LEDs and Lumens

8/2/2019 LEDs and Lumens

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LEDs and Lumens - Part 1

Contents

Part 1:

Bulb Brightness in Lumens Light Measurement Units and Relationships

Light Measurement Equivalents

Units in LED specifications

Part 2: Steradians Explained Calculating Candelas and Lumens Lumen Calculator LED Radiation Diagrams

Part 3: Comparing real bulbs to LEDs Illuminating a room with an LEDLED based power failure lights LED Watts and Light Bulb Watts - How to Compare)

Introduction

LEDs are becoming more widely accepted as a light source for illumination of living spaces and many questionshave 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 inmillicandelas, candela, or Lumen (mcd, cd, lm) in an LED specification, while most incandescent bulbs are rated

in watts and lumens.

Bulb brightness

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 beingexpressed in terms of lumens. Published lumens represent the sum of all the light emitted by the bulb in alldirections.

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Light measurement basics

Both candela and candlepower correspond to the amount of light (i.e. the quantity of photons) produced by astandard light source. Originally, the standard light source was a real candle so it is also called candlepower. But likemost measurement standards, it's now a theoretical construct having a specific wavelength of visible light at a

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specific power level. The good news is that the intensity of the standard candela is nearly identical to the originalstandard 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 flowsaway from the source. Foot-candles and LUX indicate how well the surface area of an object, wall or floor isilluminated at some distance away from the source (intensity of the light reflecting off objects like the floor, wall, or

worksurface).

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So what does this mean? If you have a light source of 1 candela, it's also 1 candlepower in intensity. If you measuredthe light on a 1 square foor surface, 1 foot away, it will have 1 foot-candle of illuminance over it's surface. In metricterms, 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 thatis 1 meter away. If the light source is in the middle of a sphere and generating light in all directions, you will findthat 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, themore light flows given the same intensity. In other words, 1000 mcd over 130° viewing angle is a lot brighter (morelumens) than 1000 mcd over 20° viewing angle. Lumens (lm or mlm) usually refers to the total light output of thedevice at the rated current.






Contents

Part 1: Bulb Brightness in Lumens Light Measurement Units Light Measurement Relationships

Units in LED specifications

Part 2:

Steradians Explained

Calculating Candelas and Lumens

Lumen Calculator

LED Radiation Diagrams

Part 3: Comparing real bulbs to LEDs Illuminating a room with an LED LED based power failure lights LEDs and Watts and Light Bulb Watts - How to Compare

LEDs are becoming more widely accepted as a light source for illumination of living spaces and many questionshave 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. Here we review how many Lumens are produced and how it canbe calculated.

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 (aspecific cone shaped solid unit angle of 65.54°) 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 surfacein the 360° 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 flowingout in all directions.

An LED specification sheet shows the luminous intensity (mcd) of theLED for a specific viewing angle of the LED. The viewing angle is theangle of the beam of light produced by the LED and lens. The angle isbounded by the edges where the intensity falls to 50% of the max intensityusually found at the optical center of the beam. Thus, a 25,000 mcd LEDwith a viewing angle of 20° can provide 25 Candelas of light intensitywithin the 20° viewing angle. The question most often posed is how manylumens 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 spheresegment has a 1 meter radius.

Calculating Candelas and Lumens

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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 pointlight 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.

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If you remember your trigonometry, we cancalculate the height of the cap h, in a 20° cone (theviewing angle) by finding the length of a 10° righttriangle. A 1 Meter radius simplifies the calculationand the height is found using the formula 1-Cos(10°). Notice we use half of the viewing anglefor the calculation. LED specs will show eitherviewing 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 20° 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 theintensity 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 20° cone covers 0.095 M2 and multiplying that small area times thecandela 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(10°))*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 widerbeam lens (i.e. 40°), will have a lower candela value. Using the above calculation, a 25,000 mcd 20° viewing angleLED has the same luminous flux as a 6,250 mcd 40° 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.

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

milliCandellas

(mcd)

Half Angle

(degrees)

Total Light Output

Lumens

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This is a simple mathematical model to calculate lumens from candelas when you know the viewing angle of theLED. It is easy to use but it is really only an approximation because it does not consider the spectral qualities of thelight.


Contents

Part 1: Bulb Brightness in Lumens Light Measurement Units Light Measurement Relationships Units in LED specifications

Part 2: Steradians Explained

Calculating Candelas and Lumens Lumen Calculator LED Radiation Diagrams

Part 3:

Comparing real bulbs to LEDs

LED Watts and Light Bulb Watts - How to Compare

Illuminating a room with an LED

LED based power failure lights

LEDs are becoming more widely accepted as a light source for illumination of living spaces and many questionshave 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 inmillicandelas (mcd) in an LED specification, while most incandescent bulbs are rated in lumens.

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 atabout 20 total lumens. The difference between an LED light source and a free standing bulb is that the 20 lumens areradiated out in a 360° 3D space. We would need 10 LEDs rated at 2 lumens each to obtain the equivalent totalluminous flux. An object in the room will receive only a portion of this flux. When the 4-watt, 20 lumen bulb isplaced in the center of a 1 meter radius 360° sphere, the surface area receives 20 / 12.57 = 1.6 lumens per M2. At 1meter, that is 1.6 lux.

