LEDs, Lumens and Brightness
Comparing LEDs and Bulbs
New products such as the LED based PowerSURE® Power Failure Light improve safety in the
dark by providing both power failure lighting and night light functionality.
Here you'll find additional information regarding LED brightness and comparing them to incandescent bulb brightness, plus how to identify the key information in LED specifications.
LEDs and Lumens - Part 3
LEDs are becoming more widely accepted as a 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 (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 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 lux.
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.
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 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 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.
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 feet), 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 6-1/2 feet) receives 1/4 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.
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 1/10th
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 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 decor 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 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 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.