirror would reflect nearly all of the light while a painted wall could reflect less than half depending on the color and surface finish. Absorption in the plastic decorative shade also reduces the total lumens of direct light delivered to the room.

There seems to be a lot of confusion over this as many people want to compare LED watts with incandescant watts. The short answer is that they are not easily comparable! An incandescant light bulb consumes 40, 60 or 100 watts of POWER. Approximately 90% of power consumed is given off as heat and the rest given off as light. The term Luminous Efficacy is used to describe the ratio of the visible light energy emitted to the total power input to the lamp. This visible light is measured in lumens.

Incandescant lamps are full spectrum devices where LEDs are not. LED's will give off a specific range of wavelengths. The power consumed to do this is very little compared to incandescant lamps.

As an example, assume a 350 mA LED with a forward voltage drop of 3.2 Volts. Using Ohm's Law, (Power = current * voltage or P=IE)you can see that this LED will consume about 1.12 Watts. CREE has XLamp XR-E LEDs that can produce over 200 Lumens at this power. Comparing that to a 25 watt incandescant lamp (approximately 180 lumens) becomes difficult because the lamp consumes 25 times the power of the Cree LED but the LED has more visible light output. One must also consider the inefficiencies of converting AC to DC power, but even a 70% efficiency demonstrates that the LED is far superior to incandescant bulbs for illumination efficiency.

**Introduction**LEDs are widely accepted as a low energy light source for illumination of living spaces and many questions have been raised by individuals attempting to understand how much light is produced by a High Brightness (HB) LED when compared to an incandescent light bulb. HB LEDs are characterized by luminous intensity, usually in millicandelas, candela, or Lumen (mcd, cd, lm) in an LED specification, while most incandescent bulbs are rated in watts and lumens. How can you compare them?

**Bulb brightness in Lumens**A 100 watt Bulb is rated at approximately 1700 lumens

A 60 watt incandescent bulb is rated at approximately 800 lumens

A 40 watt bulb is rated at approximately 400 lumens

A 25 watt bulb is rated at approximately 180 lumens

A 4 watt night light bulb is rated at approximately 20 lumens

LED ratings of 25,000 mcd become conceptually difficult to understand when we are accustomed to light being expressed in terms of lumens. Published lumens represent the sum of all the light emitted by the bulb in all directions.

**Light measurement basics**Both candela and candlepower correspond to the amount of light (i.e. the quantity of photons) produced by a standard light source. Originally, the standard light source was a real candle so it is also called candlepower. But like most measurement standards, it's now a theoretical construct having a specific wavelength of visible light at a specific power level. The good news is that the intensity of the standard candela is nearly identical to the original standard candle. All other quantitative measurements related to light are derived from the candela.

Candelas are a measure of the amount of light generated at a source. Lumens quantify how much total light flows away from the source. Foot-candles and LUX indicate how well the surface area of an object, wall or floor is illuminated at some distance away from the source (intensity of the light reflecting off objects like the floor, wall, or work-surface).

So what does this mean? If you have a light source of 1 candela, it's also 1 candlepower in intensity. If you measured the light on a 1 square foor surface, 1 foot away, it will have 1 foot-candle of illuminance over it's surface. In metric terms, if you measured the amount of light on a square meter that was 1 meter away, it will have 1 LUX of illuminance.So you could say that the light source is generating 1 Lumen in the direction of the 1 square meter that is 1 meter away. If the light source is in the middle of a sphere and generating light in all directions, you will find that the 1 cendella light source is generating 12.57 lumens as explained below.

**Light Measurement Equivalents and Common Illumination Levels:**- 1 square meter (M2) = 10.7638 ft2

- 1 LUX is defined as 1 lumen/ M2 of a surface; thus 1 LUX= 0.0929 FC,

- 1 foot candle (FC) is 1 Lumen/ ft2 of a surface; thus 1 FC=10.7638 LUX

- There are 12.57 square meters on the surface of a 1 meter radius sphere.

**In real life objects are illuminated and measured in LUX:**- Full Daylight is about 10,000 LUX (imagine 10,000 lumens each and every square meter!)

