I'd like to show you some of the light sources that we are using
nowadays in microscopy. And the very first light source that we don't use
anymore on modern microscopes is right outside. The sun has been
used for many centuries to illuminate our specimens, often using
a little mirror that then directs the sunlight onto the specimen.
So that used to be the brightest light source we could find.
Nowadays, we're more inside and we're using devices like those standing here.
So first of all, here on the left, I have a halogen lamp. The lamp
itself is sitting in a lamp house. It is a halogen lamp, very much like
what you use at home. We can put a voltage on this, we'll then
illuminate -- the spiral here will start glowing, and that light will
then through a condenser lens be transmitted into our microscope.
So some of the halogen lamp houses have a little mirror sitting here on the back
to get more light coming out of the front. Often that's not needed,
this lamp house here is just completely black inside and all you get out is the
light coming out of the front. Since we have a condenser lens here, in front
of the filaments, that lens will make a magnified image of the filament.
And I project it here on the whiteboard, and so that image of the filament,
it's the image of our light source, is something you will also encounter in
various positions throughout the microscope. Now halogen lamps
are nice for transmitted light illumination, but they have two disadvantages. One is
that as you see, it takes quite a bit of time for the light to come on. It doesn't
switch on momentarily, and the other thing is it doesn't get super
bright. And for things like fluorescence, we really want brighter light sources.
So, a light source that has been used for many, many years in fluorescence microscopy
is this mercury arc lamp. This is the bulb, and in this bulb, there is an anode and a
cathode, and a very high voltage is being put over this such that it forms
an arc. Just the same thing as when you're welding, which of course everyone
does in their spare time. And this arc is super, super bright. That arc is
then projected so that the light goes in every direction through a condenser
lens it reaches our microscope. Also, inside this lamp house, there's often a mirror,
a parabolic mirror and that reflects the light going into the back direction
so that it comes out in the front. So you get more light coming out.
So this is a mercury arc lamp. We don't like them that much anymore
and that is because you have to replace them after about 200 hours.
Also when these things break, they're a big hazard. Immediately
evacuate the room if that happens. And they're kind of a pain to
align. You have to set them up so that this arc is positioned in a right
place, with respect to the condenser lens. So that the light goes
exactly through the optical path of the microscope. Therefore, little
boxes like this one here have become more and more popular. Here,
we have a so-called metal halide illuminator, it's also a lamp.
And that thing just clicks in here, there's no alignment that needs to be done.
The light is then delivered through this thing here, which is a liquid
light guide. So it's a tube that's filled with a liquid, something like water,
and the light is coupled in on this end and makes it all the way to the
end. So when I now assemble this and fire it up here.
Makes a lot of noise, takes some time. So one thing about these
lamps is that they don't like to be switched on and off very often.
So you know, once they're on, leave them on for at least an hour or so.
So when I now press the shutter, so that it opens, you see the light
coming out of this liquid light guide. The lamp needs to warm up a bit,
and it will become brighter and brighter over time. The light source
that is more and more replacing both the halogens and things like this
is an LED. And here I have one of those light emitting diodes. So
these things are really nice because you switch them on very, very rapidly,
there's no fans, much less heat production. And nowadays, they can
become very, very bright, as well. So often when you couple this into
the microscope, you will put a condenser lens in front of it. And you will get an assembly
something like this. And we have nice light of the wavelength that we're
interested in coming out of them. So these LEDS have defined colors, but
you can get multiples of them and combine them, and electronically
select which wavelength you would like to use.
Lastly, in quite a few microscopies, such a confocal and TIRF,
we use lasers. Lasers can look like this one here. So this is a
solid state laser. So as you know, a laser is really a narrow beam
of light coming out. And I'll switch this one on. So again, this takes
a little bit of time to warm up. Lasers, since they produce this very
narrow beam of light, you have to really watch out with them. You
don't want to get that beam of light hitting your eyeball ever.
This has a safety shutter here in the front, and so you can now see
this very bright spot of green laser light coming out.
Okay, that was the various types of light sources that we use
with microscopes.