How bright is 65 lumens?
vanchierifamily
How bright is 65 lumens? How many lumens is a standard flashlight? How
many lumens is the sun as observed from the earth (for comparison)?
Thanks!
ro...@mauve.demon.co.uk
Lumens is pretty much how bright the total light output of the bulb looks.
If white, it's on the order of a quarter of a watt of light energy, or
about the light put out by a 5W halogen bulb.
A 60W frosted cheap light-bulb puts out ~700 lumens.
The sun puts out around 20000000000000000000000000000 lumens for comparison.
--
http://inquisitor.i.am/ | mailto:inqui...@i.am | Ian Stirling.
---------------------------+-------------------------+--------------------------
If it can't be expressed in figures, it is not science, it is opinion.
-- Robert A Heinlein.
Phil Hobbs
Zenith sunlight is around 100,000 lux (1 lux = 1 lumen/sq metre). The lumen is a
photometric unit, defined so that a light source of 1 lumen/m**2/steradian has the
same subjective brightness regardless of its colour. The sensitivity of the human
eye is a strong function of wavelength, reaching a peak of about 683 lumens/watt at
550 nm in the yellow-green, and falling very steeply on both sides. An ordinary
light bulb has about 15 lumens/watt, so your tactical flashlight is about as bright
as a 4W Christmas bulb.
Cheers,
Phil Hobbs
IBM T. J. Watson Research Center
Daniel P. B. Smith
"vanchierifamily" <vanchie...@cox.net> wrote:
It puts out about six times as much light as an old-fashioned two-D-cell
flashlight with an incandescent bulb. If the reflectors are similar so
the beams are equally concentrated, at the same distance it would light
up a surface as about six times as brightly as the old-fashioned
flashlight.
A lumen is a measure of the total amount of light coming out of a light
source. When you buy a light bulb, it always lists the wattage (which
is a measure of how much energy it uses) and the number of lumens (whic
is the amount of light it puts out).
There are a (large!) number of different units of light measurement
because you can take the same number of lumens and let it spread out in
all directions or concentrate it into an intense narrow beam. And you
can take the same _beam_ and shine it on something nearby and light it
up brightly or far away and light.
Lumens: total amount of light (luminous flux).
Candelas or candlepower: takes into account beam concentration.
Lux: how brightly a surface is lit. Take into account beam
concentration AND distance.
A 100-watt incandescent bulb might emit 1700 lumens total. The lumens
output is stated on the package.
A typical (old-fashioned, incandescent) flashlight bulb for a 2-cell
flashlight might emit on the order 10 lumens total.
With no reflector behind it, that 10 lumens spreads out in all
directions. With a reflector behind it, the same number of lumens is
concentrated into a narrow beam. The unit that takes into account beam
concentration is the candela (or candlepower--they are for all practical
purposes the same thing).
With a reflector that produces a loose beam you might find that the
flashlight illuminates an area of a square foot at a distance of five
feet. With a reflector that produces a more concentrated beam you might
find that you can illuminate that same square foot at a distance of
twenty feet. Same number of lumens, different candlepower.
In either case, if the same number of lumens is spread out over the same
area, the surface will be lit equally brightly.
One lumen per square meter is called a "lux," the measure of how
brightly a surface is lit. (Lux = new modern metric unit, footcandle =
old-fashioned unit, one footcandle is about ten lux).
Suppose you take a flashlight that puts out a total of 65 lumens and use
it to light up an area of one square foot = 0.10 square meters. (You
shine it on a surface that is the right distance away for it to light up
one square foot). That surface is being illuminated at 650 lux.
By comparison: moonlight is one lux, typical home interior lighting
might be 300 lux, a school or office might be 1000 lux. 650 lux is more
than bright enough to read by.
Remember, though, that flashlight is only lighting up ONE square foot to
that brightness. In your house you're lighting up hundreds of square
feet, so you need bulbs that put out thousands of lumens.
The brightness with which full sunlight lights up the surface of the
earth is about 100,000 lux.
Hope this helps.
If you're interested, and if my arithmetic is right, the sun puts out
1e5 lux over an area of four pi times 150,000,000 kilometers squared =
about 30,000,000,000,000,000,000,000,000,000 lumens (30 octillion
lumens). Its intensity is 2,000,000,000,000,000,000,000,000,000
candlepower (2 octillion).
--
dpbsmith at world dot std dot com
(replace "at" with at-sign and "dot" with period and remove spaces)
Repeating Decimal
vanchie...@cox.net wrote on 3/1/03 12:56 PM:
Unfortunately, there is a big mix-up in the units you are using.
*Brightness* has a very well defined technical meaning. Most people use
brightness rather loosely and incorrectly.
The optical output power of a light source when weighted or corrected for
the spectral sensitivity of normal photopic (daylight) vision is measured in
lumens. Brightness measures the lumens per unit area per unit solid angle at
the eye generated by a light source. Vernacular use of brightness may refer
to the total number of lumens produced.
In short, you need to learn somewhat more about photometry before you can
ask a meaningful question and understand the answer.
Bill
Phil Hobbs
> Unfortunately, there is a big mix-up in the units you are using.
> *Brightness* has a very well defined technical meaning. Most people use
> brightness rather loosely and incorrectly.
Part of the trouble is the tendency of the Radiometry and Photometry Powers That Be
to redefine common pre-existing terms, and then scold people who don't follow their
lead. "Intensity" and "brightness" are the two most egregious examples. In the
vernacular, a 200W tungsten filament is (approximately) twice as bright as a 100W
filament, but to a photometrist they have the same brightness since the surface
temperature of the filaments are about the same (the extra power goes into heating
twice the area to the same temperature).
>
> In short, you need to learn somewhat more about photometry before you can
> ask a meaningful question and understand the answer.
Humpty-Dumpty at least paid words extra when he made them do extra work. ;-) The
rest of us understood the question.
rdbis...@gmail.com
Phil Hobbs
>>> On 02/03/2003 Repeating Decimal wrote: Unfortunately, there is a
>> Cheers,
>> Phil Hobbs
>> IBM T. J. Watson Research Center
> so ashamed.
you were in the second grade back then, so don't feel so bad. ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com