- Cloudy day is about 1,000 LUX

- A lighted parking lot at night is about 10 LUX (average)

- A full moon is about 0.1 LUX

**OSHA Lighting Level Recommendations for Work Spaces:**- Public spaces with dark surroundings.................................30 LUX

- Simple orientation for short temporary visits........................50 LUX

- Working spaces where visual tasks are only occasionally performed...100 LUX

- Performance of visual tasks of high contrast or large scale.........300 LUX

- Performance of visual tasks of medium contrast or small size........500 LUX

- Performance of visual tasks of low contrast or very small size.....1000 LUX

**In LED specifications:**- lm is lumens - total luminous flux flowing from the LED. mlm = millilumens.

- cd is candelas - the luminous intensity at a specific forward current. mcd = millicandela.

Cd or mcd units are usually expressed as a value relative to the viewing angle. The larger the viewing angle, the more light flows given the same intensity. In other words, 1000 mcd over 130Â° viewing angle is a lot brighter (more lumens) than 1000 mcd over 20Â° viewing angle. Lumens (lm or mlm) usually refers to the total light output of the device at the rated current.

**Steradians - Solid Geometry Lessons Anyone?**By definition, 1 lumen is the amount of light produced by a 1 candela source radiating out through 1 steradian (a specific cone shaped solid unit angle of 65.54Â°) within an imaginary sphere surrounding the light source. One candela illuminates the entire surface of a 1 meter radius sphere at an average 1 lumen for each sq. meter of surface in the 360Â° sphere. There are 4 π, or 12.57 steradians in a sphere. Thus, the standard candle at 1 candela intensity produces 12.57 lumens of total visible light radiated in all directions. Lumens are the total quanty of light flowing out in all directions.

a steradian An LED specification sheet shows the luminous intensity (mcd) of the LED for a specific viewing angle of the LED. The viewing angle is the angle of the beam of light produced by the LED and lens. The angle is bounded by the edges where the intensity falls to 50% of the max intensity usually found at the optical center of the beam. Thus, a 25,000 mcd LED with a viewing angle of 20Â° can provide 25 Candelas of light intensity within the 20Â° viewing angle. The question most often posed is how many lumens do you get from this type of LED light?

We can find this out by understanding the relationship of candelas, steradians and spheres using the diagram below. Note that the sphere segment has a 1 meter radius.

**Calculating Candelas and Lumens**A steradian is a solid cone having 1 square meter of surface area on a sphere having a 1 meter radius. There are 4π (12.57) steradians in a sphere. The light grey area is known as a spherical cap that we will illuminate with a point light source.

The surface area of a spherical cap is calculated using the formula S=2πRh where h, the height of the cap, is completely dependent on the viewing angle (apex of the cone). A smaller angle, results in a smaller area on the surface and a smaller height of the center of the cap.

LED Viewing Angle Calculation If you remember your trigonometry, we can calculate the height of the cap h, in a 20Â° cone (the viewing angle) by finding the length of a 10Â° right triangle. A 1 Meter radius simplifies the calculation and the height is found using the formula 1-Cos(10Â°). Notice we use half of the viewing angle for the calculation. LED specs will show either viewing angle or half angle so it is necessary to distinguish between the two when reading the spec. Viewing angle is denoted by "2∅" in most LED specs.

At 1 meter, a 25,000 mcd LED with a 20Â° viewing angle covers a spherical cap area of 2*π*1*0.015 = 0.095 square meters with 25 candelas of intensity. In order to determine total lumens flowing, we must determine how bright the intensity would be if the same amount of light flow were covering an entire 1 square meter area on the surface of that 1 meter radius sphere. We know that the 20Â° cone covers 0.095 M2 and multiplying that small area times the candela value tells us how many lumens the device delivers into a 1 M2 area. This half angle cosine formula 2 π*(1-Cos(∅))*25 Candelas yields [6.283*(1-cos(10Â°))*25] = 2.39 lumens.

