Measuring Laser Energy Loss
Country: United States
Date: October 2006
I am conducting a experiment to
decide whether I should use plastic or glass
mirrors to form a design using lasers. Part of
that experiment requires finding out how much
energy is lost after each time the laser hits a
mirror. So, what I need to know is, what can my
teachers and I use to complete these measurements of
the strength of the laser that can be done in a
high school laboratory?
It makes no difference whether the mirror substrate is
plastic or glass, if you use the correct type of mirror for
this application. The type of mirror recommended for use in
accurate optical work is called a "First Surface Mirror".
This type of mirror has its reflective coating on the front
side of the mirror, so the light being reflected never "sees"
the plastic or glass. These types of mirrors are available
from many scientific sources, including Edmund Scientific.
With regards to plastic mirrors, there are two general types
of plastic used: acrylic and polycarbonate. My recollection
is that optical grade polycarbonate (this is what CDs are
made from) has slightly better optical characteristics than
acrylic. But the differences small; either type is
comparable with glass regarding its light absorbent
properties. But as mentioned above, the use of a First
Surface mirror is really the way to go. You can then
determine the amount of light being lost as a result of
reflective losses, without the uncertainty of absorption
losses in the mirror's substrate. Note that you must never
touch the reflecting surface of a First Surface mirror,
because it is very delicate and cannot be cleaned of
Another variable is what the reflecting surface is made of.
Glass mirrors have silver deposited on them as the reflecting
medium, or they may have vacuum-deposited aluminum. Nearly
all plastic vacuum-deposited layer of aluminum instead of
silver. Silver and aluminum have slightly different
reflectivities, so you may want to experiment to see what the
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Update: June 2012