This UVC wand is sold by Walmart for $20. The listing makes no mention of the potential hazard UV light poses to eyes and skin.

"What we would suggest is definitely looking at products that do have a safety certification from a nationally recognized testing laboratory, such as UL," Straka adds. "There are installers and commercial-grade products out there that do have a safety certification and which can be installed and used very safely."

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The coronavirus pandemic has businesses and homeowners alike seeking out anything that might make shared spaces feel safer and more sanitary. That has some consumers and product manufacturers turning to ultraviolet, or UV light as a potential solution -- specifically, a type of UV light called UVC, which has extra-short wavelengths and enough energy to alter the DNA and RNA of organic cells, which stops them from reproducing.

"Walmart is committed to providing safe and compliant products to our customers. One way we do this is by requiring our suppliers to comply with all rules and regulations applicable to the products they sell, whether those items are sold in stores, online, or on our third-party marketplace. If Walmart learns that a supplier is providing non-compliant items, those items are promptly removed from sale. We are actively reviewing this matter and appreciate you bringing it to our attention."

Digs – Digs are characterized as regular or irregular inclusions in or on the surface of the grating. They can be resident on the master grating, or introduced during the replication or coating process.

"This is a hazard you can't see," explains Pamela Gwynn, principal engineer for UL's Life and Health Sciences related to medical equipment. "They're not toys. They're not instantaneous. There has to be some time for the device to be in place for it to be able to do the work it's intended to.

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Protected Aluminum (Al) – Aluminum coat with a thin overcoat of magnesium flouride (MgF2) which prevents the formation of aluminum oxide which is absorbing in deep UV. It provides no benefit over bare aluminum for gratings used in VIS and IR.

"Companies selling pesticide devices that claim their product is 'EPA-approved' or 'EPA-certified' are making false and misleading claims," an EPA spokesperson adds. "In EPA's regulatory context, these terms do not hold any meaning or significance for pesticides or pesticide devices."

In an age when Clorox wipes are a precious commodity, sanitizing UVC light might seem like a promising alternative. But experts urge caution for anyone tempted to bring the tech into their home.

"We know that UVC is a proven way to help eliminate dangerous bacteria and viruses in water, air and on surfaces," says Karen Willis, NEMA's industry director for lighting systems. "Nevertheless, in the midst of COVID-19, we are concerned about proliferation of UVC disinfecting devices being sold with uncertain safety features and incomplete operating instructions."

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Diffraction gratingformula

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The simple fact of the matter is that, except in extreme cases, the performance of a diffraction grating is primarily a function of the things that you cannot see with the naked eye. The grating efficiency is a function of the shape of the groove and the reflectance of the coating. You cannot evaluate this with the naked eye. The stray light performance is primarily a function of the micro, not macro, structure of the grating surface. You cannot see a rough groove structure, or nonspecular reflective surface with the naked eye, but it’s easy to see a small dig or light scratch.

What is gratingconstant

"The damage that's being done is also not instantaneous," Gwynn adds. "So it may take several days before you realize you've damaged your eyes or caused burns on your skin."

For best efficiency, the arrow should be oriented so that the tip points back towards the source, inscribing the smallest angle possible, as shown below.

Man-made UVC light is just as intense, and just as good at ionizing organic molecules and altering their DNA and RNA. For years, scientists and medical professionals have used the invisible light as a natural disinfectant against bacteria and viruses, including the coronaviruses that cause illnesses like SARS and MERS. While scientists are still working to determine the full efficacy of UVC light against SARS-CoV-2, the coronavirus that causes COVID-19, the early indications are promising -- enough so that the FDA issued guidance in March that it "does not plan to object" to any previously cleared sterilization or disinfectant devices updating their marketing to indicate that they combat COVID-19.

When a master ruled grating is generated, the diamond tool does not actually remove material and cut a theoretically shaped groove. Rather, the coating is burnished by the tool. As a result, there is some displacement and deformation of the material on the short facet into the previously ruled groove every time a new groove is formed. The resulting profile will show some peak round-off, and not achieve theoretical depth. Actual groove depth is typically 90% of theoretical.

