To help you sort through the common choices, we’ve gathered advice and cost information from vision experts and major eyeglass retailers, and December 2019 VisionWatch market research from the Vision Council, a nonprofit trade association for the optical industry.

Dayy Photonics can advise on how to apply FWHM principles to your next project and offer solutions to solve for your optical requirements.

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The optical bandwidth of a light source is strongly related to the temporal coherence, characterized with the coherence time.

The "full width at half maximum" (FWHM) is a measure of the width of an intensity distribution at the point where the intensity is at its highest. It is often employed to determine the width of a spectral line or the range of wavelengths in monochromatic light.

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Scan the warranty. Many glasses will be backed up by some kind of warranty, but it might not cover coatings. Or warranties may vary in terms of which ones are covered and for how long. What can you expect? “Virtually all coatings today are designed to last the life of the prescription,” says Vitale, which, on average, is about 28 to 30 months.

No matter what your warranty says, if a coating starts to degrade within a year, Vitale recommends taking the glasses back to see whether you can get the problem fixed or the glasses replaced.

Some of these are strictly cosmetic, but depending on your vision needs, others may “potentially improve your comfort and safety,” says Andrew Iwach, MD, clinical spokesperson for the American Academy of Ophthalmology (AAO).

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What to know: “Everything on the market now except your most basic standard plastic CR-39 lens [which still made up about 36 percent of the prescription lenses sold in the U.S. in 2019, according to the Vision Council] is probably going to come with scratch-resistant coating already on it,” Vitale says. “And that’s because CR-39 is already pretty scratch-resistant.” To further help keep lenses scratch-free, store your glasses in a case when you’re not wearing them and use a microfiber nonscratch cloth to clean them.

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Lenses tinted yellow are sometimes marketed as “blue blockers” for their supposed ability to reduce exposure to the light emitted by electronics such as smartphones. The theory is that this may ease both eyestrain and any sleep problems associated with using the devices too close to bedtime.

Put price in perspective. Some types of coating, like those that are anti-reflective (sometimes called glare-reducing), come in a range of prices. But experts say the most expensive option offered isn’t necessarily of significantly higher quality. “I know some ‘house brands’ out there that are pretty similar to the premium versions,” says master optician Michael Vitale, vice president of membership and technical affairs at The Vision Council.

The degree of resolution of a light source directly affects our ability to detect details in an object. An image with high resolution is generally clear and crisp while an image with a low resolution may appear fuzzy or pixelated. We can influence optical resolution by manipulating various inputs, like optical lenses and display components, to achieve the desired resolution.

Think about how you use eyeglasses. Indoors and outdoors? For sports or only for reading or desk work? Driving day and night? Your lifestyle and habits should inform your coating decisions.

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In this article we will discuss why FWHM is important in the world of photonics, especially as it applies to light emitting diodes (LEDs) and other specialty light sources.

What to know: The AAO recommends getting 100 percent protection from UVA and UVB rays. Lenses that offer this should be labeled 100 percent UV protection or UV400.

The types of lenses you choose can make a big difference in the quality of your eyeglasses. Depending on what your eye doctor advises, some people can get along just fine with basic plastic CR-39 lenses, while some may want to consider thinner and pricier polycarbonate or high-index types, which may be more appropriate for stronger prescriptions.

Who might consider: Most people, but AR coating may be especially useful for those who often drive at night (it can reduce reflections from headlights at night) and read on computers. It’s strongly recommended with polycarbonate or high-index lenses, which reflect more light than basic plastic CR-39 or glass lenses.

The FWHM represents the range of wavelengths over which the intensity of light emitted by the LED is at least half of its maximum intensity. The following outlines several applications of FWHM in light-emitting diodes:

What you’ll pay: Usually included in the price of glasses, but more robust scratch resistance may be built in to some upgraded lens packages.

FWHM is a good measure of an image’s sharpness, or resolution. As conditions get fuzzy for seeing, focus, or guiding quality, the width spreads out in relation. The lower an FWHM value, the better, essentially sharper, the image. If two peaks have overlapping FWHMs, they are unresolvable, i.e., they will look like one peak.

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Optical bandwidth refers to the frequency range that an optical element or photonic device can handle. It encompasses various aspects, such as the reflection bandwidth of a mirror, the optical transmission bandwidth of an optical fiber, the gain bandwidth of an optical amplifier, or the phase-matching bandwidth of a nonlinear optical device.

Who might consider: People who just like the look, of course. But tints may also help improve contrast for those with degenerative retinal disorders or light sensitivity. “Tinted lenses may reduce discomfort in bright light,” Brodie says. In some cases, even light tints may increase contrast, which can be helpful to pilots and marksmen working in outside light, he says.

While there are several ways to adjust resolution, what we are really trying to do in such cases is change the light source’s bandwidth, and bandwidth is the measurement taken at what is knows as the light wave’s full width half the maximum (FWHM).

