The links below are a good start.
Michael Press said:Candlepower is a measure of light emitted; watts is a measure of
power output. How come inexpensive clip-on headlights are rated by
candlepower but expensive lights with separate battery units are rated
by wattage? Can you convert from one to another?
Here's specifically what I'm getting at: I use a Sigma Sport Mirage X
with 5 and 20 watt lights for winter commuting on dark country roads,
but it's too heavy and bulky to keep on the bike all the time. I want
to keep a smaller, cheaper light in my commuting bag to throw on the
bike if I work late in fall or spring when I don't have the Mirage
lights, and to put on other bikes I may take to work occasionally.
Obviously it won't have nearly the power of the Mirage lights, but
should be enough to be seen and kind of see the road. I'm trying to
get a sense for how bright one of these things will be compared to the
my 5 and 20 watt lights.
So, what's the brightest self-contained light? The Cateye EL-500
claims 1000 candlepower; how does that compare to a wattage rating?
Thanks,
Michael
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/photom.html
Power Per Unit Solid Angle
The power (flux) per unit solid angle (sometimes called pointance) is
the nearest precise terminology to the common term intensity. It
expresses the directionality of the radiated energy and is appropriate
for the description of point sources. In the case of radiant power, it
is expressed in watts per steradian. For visible light it is expressed
in lumens per steradian = candela.
http://www.fact-index.com/p/ph/photometry__optics_.html
Watts (radiant flux) vs. Lumens (luminous flux)
A comparison of the watt and the lumen illustrates the distinction
between radiometric and photometric units.
The watt is a unit of energy. We are accustomed to thinking of light
bulbs in terms of their wattage. But wattage as marked on the package,
is not a measure of the amount of light it puts out, but a measure of
the amount of energy it consumes: it tells you how much the bulb will
add to your electric bill, not how effective it will be in lighting your
home. Because incandescent bulbs sold for "general service" all have
fairly similar characteristics, wattage is a guide to light output, but
only a rough one.
Watts can also be a measure of output. In a radiometric sense, an
incandescent light bulb is about 80% efficient; 20% of the energy is
lost (e.g. by conduction through the lamp base) The remainder is emitted
as radiation. Thus, a 60-watt light bulb emits a total radiant flux of
about 45 watts.
Incandescent bulbs are, in fact, sometimes used as heat sources, (as in
a chick incubator), but usually they are used for the purpose of
providing light. As such, they are very inefficient, because most of the
radiant energy they emit is invisible infrared. There are compact
fluorescent bulbs that say on their package that they "provide the light
of a 60-watt bulb" while consuming only 15 watts.
The lumen is the photometric unit that corresponds to the watt. Although
most consumers still think of light in terms of wattage, in the U. S. it
has been a trade requirement for several decades that light bulb
packaging give the output in lumens. The package of a 60-watt
incandescent bulb indicates that it provides about 900 lumens, as does
the package of the 15-watt compact fluorescent.
The lumen is defined as one candela per steradian; while the candela, a
base SI unit, is defined as the luminous intensity of a source of
monochromatic radiation, of frequency 540 terahertz, and a radiant
intensity of 1/683 watts per steradian. (540 THz corresponds to about
555 nanometers, the wavelength, in the green, to which the human eye is
most sensitive. The number 1/683 was chosen to make the candela about
equal to the standard candle, the unit which it superseded).
Combining these definitions, we see that 1/683 watt of 555-nanometer
green light provides one lumen.
The relation between watts and lumens is not just a simple scaling
factor. We know this already, because the 60-watt incandescent bulb and
the 15-watt compact fluorescent both provide 900 lumens.
The definition tells us that 1 watt of pure green 555-nm light is
"worth" 683 lumens. It does not say anything about other wavelengths.
Because lumens are photometric units, their relationship to watts
depends on the wavelength according to how visible the wavelength is.
Infrared and ultraviolet radiation, for example, are invisible and do
not count. One watt of infrared radiation (which is where most of the
radiation from an incandescent bulb falls) is worth zero lumens. Within
the visible spectrum, wavelengths are light are weighted according to a
function called the "photopic spectral luminous efficiency." According
to this function, 700 nm red light is only about 4% as efficient as 555
nm green light. Thus, one watt of 700 nm red light is "worth" only 27
lumens.
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