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One other word of caution here: Beware of products that claim "EPA certification" as some form of reassurance about safety or efficacy. EPA certification only signifies that the company making the device has registered with the EPA, and is listed in its database of companies who make products that include an active pesticide. Being listed in that database does not certify that the company's product is safe or effective for use in the home.

Amazon didn't respond to a request for comment, though its product listings note that the company assumes no liability for inaccuracies or misstatements about products.

The efficiency of a grating in polarized light is dependent on the orientation of the plane of polarization relative to the direction of the grooves. For maximum efficiency, the grating should be oriented such that plane of polarization is oriented perpendicular (s-polarization) to the length of the grooves.

"Because disinfection kills most recognized pathogenic microorganisms, it can generally be inferred that sterilization and disinfection should minimize the viability of SARS-CoV-2 on surfaces and in the air in confined spaces," the FDA wrote. That gave manufacturers a green light to begin marketing potential COVID killers.

What is a diffraction gratingused for

Holographic master gratings are produced by exposing a thin layer of photoresist to 2 intersecting coherent, monochromatic beams. The resulting interference pattern differentially exposes the photoresist. After development, the sinusoidal variation in light intensity during exposure is transformed into a physical structure of the same profile. The addition of a reflective overcoat completes the process.

Diffraction gratingexperiment

Because no material is 100% reflective, absolute efficiency measurements will always yield a lower numerical efficiency value than a relative efficiency measurement on the same grating. Grating efficiency for a given wavelength and groove spacing is strictly a function of the groove shape and the reflectance of the coating. The optimum groove shape is then a function of the angle of incidence and order of use.

In the meantime, research continues into the potential efficacy of UVC light against the virus that causes COVID-19. Some companies that sell UVC products, like Signify and Big Ass Fans, are putting those products to the test at third-party research labs to verify effectiveness against SARS-CoV-2. Researchers at Boston University found that Signify's wall-mounted UVC lights killed up to 99% of the virus within seconds; the company is now broadening its portfolio of UVC luminaries for commercial environments.

The Haiku UV-C smart ceiling fan from Big Ass Fans uses built-in UV light to disinfect air as it circulates throughout the room. It's already making its way into homes, businesses and schools across the country.

A diffraction grating is a passive optical component that redirects light incident upon the surface at an angle that is unique for every wavelength in a given order. This redirection (or diffraction) is a result of the phase change of the electromagnetic wave as it encounters the regular, fixed structure of the grating surface. Every wavelength undergoes a different phase shift, and as a result, diffracts at a different angle, resulting in a dispersion of broadband light.

The invisible light can kill viruses and pathogens like the one that causes COVID-19, but experts are raising alarms about the potential safety risks.

Straka and other safety science experts at UL recently teamed with the National Electrical Manufacturers Association and the American Lighting Association to address these concerns in a two-page position paper (PDF) outlining the potential risks of using uncertified UVC devices. Straka calls the situation "a major safety issue," and his counterparts at NEMA and the ALA agree.

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One potential source of risk is older, mercury-based UVC lamps, as well as lamps that emit ozone, which can be toxic to the lungs -- but newer, UVC LED lamps are a potential hazard, as well. The biggest concern, UL's experts say, are uncontained devices like light wands that could potentially expose a user's eyes or skin to the invisible UVC light at close range, causing damage. UL is unwilling to certify wands like those due to the high risk in an uncontrolled home setting.

Many of the products currently being sold seem to downplay that risk, or ignore it altogether. One of the sponsored products at the top of Amazon's search ranks is a UVC light wand from a company headquartered in Shenzhen, China called 59S. The product features a child safety lock and an auto-shutoff feature if the wand isn't shining downward, but a misleading chart at the top of the product listing comparing the wand's LEDs to other types of UV lamps wrongly claims that the light comes with no side effects. You have to scan the fine print in one of the product photos to find a warning about exposure to the eyes and skin -- it isn't printed anywhere else in the listing, though it does say you should wear protective goggles. And, as an observant CNET reader pointed out, Amazon has it categorized under "Hand Sanitizers."

Now, interest in UVC light is surging. Signify, formerly Philips Lighting, which has been producing UVC lighting for more than 35 years, tells CNET that interest in the lights has spiked enough to justify an eight-fold increase in production since the start of the pandemic. In Kansas, Digital Aerolus developed a UVC drone that can fly through contaminated spaces disinfecting everything it comes across. Here in Kentucky, Big Ass Fans recently released a UVC ceiling fan designed to disinfect air as it circulates throughout the room.