Avoid the hard sell. Even if you’re buying the glasses from your ophthalmologist, “by the time you get to the eyeglass purchase point in the eye exam cycle, you’re in a retail environment and may be working with the staff, rather than the doctor,” Vitale says.

Typically applied on both sides of an eyeglass lens, this coating, also known as AR or anti-glare, “reduces the amount of light reflected off the surfaces of spectacle lenses, and may enhance the contrast of certain scenes,” says Scott E. Brodie, MD, PhD, professor of ophthalmology at NYU Langone Health in New York City. This means you’re getting the maximum light from the environment you’re in, but without any visual interference that can occur from that light bouncing off your lenses. (The term “anti-glare” is a misnomer, however, says Brodie, and glare—think oncoming headlights—is better addressed with polarized lenses than with AR.)

What you’ll pay: Tints cost an average of $74.67, according to the Vision Council’s VisionWatch market research for December 2019. Blue blocking costs $50 at Warby Parker and starts at $19 at EyeBuyDirect.

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Ask for an itemized description. In many cases, eyeglass lenses come bundled with certain coatings, commonly those that reduce reflections or bolster scratch resistance. Be sure you don’t pay extra for a coating that’s already supposed to be part of your package.

As for blue blockers, “there’s probably no harm, though anything that limits the light reaching you can lessen your vision,” Iwach says. “But the benefits have not been proven.” He has also recently seen an increase in interest in yellow-tinted lenses for night driving but cautions that these “can actually complicate things, since in low-light conditions, you want to get the most light in that you can.”

Over time, exposure to the sun’s ultraviolet rays can lead to vision problems such as cataracts and retinal damage. But many eyeglass lenses have a substantial amount of UV protection built in. You can also sometimes buy an additional treatment to safeguard your eyes fully or opt for lenses with 100 percent protection.

Be even more cautious about anything that feels high-pressure if you’re in an eyeglass store, he says. If you feel pressured, Vitale says, “you may want to look elsewhere.”

And while some coatings may be included in your eyeglass package price, others may add as much as $100-plus to your bill.

When referring to optical bandwidth, it is possible to express it either in frequency or wavelength units. However, it's worth noting that the conversion between nanometers and gigahertz is not fixed and instead depends on the center wavelength or frequency. For converting small wavelength intervals into frequency intervals, you can utilize the following equation, where 1 nm is worth more gigahertz if the center wavelength is shorter:

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What to know: Tinted lenses are often touted as “curing” color blindness, Brodie says. The truth: A tint may make some color contrasts more visible to people with partial color impairment but may make other contrasts harder to see, so it’s a trade-off.

Note that in optical fiber communications, the term “bandwidth” is often misused to describe the data rate achieved in the system. It is more appropriate to use the term data rate or data transmission capacity to avoid any confusion with optical bandwidth. Also note that there is no direct correlation between data transmission capacity and optical bandwidth.

Here, dyes are applied to lenses to cut down on the visible light (but not necessarily UV rays) entering the eye. Hues range from light tints like a pastel blue or pink to deeper shades used for sunglasses.

What you’ll pay: It’s usually included, especially with sunglasses. Otherwise, an upgrade to 100 percent UV protection may be available. At EyeBuyDirect, for instance, it’s included with an upgrade to thinner lenses. And note that polycarbonate lenses, which are natural UV blockers, offer 100 percent protection, as do many high-index lenses, Vitale says.

LEDs, unlike laser lights, have a broader bandwidth that is advantageous in stimulating numerous fluorescent probes. Compared to arc lamps that produce extreme heat and an unceasing spectrum, LEDs are also cooler, more compact, and offer a more convenient approach to cycle the source on and off. Additionally, they allow for a quick selection of specific wavelengths. During the past four decades, LEDs have advanced at a pace that rivals microprocessors.

A light source can have some optical bandwidth (or linewidth), meaning the width of the optical spectrum of the output. For narrow-linewidth lasers, the bandwidth can be extremely small—below 1 Hz, which is many orders of magnitude less than the mean optical frequency. On the other hand, ultrashort pulses with few-femtosecond pulse durations can have very large bandwidth—easily tens of terahertz.

This coating will make your glasses less vulnerable to scratching if you happen to drop them or clean them with an abrasive cloth. “Many glasses wouldn’t last a day without this,” Vitale says. “Polycarbonate and high-index lenses are very soft.”

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But once you’ve picked frames and lenses, there’s a wide variety of lens coatings to consider, each with its own properties—from light reduction to scratch resistance.

What to know: AR may get a bad rap from consumers who remember it being prone to degrading and developing cracks called “crazing” or “spiderwebbing” in its earlier days. But, according to Vitale, such problems were mostly resolved at the beginning of the 2000s. In the past, AR coatings also tended to attract dust and grime, but they now generally include an anti-static treatment that helps repel water and oil and keeps the coating cleaner. “Most of the AR coatings on the market today perform extremely well,” Vitale says.

How do you know what’s right for you? Your eye doctor may have some suggestions. “These are, by and large, personal decisions,” Iwach says. “One size doesn’t fit all.”