1. NEVER TOUCH OPTICAL SURFACES. Perspiration may be sufficiently acidic that the pattern of a fingerprint can become etched into a glass surface and be impossible to remove. Handle lenses by their edges only. Optical workers wear soft cotton gloves when handling lenses.

2. NEVER TOUCH GLASS SURFACES. The glass surfaces of projection lamps, concentrated arc lamps, photocells, etc. must not be touched. Fingerprints on the glass surfaces of lamps can cause fracture of the glass when it becomes hot. Foreign substances on the surfaces of photocells can cause electrical leakage which degrade its performance.

3. AVOID SURFACE ABRASION. Never lay a lens on a table, for this can scratch the lens surface. A convex surface is especially vulnerable to such damage, for it will be abraded at its center, its most useful part. When lenses must be set aside, lay them on soft cloth or sheets of foam plastic provided at each lab station.

4. FRONT SURFACE MIRRORS are especially vulnerable, for they have an unprotected thin coating of silver or aluminum on the exposed front (reflecting) surface. Fingerprints on this surface cannot be removed, and destroy the usefulness of these expensive mirrors. The metal is soft enough to be scratched easily even by lens tissue. THE STUDENT SHOULD NEVER ATTEMPT TO CLEAN FRONT SURFACE MIRRORS. The instructor will clean them, if necessary. If a fingerprint is fresh it may be possible to remove it by "lifting" it with appropriate sticky tape, like Scotch Magic © tape. A camel's hair brush may be gently used to remove dust specks, or dust may be blown off using a pressure can of dry, clean air.

5. LENS CLEANING. High quality lenses, like camera lenses, generally consist of a number of elements, some of which may be cemented together. Cleaning with solvents is to be avoided, for some solvents can penetrate the cemented surfaces, dissolving the cement. Even the most careful cleaning methods, if too frequently used, degrade the lens surface, especially if the lens is anti-reflection coated. Protect optical surfaces to avoid the necessity of cleaning. Do not expose them to air pollution—keep lenses capped or in a case. For example, cooking grease in the air of a home can deposit a foul film on the exposed surface of a lens. Do not use solvents or lens tissue if the only problem is dust particles. Particles may be blown off with an air blast from a small ear-syringe. Do not use air from a compressed-air pump, for it can contain water, oil, and grit.

When a surface film or fingerprints must be removed, use lens tissue designed to be used wet. Dampen the tissue with a suitable solvent, like a commercial lens cleaning solution, and gently wipe the lens (without finger pressure) using a circular motion and working from the center outward. Recent research indicates that the silicone-treated cloths and papers sold for cleaning eyeglasses are not safe for coated lens surfaces, even though they are sometimes sold in camera stores.


1. Lens holders are usually designed with a position indicator that indicates the true lens position along a calibrated scale. The experimenter should check each lens holder to be sure this is actually so. Illuminated object-source boxes often have the object position displaced from the indicator. This displacement should be checked, measured and compensated for.

2. Adjust the heights of all lens holders so that the axes of all lenses, object, and image lie on a common axis parallel to the measuring scale.

3. Periodically lubricate the machined surfaces of optical benches with a rag dampened with machine oil. This prevents corrosion and improves the smoothness of movement of the carriages.


1. Optical instruments with movable mechanical parts often have clamping screws to lock an adjustable element in position, preventing its motion temporarily. These screws should be tightened with the fingers only, just tight enough to prevent slippage, no more. Forcible tightening can put dents in rods or shafts, sometimes preventing free motion latter, or making it difficult to position the screw near the dent.

2. When moving optical apparatus, pay attention to its construction and the strength of its parts. One does not lift a spectrometer by one of its arms, for that could bend the arm. Support it under its center shaft when carrying it.


1. Never look directly at the sun with the unaided eye. Sunspots can be observed only with adequate, specially designed filtration. Smoked glass as a filter is risky if not uniformly or thickly smoked. One layer of fully exposed photographic film is not enough protection. Consult instructor.

2. Looking at the sun through binoculars or telescope is even worse, for these instruments collect many times more light than the unaided eye. The only safe way to use these is to pull out the eyelens beyond its normal position until the telescope casts a real image of the sun on a white paper screen. This image on the paper may be safely observed.

3. Polaroids, even crossed ones, are not an adequate sun filter. They do not remove the infrared and ultraviolet radiation, which does the most retinal damage. This is especially dangerous because you don't see or feel it until the damage is done. Even moderate exposure to infrared is known to be capable of causing cataracts.


1. Helium-neon lasers used in introductory optics laboratories are low power (about 0.5 mw), and are safe if used intelligently. Never look directly down the narrow laser beam, and avoid looking at beams reflected from smooth surfaces. When the beam is diverged by a lens, the energy/area is small enough to be safe.

2. Special protective goggles are available which filter the laser frequencies but transmit other frequencies of light. Use then when you are working with medium or high power lasers, but be sure the specifications of the goggles are matched to the power and frequencies of the lasers you are using.

3. When you must look at a focused laser beam through an optical instrument, as when observing diffraction patterns, use neutral density filters to reduce the beam intensity. Then reduce the filtration until the light becomes visible, but not overly "bright."

4. Lasers with exposed mirrors and Brewster windows are vulnerable to dust. One dust speck can suppress or reduce laser output. Use lasers only in a clean environment and cover lasers when not in use.

5. Older model lasers should be used for a few hours every month. If they have been idle too long, they may not lase right away. Try leaving such a laser on overnight, or until it begins to lase. Modern "hard-seal" lasers do not have this problem.


1. Mercury, hydrogen, and some other gas discharge sources may emit considerable ultraviolet. Carbon arcs and concentrated-arc sources are serious ultraviolet hazards. Shield the eyes from direct and reflected light from these. Safety goggles may be required, but be sure the goggles are designed to absorb the correct wavelengths with sufficient attenuation.

© 1991, 2004 by Donald E. Simanek