In our last post, we introduced the infrared portion of the electromagnetic spectrum and talked briefly about the imaging and healthcare applications for that part of the spectrum. Now let’s discuss the optical properties of infrared materials–how materials transmit, focus, or reflect infrared light–and why different materials may be better suited to various optical applications.
It’s time to jump into the world of technical terms and equations that describe these properties.
And hopefully you will become “curiouser and curiouser…”
Pure fused silica (SiO2) glass transmits (allows to pass through) light all the way from the ultraviolet through the near-IR, but absorbs almost all light above 2 microns (2000 nm).
Whether you want to see the world outside your house through a window on a sunny day (visible light), or send an email across the ocean (near infrared), the material that is best for transmitting the wavelengths of light used in these applications is silica glass.
Using a technique called spectrophotometry, you can see the full transmission window of silica glass in the UV, visible, and infrared (IR) in the first graph. This material lets more than 90% of the light through over this wide range of wavelengths.
In the next graph, we compare the transmission of silica and one of the glasses our company produces, IRRADIANCETM CLASSIC-6 chalcogenide glass, which does not transmit at all in the visible but transmits light out to 18 microns (this is the far-infrared region).
This difference in transmission is even easier to see if you compare a visible photograph of our CLASSIC-6 glass (it looks metallic, doesn’t it?) to a photograph taken with an infrared camera (now you can see through it!).
The glass transmits the infrared light so you can see the image behind the material. This chalcogenide glass is the material of choice for many of those infrared applications we talked about in the last post.
The refractive index (or, often, just index) of an optical material is a measure of how much that material slows light as it passes through. The equation that describes the relationship between refractive index and the velocity of light is
where n is the refractive index of the material, c is the velocity of light in vacuum, and v is the velocity of light in the material.
The refractive index of an optical material is critical for optical designers, because the refractive index determines how the material reflects, refracts, and disperses light as it moves through materials like glass at different wavelengths and at varying temperatures.
where R is the reflection coefficient, n1 is the index of the air (~1), and n2 is the index of the optical material.
The refractive index of fused silica is ~1.5 in the visible spectrum, which leads to a 4% reflection of light at a glass surface. This is why you can see a ghostly reflection of yourself in very clean window glass, and why the pure fused silica only has a 92% transmission in the graphs above. (There are two surfaces, each reflecting 4% for each window or lens.)
In contrast, the chalcogenide glass composition, CLASSIC-6, has a refractive index of 2.8, meaning that more than 20% of the light is reflected from each surface, leading to a loss of almost 40% in reflected light. This high refractive index makes anti-reflective (AR) coatings important for infrared applications.
Refraction is the process of the light ray (or beam) bending as it moves from air into an optical material. This process sounds complicated, but has been experienced by anyone who’s ever played in the water. The reason that rocks or fish aren’t where they “seem” to be in the water is that the light has been bent as it leaves the water, giving you a false perspective.
The path that the light travels is bent according to Snell’s law, which says:
where n1 and n2 are the indexes of the water and air, and θ1 and θ1 are the angles of the light ray measured to the plane perpendicular to the water surface.
Dispersion is a description of the wavelength dependence of the refractive index of a material. As the wavelength (usually represented by the Greek letter lamba or λ) of light increases, the refractive index of a material typically decreases.
This change in index is especially important to consider in broadband optical applications, where the change in index may cause changing wavelengths to behave differently in your optic–all the “colors” will not focus to the same point, resulting in distortion in the image (known as chromatic abberation).
This next figure shows the refractive index dispersion for IRG’s CLASSIC-6 glass, showing how the index starts high at lower wavelengths, and then rapidly drops off before starting to flatten out at longer wavelengths. This low refractive index change at wavelengths of interest (like the mid-wave infrared of 8-12 micrometers) means that lens systems can be designed to operate over all these wavelengths. Images are clear no matter what wavelength of light in that range is entering the optical system.
This same dispersion behavior is seen in essentially all optical materials, and is the reason that a glass prism splits light into a rainbow of color. The shorter wavelengths (the blue light) experience a different refractive index than the longer wavelengths (the red light). As a result the beam of white light splits into its component parts (the spectrum) as it passes through the prism.
The final factor that impacts the refractive index is the temperature dependence of the refractive index, or dn/dT (optical designers will call this the thermo-optic coefficient). This factor isn’t as important if you work in a nicely-controlled laboratory space, but for those applications with demanding environmental consideration (e.g. spaceflight), understanding how the refractive index of the material changes with temperature can be mission-critical.
You can sometimes see a change in the refractive index of air in really hot environments, like in the heat trail of a jet plane.
Another example of this is a mirage–a naturally occurring phenomenon that is caused by this change in the refractive index of hot air versus cold air. As the temperature of the air varies, the light bends differently through the air due to this difference in refractive index of the air at changing temperatures.
In complex IR optical systems, the dn/dT of germanium, a commonly used infrared crystal, is 396 ppm/°C. This means that a change of 25 °C in operating temperature can lead to a refractive index change of 0.01–which is enough to throw off most optical systems. In contrast, chalcogenide glasses typically exhibit low dispersion in comparison to germanium.
For many infrared optical applications, IRRADIANCETM CLASSIC-6 chalcogenide glass provides high transmission (with anti-reflective coatings), low dn/dT (change in refractive index with temperature), and small dispersion (chromatic distortion).
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