Infrared introduction
Infrared (IR) radiation is characterized by a wavelength range from 0.750- 1,000 nm.Due to the limitation of the range of the detector, infrared radiation is usually divided into three smaller regions: 0.750-3 suntan m, 3-30 suntan m and 30-1000 suntan m-- defined as near infrared (NIR), mid-wave infrared (MWIR) and far-infrared (far-infrared) infrared (FIR).Infrared products are widely used in various applications, from infrared signal detection in thermal imaging to element recognition in infrared spectrum.As the growth of the demand for infrared applications and the progress of technology, manufacturers began in planar optical design (namely, window, mirror, polarizer, beam splitter, prism), spherical lens (i.e. plane concave/convex and concave/convex, meniscus), aspheric lens (paraboloid and hyperboloid, mixed), achromatic lens and the component (namely the imaging lens, the beam expander, the eyepiece, objective lens).The physical properties of these infrared materials or substrates vary.Therefore, to understand the advantages of each IR application, the right materials can be selected for any IR application.
The importance of using the right materials
Since infrared light consists of wavelengths longer than visible light, the two regions behave differently when propagated through the same optical medium.Some materials can be used in infrared or visible light applications, most notably fused silica, BK7 and sapphire.However, the performance of the optical system can be optimized by using materials more suited to the task at hand.To understand this concept, consider transmittance, refractive index, dispersion and gradient index.For more details on specifications and performance, see optical glass.
transmission
The most important property defining any material is transmission.Transmittance is measured by a quantity and given as a percentage of incident light.Infrared materials are usually opaque in visible light, while visible materials in infrared are usually opaque.In other words, they show transmission rates closer to 0% in those wavelength areas.Consider silicon through IR, for example, rather than visible light.
The refractive index
Although the material is mainly classified as IR or visible material transmission, another important property is refractive index (nd).Refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given material.This is a means of quantifying the effect of "deceleration" when light passes from a low exponential to a high exponential.This also shows how much light is refracted when the tilt meets a surface, with more of it refracted and added.
Refractive index range from about 1.45-2 visible materials and 1.38-4 infrared materials.In many cases, the refractive index and density are positively correlated, which means that the infrared material may be heavier than the visible material;However, higher refractive indices also mean that diffraction limits can be achieved with fewer lens components - reducing the weight and cost of the entire system.
The dispersion
Dispersion is a measure of the refractive index of a material against the change of wave length.It also determines the wavelength separation known as chromatic aberration.Quantitatively, dispersion is given by the index of refraction abbe number (vd), which is a function of the refractive index of the material at wavelengths f (486.1nm), d (587.6nm) and c (656.3nm).
Materials with an abbe number greater than 55 (smaller dispersion) are considered coronal and materials with an abbe number less than 50 (more dispersion) are considered to be flint materials.The abbe number of visible materials is between 20-80 and that of infrared materials is between 20-1000.
Index gradient
The refractive index of a medium varies with temperature.This index gradient (dn/dT) can be problematic when working in an unstable environment, especially if the system is designed to run an n value.Unfortunately, infrared materials usually have a greater dn/dT value than visible materials (compare n-bk7, which can be used in visible and germanium, which is transmitted only in the infrared in the key material properties table in the infrared comparison).
How to choose the right material
Three simple points need to be considered when choosing the right IR material.While the selection process is easier because the actual selection of materials used in infrared is much smaller than that of visible light, those materials also tend to be more expensive due to manufacturing and material costs.
Thermal properties - optical materials are usually placed in environments affected by different temperatures.Also, a common problem with IR applications is that they tend to produce a lot of heat.The material's exponential gradient and thermal expansion coefficient (CTE) should be evaluated to ensure that the user achieves the desired performance.CTE is the rate at which a material expands or contracts when the temperature changes.Germanium, for example, has a very high refractive index gradient and may degrade optical performance if used in thermal volatile environments.
Transmission - different applications run in different areas of the infrared spectrum.Some infrared substrates perform better depending on the wavelength at hand (figure 4).For example, if the system is used for MWIR, germanium is a better choice than sapphire, which works well in NIR.
Refractive index - the refractive index of infrared materials is much higher than that of visible materials, so more changes can be made in the system design.Unlike visible materials (such as n-bk7) that work well across the entire visible spectrum, infrared materials are typically limited to a small band within the infrared spectrum, especially when applied to anti-reflection coatings.