With the LED becoming more widely accepted as a light source for illumination of living spaces, many questions have been raised by those individuals attempting to understand how much light is produced by an LED compared to an incandescent light bulb.  LEDs are characterized by luminous intensity, usually in millicandelas (mcd) in an LED specification, while most incandescent bulbs are rated in lumens.

A 60 watt incandescent bulb is rated at approximately 800 lumens while a 40 watt bulb is rated at 400 lumens. LED ratings of 25,000 mcd become conceptually difficult to understand when we’re 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

The candela (luminous intensity) corresponds 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. 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 the source. Lumens quantify how much of that light flows away from the source in all directions. Foot-candles and LUX indicate how well the surface area of an object, wall or floor is illuminated.

It is helpful to know the following basic information:

- 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

In real life:

- Full Daylight is about 10,000 LUX
- 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

In LED specifications:

- lm is total luminous flux flowing from the LED in lumens (mlm=millilumens)
- cd is the luminous intensity in candelas of the light (mcd=millicandela)

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 65.54°) 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 surface in 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 quantify how bright the light source is in terms of total light flowing out in all directions.

An LED specification sheet shows the luminous intensity of the LED for the 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 20° can provide 25 Candelas of light intensity within the 20° 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 on the right. Note that the sphere 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.

If you remember your trigonometry, we can calculate the height of the cap h, in a 20° cone (the viewing angle) by finding the length of a 10° right triangle.  A 1 Meter radius simplifies the calculation and the height is found using the formula 1-Cos(10°). 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 “2Ø” 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 the intensity would be if the same amount of light flow were covering a 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 the candela value tells us how many lumens the device delivers into a 1 M² area. This half angle cosine formula 2π*(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 wider beam lens (i.e. 40°), will have a lower candela value. Using the above calculation, a 25,000 mcd 20° viewing angle LED 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.

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.

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

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 60° half angle or 120° viewing angle where the light drops to 50% of max intensity. This is the forward radiation. 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.

So how does all this compare to real light bulbs?

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 360° 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 360° sphere, the surface area receives 20 / 12.57 = 1.6 lumens per M². At 1 meter, that is 1.6 flux.

An LED having a 20° 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.  

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 (about 10’), the far wall receives about 1/9th of the luminous flux in the beam projected. Luminous flux at 1 Meter drops 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 the wall that is directly illuminated. A chair that is 2 meters away (about 6 ½ ‘) receives ¼ of the luminous flux and 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 a wall 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° beam gives us over 21 square feet of illuminated area on the wall at 0.1 LUX – as bright as a full moon on a cloudless night.

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

The case for 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 objects are visible unless they are black on black. When night falls and your house is dark, a single LED can easily illuminate 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 house without 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. When powered by 120 volts AC, a single 5mm LED driven night light can use about 0.3 watts while the night light bulb uses 4 Watts. This is about 1/10^th 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 lights use less electricity than 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 fires cause about $300 million of property damage and 140 tragic deaths annually, one third of which are attributed to using candles for lighting during power failures.  LED driven automatic night light power failure lights can be a life saver or simply provide comfort in knowing you can accomplish many needed tasks without ever lighting a candle.

Benefits of a Standardized Power Failure Light

PowerSURE® brand Power Failure lights replace standard wall switches and receptacles. They can be installed in nearly every room of every home without needing new wiring. They will not impact room décor because they look and act like standard wall switches and receptacles. They fit in standard wall boxes also use standard wall plates to match any décor. 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 benefits of convenient dusk-to-dawn night lighting for a fraction of the energy cost of regular night lights. The homeowner also 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 candles during power failures and blackouts which saves lives and hundreds of millions of dollars in property damage annually.

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

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