Scratches – Scratches are characterized in the conventional sense, deformation lines running in any direction other than the direction of the grating grooves.

Ruling Glitches – Ruling glitches appear to be scratches that are perfectly straight and perfectly parallel to the groove direction. They appear only on ruled gratings, and are an artifact of the ruling process. During the ruling operation of the master grating, a small bit of the aluminum coating on the master blank will occasionally seize onto the diamond stylus and deform a few grooves before clearing itself from the tool. The deformed grooves are parallel all others, and are ruled at the same pitch as all others. If the deformed grooves are extremely ragged, it can be argued that they could degrade stray light performance, but their most likely affect is to simply redefine the blaze properties for those few grooves. Ruling glitches are not considered to be functional defects unless extremely excessive in quantity.

Holographic master gratings generally exhibit better stray light properties than ruled master gratings. Blazing is not as easy with holographic gratings however, and with certain notable exceptions, they will not be as efficient as ruled, blazed gratings.

Most holographic grating masters are generated initially with a symmetric groove profile. It is important to note that a symmetric profile holographic diffraction grating will only have symmetry in efficiency on either side of zero order when the light is incident at 0 degrees (normal incidence). This explains why some symmetric holographic gratings can achieve greater than 50% absolute efficiency in a given order, although most do not. Special techniques can be employed to give some holographic gratings an asymmetric profile, and hence, blaze properties. These gratings combine the beneficial low stray light properties of holographic grating with the high efficiency of ruled gratings.

Dispersion is the ability of a grating to angularly separate adjacent wavelengths of light. The higher the separation, the higher the dispersion.

Those are all high-end, commercial-grade products -- but interest in cheaper, simpler UVC products that people can bring into their homes is on the rise, too. That's where safety experts are beginning to sound the alarms.

Pinholes – Pinholes in a reflection grating serve only to reduce the total amount of light available for diffraction by the ratio of their area to the total area illuminated. This is insignificant. Any light passing through a pinhole in the coating is automatically rejected from the optical path of the system.

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As the angle of diffraction approaches 90 degrees, the angular dispersion increases. Decreasing the groove spacing, increasing the angle of incidence and operating in higher orders are all effective ways to increase dispersion. Any set of conditions allowable by the grating equation that increases the angle of diffraction will increase angular dispersion.

Quality vs. Function Here is where subjectivity comes into play. Everyone will have a different definition of quality. Some will include appearance, some will include only function, and some will include a combination of function/appearance/consistency relative to cost. When a universal definition is adopted, there will be no more debate on this matter. Until then, the debate continues.

What is a diffraction gratingin physics

Elsewhere, one recent study out of Columbia University looked at "far-UVC" light that shines at a less intense wavelength -- still strong enough to zap viruses, but potentially incapable of penetrating eye and skin tissue.

Run a quick search for "UVC" on Amazon or at Walmart and you'll find page upon page of products that promise to sterilize your home. With names like "Germ Guardian," "Bio Shield" and "The Germ Reaper," most claim to kill 99.9% of viruses and other pathogens they come into contact with.

Ultraviolet light is invisible electromagnetic radiation that falls between 180 and 400 nanometers in wavelength. UVC light is ultraviolet light that falls specifically between 180 and 280 nanometers -- the shortest, most intense part of the ultraviolet light spectrum. Unlike less intense UVA and UVB light, which will burn your skin if you sit out in the sun too long, UVC light can burn skin within seconds. Natural UVC light from the sun is completely absorbed by the Earth's atmosphere.

"Under the pandemic, we've seen a great increase in the interest around UV germicidal-type devices, including those that are now more consumer-facing," says Todd Straka, global industry director of lighting at Underwriters Laboratories, where UVC products are tested for safety certification. "These can be very dangerous if you don't use them properly."

What is gratingin physics

The master gratings are produced by forming the surface of a soft metallic coating with a diamond form tool. The resulting groove profile has a well defined and controllable groove profile that directs energy efficiently into the desired wavelength range.

