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Laser Demonstration Safety

When doing laser demonstrations at schools and lecture halls, some instructors have the option of equipping each student with laser safety goggles, but more often than not, a school’s budget does not allow for such finery.  Here’s a simple way to do laser demonstrations and experiments in such a manner as to not risk the health of the students or break the equipment budget of the school:  Simply purchase a large sheet of dichroic filter gel, like the type used in stage lighting systems, and suspend it between the students and the target at which the laser will be fired (i.e., balloon, match, etc.).  Hang it close to the target, so that any light reflected off the target will not reach the eyes of the audience (without first being attenuated by the filter gel).  At some point beyond the target, have some means of containing any secular reflections from the point where the beam impacts the wall or other surface.

 Something as simple as a cardboard box might suffice.  When a classroom is set up in this manner, students can clearly see the target (through the gel), and watch how it is affected by the laser - all in comparative safety.  A couple of excellent sources for dichroic gels are:


A good choice for filtering the light from a red laser might be a sheet of Roscolux #94.  A good choice for filtering green laser light might be Roscolux #41, #324, or #19.  For filtering blue lasers, Roscolux #19 might be a good way to go.  When choosing filters, it would be wise to contact Roscoe or Lee directly and consult their technicians about it.


In building remodeling or appraisal, it is sometimes important to determine how windows are put together, how many panes they contain, or how thick they are.  In zoological aquariums there may be a need to determine how thick an aquarium window is, or how it is constructed, or what its internal integrity is.  Sometimes these kinds of measurements can be done with simple observation or with conventional tools, but such methods are often inaccurate or difficult.  Enter the laser way!
By aiming a laser beam into a window, one can quickly and accurately ascertain how thick the window is, how many panes it contains and the spacing between them, how thick each pane is, whether the panes are glass or plastic, and whether the panes have air gaps or are glued together (laminated).  Accurate measurements are only possible if all the panes which make up the window are parallel to one another.  These measurements are made possible because of the refraction index differences between air and glass, air and plastic, or glass and plastic.

Trigonometry is required for accurate thickness determination, but if you are averse to trig, simply purchase or fabricate an equilateral triangle.  It could be made of wood or plastic, and the only thing critical is that each corner of the triangle be 60 degrees.  Using the triangle as a laser mount and angle guide, aim the laser into the window.  Where the beam enters the window, a dot will be observed, thanks to refraction at the interface of the pane and the air.  Where the beam enters the next pane, another dot will be seen.  This process continues thru each pane in the window until the furthest pane is reached.  At the outside surface of the furthest pane (the back side of the window), the laser beam is reflected back along a new path until it exit’s the same pane it entered.  What will be observed is a series of dots going into the window and another series coming out; the number of dots being determined by the number of panes in the window, and their composition.  If the laser beam enters the window at exactly 60 degrees, the distance between where the beam enters and exit’s the front pane will be the overall thickness of the window.  Note that as the laser beam is reflected off the different surfaces within the window, each surface will start its own reflection path. For instance, if a window is comprised of three panes of single strength glass, and the end of the laser is kept a few millimeters from the front pane, a beautiful array of twelve dots will be observed.  On the outside of the pane that the beam entered, there will be three dots.  The distance between the entry dot and the nearest exit dot will be the overall thickness of the first two panes.  The distance between the entry dot and the furthest exit dot will be the overall window thickness.

In windows made up of thick plastic panes, a total of six dots per pane may be observed, and there may be several dots visible on the front pane, but the method used to measure thickness remains the same.  If the laser beam enters the window at exactly 60 degrees, accurate measurements can be made irrespective of the composition of the window; and the technique will often work with frosted windows.  Bear in mind that the beams of most pocket lasers are not aligned in the same exact axial plane as the tubular housing containing the laser’s components.  This must be taken into account, and appropriate angular adjustments made, in order to obtain accurate measurements.

Besides being used to measure thickness, this technique can be used for insights into the window’s construction.  Telling the difference between glass and plastic panes is usually easy because the laser beam will be visible as it passes through thick plastic panes.  In thick, laminated, plastic aquarium windows, it only takes seconds to determine how many panes were glued together to construct the window, and how thick each pane is.  Unfortunately, thanks to the similar refraction indexes of ice and water, this technique cannot be used to determine the thickness of ice, so it can‘t be used as a means of determining where to safely to cross frozen rivers and lakes.

Window measurements should normally be done with low powered lasers which pose no visual hazard.  The color of the laser matters little.  It helps greatly, and much more detail will be seen, if the diameter of the laser beam is very small.