Brightness: Laser versus Flashlight
Which light do you think shines brighter; a laser pointer
or a flashlight?
It depends on the target which feels the light:
how big is the whole target, and how far away is it?
If the target is far away, I'm pretty sure the laser pointer is brighter,
because it spreads out much less than the flashlight.
If the target is close up, then the laser is still brighter on any tiny target, like an ant,
but the flashlight is brighter for a large target like lighting up a wall or the inside of a box.
A flashlight can get maybe 2 watts of electricity from the batteries.
The light-bulb is typically 5-10% efficient, so it makes 0.1 to 0.2 watts of real light.
The laser-pointer makes 0.002 to 0.005 watts of real light.
We like to call it 2 to 5 milli-watts.
If you try to light up a room with laser and flashlight,
the laser is about 50 times dimmer.
But then there is beam-width and beam-angle.
Close-up, the beam-width counts more.
The flashlight beam-width is about 50 mm (milli-meters) or 2 inches wide,
right where it leaves the flashlight.
Some are smaller, others are bigger.
The laser beam-width, close-up, is about 2 mm or 0.08 inches wide.
The area of the laser's small spot is a little less than 2mm times 2mm = 4,square millimeters.
I guess it is about 3 square millimeters.
3 milli-watts divided by 3 sq.mm is
= 1 milli-watt per square millimeter.
The part of the flashlight's beam that falls in that 3sq-mm is
(2mm/50mm)^2 times 200 milli-watts = 200/1250 milli-watts
= 1/6 milli-watt per square millimeter.
Right there in the small spot, the laser is about 6 times brighter.
So if you aim a flashlight and a laser-pointer onto the same place on a white paper,
the laser's small color-spot should stand out nicely within the flashlight's wide white spot.
Try taking a flashlight and laser-pointer out into mid-day sunlight, and point at a paper on the ground.
I think the laser's spot on the paper will be fairly visible,
but the flashlight's spot will not show up much.
The flashlight is several times dimmer than sunlight,
so it only makes the paper maybe 10% brighter,
and a 10% difference never looks like much to our eyes.
But the laser is either equal to the sunlight or brighter than the sunlight, within it's small spot.
So it makes 100% difference or more, and it is clearly visible.
Far away, beam-angle counts more, and the laser gets an advantage there too.
All beams spread out like a fan or a cone.
The flashlight makes a fairly wide fan.
The dim outer part of it is maybe 60 degrees wide,
and the brighter middle part of it maybe 5 degrees wide.
The laser makes a very narrow cone, maybe 0.1 degree wide.
So far away, the laser will cover 50 times less horizon than the flashlight,
and it will gain a boost in apparent brightness of 50x50 = 2500.
When it does not miss the target.
The (flashlight / laser ) ratio of brightness will be
( 50 times more power ) divided by ( 2500 times more square-angle )
= 1 / 50.
So hundreds of yards away, the flashlight will be 1/ 50th as bright as the laser-pointer.
But only if the laser-pointer is aimed really well, so it doesn't miss the target.
It can be hard to aim a little thing to 0.1 degree accuracy.
Try getting a little white bicycle reflector and go out at night with your flashlight and laser-pointer.
Prop up the reflector against a curb or wall, then back up 10 feet, then 100 feet.
The reflector makes it faster and easier to know when you are hitting the target.
Hold the flashlight or laser right in front of your nose while you aim,
because the reflected beam from those reflectors can be very narrow,
and it might miss your eyes and never look bright if you "shoot from the hip".
But do not point the laser at anybody, or at any moving cars or airplanes.
When a laser goes into a little target like an eye, it is just too bright.
It might actually hurt something,
and it's certainly too bright for them to keep watching where they drive.
Color helps too. A narrowly defined color is another way to be brighter,
for targets that care about color.
Lasers have yet another advantage that way.
If you are looking through a clear-red filter
at the wall where you aim your flashlight and red laser-pointer,
the flashlight (and any sunlight) will be 4-10 times dimmer than before,
but the laser will be almost as bright as before. It will stand out a lot more.
When I want to know how big a beam-spot at a distance will be,
I use Radians. They are like degrees of angle, except 1 Radian is about 57 degrees.
I usually just think of it as 60 degrees wide.
The good thing about Radians is they are about equal to
spot-width divided-by spot distance.
So if the angle is 1/10th Radian,
then 10 feet away the spot will be 1 foot wide.
I can figure out light-beams that way.
I hope they have been teaching you division or ratios in your grade.
I just used so much of them...
Thanks for the nice question.
The answer to your question depends on what you define as being "brighter''.
In other words, is "brightness" defined as a light source's intensity or
illuminated power over a focused area? Or is "brightness" defined by a
light source's ability to illuminate an entire room? [Paschotta, Rudiger
Dr. "Encyclopedia of Laser Physics and Technology" 1st Edition October
2008 RP-Photonics.com. < http://www.rp-photonics.com/brightness.html> 16
Consider the fact that lasers are controlled and organized photons and are
concentrated with a given direction and focal area. Flashlights, on the
other hand, radiate photons in random directions, is weaker and diffuses.
[Weschler, Matthew. "How Lasers Work" 01 April 2000. HowStuffWorks.com.
16 December 2010.]
If the laser and flashlight were both pointed in the same direction,
and are both targeted at a focal point on a wall, you probably would
say the amount of light for the given area, at the designated focal point,
on the wall was "brighter" for the laser.
But if you were in a dark room, and had both the laser and flashlight on
the floor, both pointing up, and asked yourself which of the two best
lights up the room and is therefore considered brighter - you'd probably
say the flashlight is.
So before answering the question, ask yourself, what is "Brightness"?
Brightness is not a simple property of light. It depends upon how
you measure it. Our eyes see only a small part of radiation. Most "light" we
can't see. The brightness also depends upon the geometry of the light beam.
A laser pointer is "weaker" than a flashlight, but all the light is moving
in the same direction. It does not spread out like a flashlight beam. Also
the brightness may be measured by the electrical power used to power the
light source. That is a different definition.
So it is difficult to compare the brightness of a light source
without carefully defining what it is you wish to use as the measure of
Click here to return to the Physics Archives
Update: June 2012