SCHOTT solutions no. 2/2013 > Filter Glasses
Infrared lasers are being used more and more often in medicine and industry. As a result, the demand for special filter glass, which is essential to preventing eye injuries, continues to grow. At only 1 mm thick, the bandpass filter VG20 from SCHOTT offers strong absorption in the near infrared region (NIR). Photo: SCHOTT/C. Costard
Don’t be Blinded
The new VG20 filter glass from SCHOTT effectively protects both people and devices against harmful infrared radiation.
The small, bluish-green glass pane that Dr. Ralf Biertümpfel, Application Manager for Filter Glass at SCHOTT Advanced Optics, holds in his hand immediately reminds us of the world-famous blue Chagall glass windows of St. Stephan’s Church in Mainz, Germany, which is located not far from SCHOTT’s headquarters. However, despite being of similar beauty, this new VG20 Filter Glass is actually an advanced technology – no other filter glass protects as well as this latest product from SCHOTT. Supplied as a polished filter, as well as with an interference coating that’s optimized to suit a given application, VG20 begins absorbing 50 percent of the radiation, a yellowish green, at 565 nm. At an infrared wavelength of 850 nm, it blocks radiation a thousand times better than conventional glass.
Intensive radiation in this frequency range can be extremely harmful to the human eye. This is especially true if it’s emitted by GaAlAs, InGaAs or Nd:YAG lasers like those used in industrial manufacturing or medical applications. In both of these cases, it’s not only important to protect the users, but also to ensure that their safety eyewear impairs them as little as possible. VG20 is ideally suited for these applications, as its strong filter effect permits the protective glass layers to be one-third thinner than conventional filter glasses. This allows safety eyewear to be more comfortable and easier for users to wear.
In addition to safety, it’s also critical that users still see things in their natural colors. This is of particular importance in the area of medicine. While the bluish glass of VG20 darkens the colors of the spectrum somewhat, it doesn't falsify them – one can recognize all of the colors, and even see that a white area is in fact white, when looking through the glass.
Illuminated displays can be quite annoying if residual light intensifiers are used without filters because their intensive infrared radiation is also intensified, increasing proportionately like that of people or animals standing at a distance. If the displays in an aircraft cockpit (photograph) are covered with VG20 filter glass, however, they can be read very easily. Photo: iStockphoto
Climate resistant under harsh conditionsThe outstanding filter effect of VG20 is a result of the free copper ions in the glass matrix. ”Glass is really a liquid, therefore salts can dissolve in it much like cooking salt dissolves in water,” Dr. Biertümpfel explains. Yet, while copper can also be found in simple blue-tinted window glass, it is still extremely difficult to dye glass very evenly. If dyes are added to the glass melt, striae that interfere with the optical quality can form very quickly. In the past, this could only be avoided by adding other ingredients that also have a negative impact on the properties and are sometimes ecologically questionable. Nevertheless, by further improving its production technology, SCHOTT has managed to make its glass extremely homogeneous and free from striae, despite the high pigment concentrations.
Since SCHOTT avoids problematic additives that encourage reactions, VG20 is also extremely climate resistant. It remains transparent and doesn’t corrode, even after hundreds of hours in an 85 °C environment with 85 percent humidity. For this reason, VG20 can also be used in applications where the environmental conditions are more severe than those encountered in an operating room. This can include outdoor settings, where, for example, it is paired with image intensifier tubes in night vision equipment. These types of devices are becoming increasingly common among hunters, police officers, rescue teams and helicopter pilots, aiding them in their activities. Image intensifier tubes, which are particularly sensitive within the infrared range, help the user find his way in the dark or search for either individuals or wild game. But the illuminated displays of other devices can become a problem.
Even the light from something as small as a watch, smartphone or flashlight can emit intense infrared radiation, increasing just as proportionately as that of a person standing farther away. The image intensifier tube turns this into a blinding bright spot. However, if VG20 filter glass is used over a hunter’s watch display or a helicopter’s gauges, it provides the necessary protection while ensuring the devices can still be easily read.
CCD / CMOS sensors that record images in digital cameras and smartphones also require an effective infrared filter like VG20 because they react more sensitively to red and infrared radiation than the human eye. As a result, hot objects are displayed in the wrong color. Because VG20 blocks out red radiation to some degree and infrared radiation entirely, the spectral sensitivity of the sensor adjusts to match that of our eyes. Also, because of its strong filter effect, VG20 allows the protective glass layers to be thinner and optical systems as a whole to be more compact. This particularly favors the changing needs of the electronics industry, as it pursues even greater miniaturization of its technologies. <
PROTECTION IN THE NEAR INFRARED (NIR)
This graph shows the pure transmission of VG20 optical filter glass as infrared light is absorbed. The visible portions of the spectrum are allowed to pass through the filter and ensure that colors are displayed naturally while the share of near infrared light is absorbed. VG20 optical filter glass protects against red and near infrared (NIR) light at wavelengths above 650 nm, the range typically used in lasers and intensive light sources (ILS). <
Seeing in the Dark
Image intensifier tubes make it possible for individuals to see clearly at night by intensifying the remaining visible infrared radiation. The process requires a lens to project the incident light onto a photo cathode, causing the release of electrons. These electrons are accelerated, often in multiple steps, by applying a high-voltage charge (between 10 and 17 kV) in the direction of a fluorescent screen. On the screen, a much brighter monochrome image is produced with intensity proportional to the initial illumination, allowing the user to view the surrounding environment in the dark. <