When it comes to video lighting, the redhead or open-face unit is the
workhorse. In essence it's a can with a 800w lamp and a reflector.
There's no lens in front of the lamp (don't call it a bulb, bulbs grow...
lamps glow) which is why it's "open-face". Usually, for safety reasons, it
will have a metal grid to catch the larger pieces of glass if the lamp
explodes, which they do occasionally.  
The original units made by Mole-Richardson had a red housing, hence
the name redheads. The next size up, the 2kw lamps had a yellow
housing and open face units of this size are still called "blondes".
This particular unit by Arri, the Arrilite 800, is probably the best made,
and most common, redhead in television and movie use (I know it's
blue, not red, weren't you paying attention?). When buying video
lights, never go for cheap, always go for quality and expect to pay
about £200 / €300 for an Arrilite 800.
Darkness Visible:
Part one - Lamps and Stuff
Magazine

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Open face units are difficult to control. The barn doors are more to prevent spill rather than control the
shape of the light. The open-face unit has a simple way to alter the beam size by moving the lamp. It's a
very rough-and-ready way of controlling the light though. In an ideal world there would be a lens in front
of the lamp to focus the light. The problem is that video lamps put out a lot of heat, which can crack and
shatter glass lenses and melt plastic. Dedolight( see below) has come up with one solution but one way
of dealing with the problem is to reduce the thickness of the lens.
This type of lamp unit, the
fresnel  invented in the 1920s,quickly became the most common overhead
studio light and is only now being replaced by more efficient methods of controlling light.
Redheads, because of the way they reflect light,  have a tendency to give slight double-shadows. This i
isn't a problem with fresnel units such as the Arri Junior range but the main advantage of the fresnel
over the open-face is that it is much easier to focus and control with barndoors.
The metal flaps on the front of
the unit are called barndoors.
They are not always included
in the basic price of the unit.
They can get
very hot in use.
It's quite easy to control light,
you just need a lens.
Unfortunately this means
weight and a  lens large
enough to focus a movie lamp
would weigh a ton!
However, since it's the surface
of the lens which does the
refracting, a chap called
Fresnel came up with an idea....
By cutting out the glass which
doesn't actually do much in the
way of controlling the light, you
can drastically reduce the
weight and mass of the lens.
The whole thing is then
flattened. The result isn't good
enough for a camera lens but
is certainly up to the job of
focusing a beam of light.
In effect you get a thin lens
which dissipates the heat,
doesn't crack, and is still an
affordable focusing method.
Unfortunately it's not very
efficient energy-wise and, in a
typical fresnel unit on "spot"
only about 10% of the light is
available. This was the case
until the arrival of the Dedolight.
The Dedolight: The next development in lighting.
As film making techniques changed, emulsions improved and video burst onto the scene, the demands of
lighting camera(wo)men and DoP (Directors of Photography) increased. One particular DoP, Dedo Weigert,
decided to create his own light and commissioned engineers to produce a lightweight, controllable and
versatile unit. The Dedolight has since gone on to win awards and adulation all over the world.

The theory behind the Dedolight is simple. You take a low voltage, high output lamp and add top quality
optics. Because the lamps don't burn at the high temperatures of the previous units, decent lenses and
reflectors can be used without excess weight or the danger of cracking. This results in a much more efficient
light output and a beam which is easier to control.

The unit comes with a dimmer and has the ability to have all sorts of projection units, beam shapers and
effects added. It is, however, quite a bit more expensive than the older fresnel and open face units but hit
the video and film making industry like a bomb.
The Fresnel, a bit more control.
A soft touch: The Kino Flo...