"There are emerging studies to understand the effectiveness of UVC against COVID-19," Straka adds, "but in terms of research, there's a lot still pending on that. So I would caution consumers ... if they're seeing really robust claims about efficacy, to keep in mind that that is still very much in the works."

The grooves of a ruled grating have a saw tooth profile with one side longer than the other. The angle made by a groove’s longer side and the plane of the grating is the “blaze angle.” The blaze angle for a blazed grating is generally the biggest factor in determining where the efficiency curve peaks under a certain set of conditions.

Now, as research into UVC and the coronavirus continues, regulators, industry leaders and safety science professionals are urging caution. Here's what you need to know -- the science, the safety risks and everything experts say you should take into consideration before bringing any UVC-powered light source into your home.

All of Optometrics’ gratings are marked on one edge with a blaze arrow. The figure below shows a typical arrow, and its relation to the blaze angle of the grating. For best efficiency, the arrow should be oriented such that the tip of the arrow points towards the source, inscribing the smallest angle possible, as shown.

Of all of the topics that can be discussed relating to a diffraction grating, visual appearance is probably the most subjective, misunderstood, and maligned property one can think of. The reasons are understandable. When someone looks at a grating and sees what appears to be a flaw, the natural impulse is to imply a negative affect on performance. This may or may not be the case in theory, but is hardly ever the case in practice. A grating’s visual appearance, unless obviously grossly damaged, should never be used to assess its functionality.

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What is gratingelement

Another wand listed on Walmart features no child lock or auto-shutoff at all, and the listing never warns of any hazard to the eyes or skin -- though it does boast of lights that are twice as intense as a previous model. There's no mention of safety certification other than 3C certification via Chinese regulators, either.

"An EPA establishment number on a pesticide device label identifies the EPA-registered location where the product was produced," an EPA spokesperson tells CNET. "Unlike chemical pesticides, EPA does not routinely review the safety or efficacy of pesticidal devices such as UV lights. Therefore EPA cannot confirm whether, or under what circumstances, such products might be effective against viruses such as SARS-CoV-2, the virus that causes COVID-19.

Devices like those are nothing new, but the pandemic-borne surge of interest in them is raising alarms, because UVC light is a known carcinogen, and even a few moments of direct exposure can be hazardous to the eyes and skin.

n = the order of diffractionλ = wavelength of lightd = distance between adjacent groovesi = angle of incidence with respect to grating normali’ = angle of diffraction with respect to grating normal

How doesa diffraction gratingwork

"UL has made a conscious decision not to certify the consumer-facing, portable, handheld wand-type devices due to the inability to manage the risks," Straka tells CNET.

"At this point, though there are some promising studies, it's not yet widely determined that [far-UVC light] is indeed safe," says Gwynn. "It's still just considered UV, and there's no exception that [the far-UVC] wavelength is not a concern."

"UVC lamps used for disinfection purposes may pose potential health and safety risks depending on the UVC wavelength, dose, and duration of radiation exposure," the guidance reads. "The risk may increase if the unit is not installed properly or used by untrained individuals.

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UVC light has a long history as a proven disinfectant at places like hospitals and laboratories, and businesses including airlines, hotels and retail spaces have been relying on the invisible light more than ever in 2020. The pandemic has also produced a flood of cheap, unregulated, consumer-facing devices for use in the home, including handheld wands that let you wave exposed UVC light across items and surfaces.

Relative grating efficiency is defined as the percentage of monochromatic diffracted light in a given order compared to the reflectance of the monochromatic incident light from a mirror coated with the same material.

Absolute grating efficiency is defined as the percentage of monochromatic light diffracted in a given order compared to all of the monochromatic light incident on the grating.

Ruled blazed gratings are very efficient, and are generally the best choice for applications requiring high signal strength. Because of the mechanical nature of the mastering process however, there can be random and periodic spacing errors that could detract from the purity of the diffracted spectra.

Similar tests for the Haiku UVC ceiling fan from Big Ass Fans produced encouraging results, with independent researchers finding that the fan was able to reduce the presence of SARS-CoV-2 released into a test chamber by 99.99% within 20 minutes.

Gold (Au) – Superior performance over aluminum in the NIR region. Below 600nm the reflectance of gold falls off significantly and is a poor choice. Above 1200nm, gold offers very little advantage for a single pass application.