Now that we've standardized our light output in lumens per square meter, the same LED die, when used with a wider beam lens (i.e. 40Â°), will have a lower candela value. Using the above calculation, a 25,000 mcd 20Â° viewing angle LED has the same luminous flux as a 6,250 mcd 40Â° viewing angle LED. This is the same LED die with a different lens. Recent LED specs have begun to include lumen values to help in this determination of total light output.

This mathematical method is a close approximation of lumens using the candela value and the viewing angle of the LED. Other factors in the construction of the LED can change the luminous flux relationship with the luminous intensity of the die. For example, some LEDs will have less light lost to the sides and rear of the die and reflect more of the intensity out through the lens.

**LED Radiation Diagrams**A radiation diagram for a specific LED shows how the viewing angle is distinguished from all the other light emitted by the LED. The red arrows indicate the 60Â° half angle or 120Â° viewing angle where the light drops to 50% of max intensity. This is the forward radiation. Note that the most light is usually on the 0Â° X axis and this represents the maximum candela output. Note also that as the light spreads out across the viewing angle, the relative candela output drops to zero (in this particular case) at 90Â°.

Any light that is absorbed into the substrate of the LED base is not seen or measured. You can see why altering the construction of the LED to reduce light lost in other directions would cause a change in the lumen output.

When an LED spec sheet specifies the lumen output rather than candela or millicandela, the Total Luminous Flux (lm) number represents all the light that is emitted from the device as measured by an integration sphere. The radiation diagram is used to see what the relative level of light is as it radiates out in any given direction.

**Comparing real bulbs to LEDs**Let's compare a standard 4-watt night light bulb with a high brightness white LED. The night light bulb is rated at about 20 total lumens. The difference between an LED light source and a free standing bulb is that the 20 lumens are radiated out in a 360Â° 3D space. We would need 10 LEDs rated at 2 lumens each to obtain the equivalent total luminous flux. An object in the room will receive only a portion of this flux. When the 4-watt, 20 lumen bulb is placed in the center of a 1 meter radius 360Â° sphere, the surface area receives 20 / 12.57 = 1.6 lumens per M2. At 1 meter, that is 1.6 lux.

An LED having a 20Â° viewing angle illuminates 0.095 M2 at a distance of 1 meter. The LED is required to have a luminous intensity of 1.6 lumens / 0.095 or 16.8 candelas to deliver 1.6 lumens over the entire square meter to equal the 4-watt bulb's light output. With luminous intensities ranging from 20 up to 40 candela, today's hi brightness LEDs can easily produce this light output.

When a night light is plugged into a wall receptacle, typically half is directed into the room while the other half is reflected off the wall. Any reflection off the wall increases this with indirect lighting. A mirror would reflect nearly all of the light while a painted wall could reflect less than half depending on the color and surface finish. Absorption in the plastic decorative shade also reduces the total lumens of direct light delivered to the room.

**Comparing LED Watts and Incandescant Lamp Watts**There seems to be a lot of confusion over this as many people want to compare LED watts with incandescant watts. The short answer is that they are not easily comparable! An incandescant light bulb consumes 40, 60 or 100 watts of POWER. Approximately 90% of power consumed is given off as heat and the rest given off as light. The term Luminous Efficacy is used to describe the ratio of the visible light energy emitted to the total power input to the lamp. This visible light is measured in lumens.

Incandescant lamps are full spectrum devices where LEDs are not. LED's will give off a specific range of wavelengths. The power consumed to do this is very little compared to incandescant lamps.

As an example, assume a 350 mA LED with a forward voltage drop of 3.2 Volts. Using Ohm's Law, (Power = current * voltage or P=IE)you can see that this LED will consume about 1.12 Watts. CREE has XLamp XR-E LEDs that can produce over 200 Lumens at this power. Comparing that to a 25 watt incandescant lamp (approximately 180 lumens) becomes difficult because the lamp consumes 25 times the power of the Cree LED but the LED has more visible light output. One must also consider the inefficiencies of converting AC to DC power, but even a 70% efficiency demonstrates that the LED is far superior to incandescant bulbs for illumination efficiency.

Last edited by a moderator: