Visual Light Transmittance Is Not So Transparent

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Every user of laser protective eyewear is told to select his or her gear based on a limited number of factors. These might include the wavelengths the eyewear covers, its optical density/beam attenuation capabilities, its fit and angular coverage, and its visual light transmittance (VLT).

The guiding principle for VLT is that laser protective eyewear should transmit enough light in the visible range so as to not impair vision while users navigate the laser use area. Poor visibility is a prime reason behind a large number of laser eye injuries because it compels users to either look above their eyewear, remove it completely, or to refuse to even wear it. So, it is clear that the higher the VLT, the better that users can see. Or is it?

The fact is that this oversimplification has led many users to bypass filters with superior vision properties. Understanding the filter choices underlying VLT is a more complex task than it appears. One consideration that laser users and laser safety officers often overlook is the illumination levels in the area where the eyewear will be used. In darker rooms, high VLT becomes an increasingly important factor. For this reason, VLT considerations can also vary widely between indoor and outdoor conditions.

Courtesy of NoIR LaserShields.

Courtesy of NoIR LaserShields.

Sunblock offers a good analogy. People often look for the product with the highest sun protection factor (SPF) number, thinking it will give them the most protection. But other factors also matter, such as the volume, uniformity, and the frequency with which the sunblock is applied — all of which more greatly influence its effectiveness than the absolute SPF number.

Most users of laser protective eyewear, when narrowing down their filter selection, have been taught to look for the highest VLT number. As with sunblock, however, the highest protection number may not yield the best protection. The laser safety officer and the user both need to understand the many facets of VLT that extend beyond filter selection.

VLT by the numbers

The definition of VLT, found in Standard Z136.1 Safe Use of Lasers, is: “The amount of visible light passing through a filter, weighted for the response of the human eye, expressed as a percentage.”

It is curious that Standard Z136.7 for Testing and Labeling of Laser Protective Equipment has no definition of VLT. But this standard does describe a number of conditions, including photopic, mesopic, and scotopic (dark) transmittance.

Photopic (light) luminous transmittance is defined by a filter that is weighted by the spectral sensitivity of the day-adapted eye.

Mesopic (twilight) luminous transmittance is defined by a filter that is weighted by the spectral sensitivity of the eye when the eye is adapted to a luminance below the lower range of photopic sensitivity of approximately five candelas per square meter, but above the upper limit of scotopic sensitivity of approximately 0.005 candela per square meter. In this range, spectral sensitivity is a function of the luminance.

Scotopic (dark) luminous transmittance is defined by a filter that is weighted by the spectral sensitivity of the night-adapted eye.

In all cases, VLT is a measure of the level of light before and after it passes through a filter. The scale number listed on EN 207-compliant laser protective eyewear takes optical density (OD) and damage threshold into consideration. This simple measurement approach, however, can yield misleading VLT results.

As a rule, the higher the VLT, the better. So, a VLT of 65% to 35% is generally high enough to be useful in all environments, while VLTs in the 20% range and below should be used only in well-illuminated working environments. Still, VLT calculations are static, and the human eye and brain are not.

Functional VLT

The concept of functional VLT sheds some new light on the situation. Functional or perceived VLT refers to how it “feels” to look through a lens rather than relying on calculated values. Numbers do not always represent the reality of the eyewear as worn.

As an example, the typical green plastic 1064-nm lens with a calculated VLT of 59% should seem lighter than a plastic, gray 1064-nm filter with the same OD and a calculated VLT of 55%. But looking through the lenses, the more neutral gray feels significantly lighter — more like 75% — in color and brighter in VLT as a result of the neutral color transmitting more of the visible spectrum on average than the green filter.

As another example, amber-orange lenses often have higher functional VLTs than their calculated values because attenuating shorter wavelengths enhances the greens and yellows, where the eye is most sensitive. Conversely, filters blocking the yellows will feel even darker than calculated, since they block the visible wavelengths that the eye perceives most.

Multiple-wavelength coverage in a single protective eyewear filter is a common requirement. Having one filter to cover all the laser wavelengths used in an environment is better than having multiple pieces of protective eyewear equipment because this helps to avoid the possibility of the wrong eyewear being used. But broader coverage is not always better because there is usually a VLT penalty for each additional protected wavelength. So, the goal should be for laser protective eyewear to cover only the wavelengths in use and no more.

In other words, it is best to select filters designed for the specific lines used and for the right ODs. Don’t overcover or overprotect, because each percentage point of visible light can matter to the user. Don’t be fooled by filter color. Look for narrowband absorbers, and always consider the functional VLT. Look through the filter, when possible, rather than selecting filters based strictly on their numerical VLT. And, in practice, users should allow their eyes to adjust to the filter — because they will, in the same way our eyes adjust when reading the menu in a dimly lit restaurant.

One final thing for laser safety officers to consider is any color distortion users might have. Gray and amber filters do not distort the perception identification. Brown filters can.

VLT may seem like a simple component of selecting laser protective eyewear. Like most things in life, however, it is not as straightforward as it appears. Laser safety is more likely to become regular practice if it is made as easy, convenient, comfortable, and practical as possible. But, no matter what, putting on protective eyewear before turning on a laser should be as automatic as putting on a seat belt before starting a car.

Published: January 2023
columnsLasersLaser Safety

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