Light measurement
A LUMEN is a unit
of measurement of light. It measures light much the same way. Remember, a
foot-candle is how bright the light is one foot away from the source. A lumen is
a way of measuring how much light gets to what you want to light! A LUMEN is
equal to one foot-candle falling on one square foot of area.
So, if we take your candle
and ruler, lets place a book at the opposite end from the candle. We'd have a
bit of a light up if we put the book right next to the candle, you know. If that
book happens to be one foot by one foot, it's one square foot. Ok, got the math
done there. Now, all the light falling on that book, one foot away from your
candle equals both…….1 foot candle AND one LUMEN!
The difference between RADIANCE & ILLUMINANCE.
RADIANCE is another way of
saying how much energy is released from that light source. Again, you measure it
at the source. Unless you're talking about measuring the radiance of something
intensely hot, like the Sun. Then you might want to measure it at night, when
it's off.
ILLUMINANCE is what
results from the use of light. You turn your flashlight on in a dark room, and
you light something up. That's ILLUMINANCE. Turning on a light in a dark room to
make the burglar visible gives you ILLUMINANCE. It also gives you another
problem when you note the burglar is pointing your duck gun at your bellybutton.
Illuminance is the
intensity or degree to which something is illuminated and is therefore not the
amount of light produced by the light source. This is measured in foot-candles
again! And when people talk about LUX, it's illuminance measured in metric units
rather than English units of measure. To reinforce that, LUX is the measurement
of actual light available at a given distance. A lux equals one lumen incident
per square meter of illuminated surface area. They're measuring the same thing,
just using different measurement units.
Pretend you're an old
photographer, like O. Winston Link, or Ansel Adams. These two gods of black and
white photography (and a print made by either can fetch quite a hefty sum of
money these days) used a device called a light meter to help them judge their
exposure. These light meters were
nifty devices. You could use it to show how much light was falling on an object,
light from the sun, and reflected light energy from every thing else. Or you
could use it to show how much light energy was reflected off the object itself.
This brings us back three
points.
The first is if we
measure the output of a light at the source that gives us one thing.
The second is that
we use an entirely different unit of measure if we are measuring the results of
that light's output.
The third is the
instructor is right off his trolley, isn't he?
Now back to the book at
the end of the ruler.
We've measured two
different things. We have a unit of measure for how much light is produced. We
Yankees express that as a foot-candle. Being lazy, we use it all over the
place.
Candlepower!
Candlepower is a way of
measuring how much light is produced by a light bulb, LED or by striking an arc
in a Carbon-Arc spotlight. Is it a measure of how much light falls upon an
object some distance away? No. That's illuminance. Is it a measure of how well
we see an object that is illuminated by that light source? No. That's something
all together different, and we are not going there!
Nowadays we use the term
CANDELA instead of candlepower. Candlepower, or CANDELA is a measure of how much
light the bulb produces, measured at the bulb, rather than how much falls upon
the thing you want to light up. Further confusing the matter is beam focus.
That's how much candlepower can be focused using a reflector/lens assembly.
Obviously, if you project all your light bulbs intensity at a given spot, or
towards something, it will be more intense, and the illuminance will be higher.
And here comes the
confuser! A candlepower as a unit of measure is not the same as a foot-candle. A
candlepower is a measurement of the light at the source, not at the object you
light up.
And a candela is the
metric equivalent of the light output of that one candle, based on metric
calculations. And since using a candle is rather imprecise, the definition was
amended to replace a light source using carbon filaments with a very specific
light source, see the following:
The candela is the luminous intensity, in a given direction, of a source that
emits monochromatic radiation of frequency 540 x 1012 hertz and that has a
radiant intensity in that direction of 1/683 watt per steradian.
The above from the National Institute of Standards Reference on Constants,
Units, and Uncertainty.
Candlepower is a measure
of light taken at the source-not at the target. Foot-candles tell us how much of
that light is directed at an object we want to illuminate.
Now, lets convert the
lumens, a metric unit of light measurement, to candlepower.
We understand a candle
radiates light equally in all directions, its output, in this consideration is
not focused by any mechanical means (lenses or reflectors). Pretend for a moment
that a transparent sphere one meter in radius surrounds your candle. We know
that there are 12.57 square meters of surface area in such a sphere. Remember
your Solid Geometry classes?
That one candle (1
Candlepower/Candela) is illuminating equally the entire surface of that sphere.
The amount of light energy then reflected from that surface is defined thusly:
The amount of energy
emanating from one square meter of surface is one lumen. And if we decrease the
size of the sphere to one foot radius, we increase the reflected energy 12.57
times of that which fell on the square meter area.
LUX is an abbreviation for
Lumens per square meter.
Foot-candles equal the amount of Lumens per square feet of area.
So, that one candlepower
equivalent equals 12.57 lumens.
And for you figuring out
LED equivalents, first you must know how many lumens your LED's each produce.
Then divide that value by 12.57 and you have candlepower of the LED. You don't
have foot-candles, remember foot-candles are illuminance. And we are measuring
radiance.
Summary
Candlepower is a rating of
light output at the source, using English measurements.
Foot-candles are a measurement of light at an illuminated object.
Lumens are a metric equivalent to foot-candles in that they are measured at an
object you want to illuminate.
Divide the number of lumens you have produced, or are capable of producing, by
12.57 and you get the candlepower equivalent of that light source.
We've now converted a
measurement taken some distance from the illuminated object, converted it from a
metric standard to an English unit of measure, and further converted it from a
measure of illumination to a measure of radiation!
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