Cleanging A Laser’s Lens

Cleanging A Laser’s Lens

Before attempting to determine the power output of a laser, first check for dust or grease on the elements in its lens system.  Optimal power output can never be obtained if the light is attenuated by junk on the lens.  There are several ways to check for such obstructions, and for cracks or defects in the lens themselves.  The simplest and least useful way is to examine the lens (while it is still in the laser module) is with a magnifier or otoscope.  When using this method, be sure and remove the laser’s power supply so that you don’t repeat the stupid mistake mentioned in the first chapter of this tutorial.

A much better way to examine the lens is to expand the diameter of the laser beam with a auxiliary lens.  A good, high magnification convex lens, with a 25 mm or so focal length, would be a good choice for this purpose.  However, in order to do a thorough inspection, an assortment of different convex and concave lens are of value.  For this purpose acromat lens (multi-component lens) are not as desirable as lens made from a single piece of glass because of the optical characteristics of the glue holding the lens elements together.  To use this method of inspection, simply hold the auxiliary lens in front of the laser and aim the laser at a light colored, non-reflective surface. Projected on the surface will be a luminescent disk who’s diameter is determined mainly by the power of the auxiliary lens.  Depending upon what type lens is used, any dirt or grease on the laser’s lens will show as a shadow or dark spot on this luminescent disk.  Optimally, if the laser’s lens is clean and not damaged, the luminescent disk should have a smooth and even appearance all across its surface.  It is important that the auxiliary lens be very clean, and the examination be done in a subdued light environment.  Note that every laser will project a different looking luminescent disk - call it a “laser fingerprint” - which can help to understand much about the lens system and the laser diode itself.   Note also that the expanded beam of certain green diode lasers will appear as two half-circles, back-to-back.  This is normal, and has to do with the frequency-doubling components in a laser module, which convert infrared light to green light.

The focusing lens in some lasers is made of glass and others are made of plastic, generally acrylic or polycarbonate.  Caution must be used when cleaning lens in order to avoid scratching them, especially those made of plastic.  Most inexpensive laser pointers use plastic lens, and often inferior grades of plastic at that.  Very high quality or high power laser modules normally use glass, because it is less likely to deform from heat buildup as the powerful laser beam passes through the lens system.  If a laser is used properly, and stored in a dust-tight container, the necessity for lens cleaning will be an infrequent event.

There are many ways to clean lens, but the first way to try is to simply blow the dust off with a blast of air or compressed gas (out of a can of dust remover).  Various brands of dust remover are sold at stores which sell cameras or computers.  If the dust is stubborn, or if there is grease on the lens, more aggressive means must be used.  If the end cap of a laser module can be easily removed, the lens can be cleaned in the same way as a camera lens, using lens cleaner and lens tissue or microfiber lens wipes.  Where important, care should be taken to avoid rotating the lens and changing the laser’s focal point.  Unfortunately, most pocket lasers have end caps which do not unscrew, and are therefore difficult or impossible to remove.  What lens cleaning is done must be done through the same small orifice that the beam comes out of.  One way to clean through this orifice is to twist a piece of lens tissue into a long, thin wick, lightly saturate it with lens cleaner, stick it through the orifice, and rotate it against the lens.  If adequate cleaning can’t be accomplished by this method, you might try wrapping a lens wipe around the end of the thin plastic straw which came with the can of dust remover, and use this combination as a cleaning tool.  The reason to use the soft plastic straw is because (normally) it is a much softer plastic than that of a laser’s lens, and will therefore be less prone to scratch the lens should the wipe slip off the end.  Never use toothpicks, nails, ink pens, or other hard items when cleaning a lens system.  And remember, though glass is much harder than plastic, it too can be scratched by abrasive dust particles on the lens, or on items used to clean the lens.  Always used compressed gas to first remove these hard particles before commencing other cleaning methods.