Its What You Dont See …
Care and Cleaning of Radiometer Optics
By Jim Raymont and Paul Mills
EIT Instrument Markets
The left and center photos show radiometers in need of better care. The photo on the right shows a radiometer that took a swim in coatings.
Cleaning a radiometer with the wrong tools can cause more harm than good. The photo on the left shows contamination on the instrument, while the photo on the right shows a view of optical window under magnification that was exposed to coatings and improperly cleaned.
The effect of improper cleaning on precision optics: From left, a normal optical window; an optical window that was cleaned dry and with abrasive cloth; and a scratched optical window.
Invisible optical brighteners, such as those coating the fibers of this cloth, can cause inaccurate UV readings.
If the optics on a radiometer look dirty, the first reaction of many people is to clean them with whatever is handy – a shop rag, a shirt sleeve or finger. Sometimes optics get cleaned with a dry cloth and other times they are cleaned with whatever solvent is handy. A little to a lot of elbow grease also might be applied. Other radiometers just cant resist the opportunity to go for a quick swim in the coating, ink, adhesive or resin. Beware – this kind of cleaning is likely doing more harm than good.
Its hard to imagine treating eyeglasses, or the lens on a brand new digital SLR camera, the same way the radiometers above were treated.
The outer optical window of a UV radiometer is a critical to providing accurate UV measurements. It protects the internal optical components in the instruments optical stack. While human eyes can see things in the visible spectrum, the radiometer is designed for measuring wavelengths that humans cant see. This means that even though that recent cleaning may sparkle to the eye, it could be causing havoc with UV measurement.
The cloth used to wipe it clean may contain optical brighteners. Optical brighteners, optical brightening agents, fluorescent brightening agents or fluorescent whitening agents are chemical additives that absorb light in the ultraviolet and violet region (usually 340-370nm) and re-emit light or fluoresce in the blue region (typically 420-470nm). These additives are used in fabrics to create a “whitening” effect, making them look less yellow by increasing the overall amount of blue light reflected. Absorbance and re-emission of light can distort UV irradiance measured by a radiometer.
Another mistake is the use of cleaning solvents that can react with wipes and swabs, dissolving materials, such as fibers and glue, and transferring these contaminants to the optical surface. Some aggressive solvents also can attack the radiometer components causing destruction and decomposition. Using an abrasive cloth or grinding loose dirt particles into the optical surface causes small scratches, sometimes barely visible to the eye. Since the optical components are precision materials designed to scatter light uniformly across the detector, an uneven surface can translate into inaccurate UV readings.
To help correctly maintain an instrument, weve developed a recommended procedure for cleaning radiometer optics using either cotton swabs or prepackaged instrument wipes that use a lint-free nonabrasive cloth and do not contain any detergents or additives. These will help to avoid common cleaning pitfalls and enhance the accuracy and reliability of the device. A general overview of cleaning optics is below with additional information, including YouTube videos posted on the EIT website. Also refer to information from the manufacturer of the instrument.
Visually INSPECT the optical surface for signs of contamination, residue, fingerprints and loose debris on or near the optical window. BLOW any loose particulates free by holding the device vertically. Using an air bulb, clean, dry compressed air, or residue-free aerosol chemical de-duster, remove loose dirt. When using de-duster, position the nozzle 10-12″ from the optical surface so that the de-duster doesnt build up on the window surface. Using the rapid moving air, not the chemical itself to blow the surface clean. If using cotton SWABS, apply a liberal amount of isopropyl alcohol to the tip. Use sufficient alcohol to thoroughly wet the swab. Moving quickly (since the alcohol evaporates), use a back-and-forth motion while rotating the shaft of the swab between your fingers, clean the surface. Never reuse a swab. If more cleaning is needed, use another swab. Used swabs will contaminate the surface. Use a clean, dry swab to remove excess alcohol. If using prepackaged chemical WIPES, put on clean, disposable rubber gloves. Handle gloves from the ends. Never touch the fingers of the glove with bare hands. Gloves protect the skin and prevent oils from the skin contaminating wipe. Tear open the foil packaging, remove the wipe and unfold it gently. Gather some of the wipe between the thumb and forefinger for cleaning. Using the gathered material, CLEAN the surface using a firm but gentle motion. Re-gather the wipe material occasionally so that clean, wet portions of the wipe are used to clean the surface. Repeat this procedure until the surface is clean. Always DISPOSE of the wipe and packaging safely, according to the instructions on the package.