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An LED having a 20° viewing angle illuminates 0.095 M2 at a distance of 1 meter. The LED is required to have aluminous intensity of 1.6 lumens / 0.095 or 16.8 candelas to deliver 1.6 lumens over the entire square meter to equalthe 4-watt bulb's light output. With luminous intensities ranging from 20 up to 40 candela, today's hi brightnessLEDs 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 isreflected off the wall. Any reflection off the wall increases this with indirect lighting. A mirror would reflect nearlyall of the light while a painted wall could reflect less than half depending on the color and surface finish. Absorptionin the plastic decorative shade also reduces the total lumens of direct light delivered to the room.

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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 wattsof POWER. Approximately 90% of power consumed is given off as heat and the rest given off as light. The termLuminous Efficacy is used to describe the ratio of the visible light energy emitted to the total power input to thelamp. 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 am 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 LEDsthat can produce over 200 Lumens at this power. Comparing that to a 25 watt incandescant lamp (approximately 180lumens) becomes difficult because the lamp consumes 25 times the power of the Cree LED but the LED has morevisible 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.

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Illuminating a room with an LED

Luminous flux (Lux) decreases with the square of the distance from the light source. In a 3 meter long room (about10 feet), the far wall receives about 1/9th of the luminous flux in the beam projected. Luminous flux at 1 Meterdrops by a factor of four when the distance doubles to 2 Meters, by 9 when it triples, by 16 when it quadruples, etc.In the case of the night light, the 1.6 LUX at 1 meter distance drops to 0.15 LUX ten feet away on any area of thewall that is directly illuminated. A chair that is 2 meters away (about 6-1/2 feet) receives 1/4 of the luminous fluxand is illuminated at 0.4 LUX.

The 20° beam of a 25 candela LED will produce an illuminated area of light having an area of 9.5 square feet on awall 10 feet away at 0.22 LUX. The chair, at 2 meters, will be illuminated at 0.6 LUX. All objects in that path of light from the LED would be illuminated 1.5 times brighter than an equivalent 4-watt night light. A wider 30° beamgives us over 21 square feet of illuminated area on the wall at 0.1 LUX - as bright as a full moon on a cloudlessnight.

In summary, today's LEDs are extremely bright "point sources" of light. A single LED with a proper lens can deliverenergy efficient night light level lighting to a room area to see obstacles and safely walk about.

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LED based power failure lights

To put this all into perspective, we've all witnessed how a full moon can light our neighborhood. Nearly all objectsare visible unless they are black on black. When night falls and your house is dark, a single LED can easilyilluminate many, if not all of the objects in a room and can prevent stumbling around in the dark looking for a light

switch. Installing one LED based Power Failure light in each room will allow a person to navigate the entire housewithout ever turning on a main light fixture or leaving one on for hours after they've passed through.

Think of the energy savings to be gained because LEDs do not consume much power to light a room. Whenpowered by 120 volts AC, a single 5mm LED driven night light can use about 0.3 watts while the night light bulbuses 4 Watts. This is about 1/10th of the energy. We get one and a half times more illumination while using one-tenth the power. In energy savings 'green' terms, ten PowerSURE Power Failure safety lights use less electricitythan one 4-watt night light, and illuminates 10 different areas compared to only one room area for a night light.

When a power failure occurs, many people search for candles to use as a source of light. Unfortunately, candle firescause about $300 million of property damage and 140 tragic deaths annually, one third of which are attributed tousing candles for lighting during power failures. LED driven automatic night light power failure lights can be a lifesaver or simply provide comfort in knowing you can accomplish many needed tasks without ever lighting a candle.

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Benefits of a Standardized Power Failure Light

PowerSURE® brand Power Failure lights replace standard wall switches and receptacles. They can be installed innearly every room of every home without needing new wiring. They will not impact room decor because they look and act like standard wall switches and receptacles. They fit in standard wall boxes also use standard wall plates tomatch any decor. This allows single or multi-ganged installations (i.e. several wiring devices in the same wall box).

Installing several PowerSURE Power Failure Lights in key locations throughout a home will provide energy

efficient night lighting and power failure lighting protection around the clock. The homeowner receives the benefitsof convenient dusk-to-dawn night lighting for a fraction of the energy cost of regular night lights. The homeowneralso gains the comfort and convenience of long lasting and automatic power failure lighting protection.

We all know that we can't prevent power failures. Fire risk is reduced by eliminating the need to rely on candlesduring power failures and blackouts which saves lives and hundreds of millions of dollars in property damageannually.

PowerSURE Power Failure Lights are the only standardized residential Power Failure light solution on the marketthat will make almost all homes safer in the future.





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LED Radiation Diagrams

A radiation diagram for a specific LED shows how the viewing angle is distinguished from all the other lightemitted by the LED. The red arrows indicate the 60° half angle or 120° 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 0° X axis and this representsthe maximum candela output. Note also that as the light spreads out across the viewing angle, the relative candelaoutput drops to zero (in this particular case) at 90°.

Any light that is absorbed into the substrate of the LED base is not seen or measured. You can see why altering theconstruction 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. Theradiation diagram is used to see what the relative level of light is as it radiates out in any given direction.

So how does all this compare to real light bulbs?



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LEDs and Lumens