So, there are two kinds.
Very big kinds in modern technology of light sources.
And one black body is a reasonably good description for lamps, sunlight,
the light of the natural world is very broadband,
and you can see this is a lamp spectrum but notice it
generally has the shape we were just seeing of that black body.
And the other with the advent of modern laser technology is dramatically different,
and it is extremely narrow in spectrum.
I know, not all lasers are but most lasers are very narrow in spectrum.
And notice that my bottom plot here is going more than the entire visible,
the visible spectrum is maybe mid four hundred up to eight hundred-ish.
And this spectrum here is a 820 to 920.
So it all fits right in one of
these tick marks on the visual lamp spectrum below, and it's really narrow.
And it's on a log scale.
So if you plug this on a linear scale,
you'll just see a line here.
So a, laser's don't have to be but generally are much narrower in spectrum.
And an important thing for
an optical or electrical engineer is to know something about efficiency.
Most lamps are pretty low efficiency.
Now, we're getting better and better at that.
And we're moving from incandescent, to fluorescent,
to solid state like LED lighting,
chiefly because of that efficiency.
But incandescent lamps can be horribly inefficient. Just a few percent.
And that's what quite efficient,
amount of electricity in to total amount
of optical power out the rest of course being shuttle's heat.
Lasers don't have to be efficient but they can be.
And diode lasers, the same fundamental technology as LEDs,
can be well above 50% efficiency.
So, in my very first part two and I actually
showed 100 watts as if it was a 100 watt light bulb.
That 100 watt light bulb might only radiate 5 watts of optical power.
And so, be careful I guess,
is the point when you're talking about power.
What do you mean the electrical power in or
the total optical power out of your light source? That's kind of an assign.
The main thing I want you to remember is radiance here,
our capital L. Let's calculate the typical radiance
or INFOTO metric units brightness of a laser versus a lamp,
just sort of typical numbers.
And let's assume just for a moment,
that our laser can be described by
a Gaussian beam because that's often the case of lasers,
sort of certain amount of total power feed in Watts.
And then let's rate the area of my laser beam,
and because radiance is in power per area plus the radian,
let's just take pi r squared as a quick estimate of the size of my laser beam,
and for the radius, I'll take the Gaussian rest radius.
Then I know that the angle that my Gaussian beam would be described by,
is inverse with that Gaussian rest radius.
So I can just plug in my expression of a theta note,
the angle of the Gaussian beam.
And of course, the obvious point is those two are inverses and that's
what's interesting about the shape of light with those two inverse.
And I've thrown in the factor M squared here just so you've seen it.
This is unfortunately, not magnification.
M squared is a factor that is often used to describe
a typical high-power lasers and it's basically how
much bigger is the angle of radiation than
you'd expect for the diffraction limited Gaussian beam.
So when you get to very high-power lasers,
they can diverge a little bit more than
the perfect collimated single mode Gaussian laser beam.
So there's this factor M squared here which can be bigger than the one,
that describes their divergence is bigger than you might expect.
And therefore, their overall brightness is lower, just so you've seen the number.
Let's assume we have a high quality laser and M squared here is one.
In general we find,
that the brightness of a laser is like it's power divided by its wavelength squared.
The number of wavelength is on the order for this type of lasers over micron or less.
And so, if you had let's say a one watt optical power laser,
you'd expect a brightness in the area of 10 to 12th watts per meter squared poste radian.
And the 12, so big number.
But I don't know how big, so let's compare it to a lamp.
Let's take a bulb with total optical power v,
maybe a one millimeter square filament,
that might not be fair but it's not too far off.
Maybe this is a halogen ball or something like that.
And it's going to radiate.
And here's the big difference into all angles for poste radians.
LEDs, light bulbs, black bodies,
don't know anything about direction.
And so, the photons come off of those hot objects in every possible direction,
and that is a big,
big difference between the angular divergence of a laser beam of course.
So if we plug in some typical numbers here maybe a 100 watt bulb,
we come up with a brightness that's seven orders of magnitude lower.
And that is the real point here.
Is lasers are bright.
They have extremely high radiance in comparison to light bulbs.
That's one of the things that makes them
interesting for photo physics and things like that.
So you need to keep in mind that lasers versus light bulbs are dramatically different in
their radiance and that's super
important when you go to use either one in an optical system.
Amy SullivanResearch Associate
Robert McLeodProfessor
