Distinctive design features of infrared optical systems

Published on by Sergey Sidorovich.

A designer of an  optical systems intended for IR light needs account for key specifications with greatly  impact on the complexity of the device construction and its production cost. The most important requirements are described below.

The maximum achievable theoretical resolution in wavelength infrared region

The maximum achievable theoretical resolution of the optical device is estimated with help of Rayleigh criterion:

r=0.61λ/NA.

  • r - minimum size of the object that can be recognized with help of optical device;
  • λ - wavelength of the used radiation;
  • NA - numerical aperture of the optical device.

In accordance with this criteria resolving power of optical device which works with thermal radiation (λ – about 10 µm) is 20 times lower than resolving power of optical device which works with visible radiation (λ – about 0.55 µm).

According Rayleigh criterion for increased resolution of the optics increasing of numerical aperture (NA) is required. In practice the real resolution always is lower than theoretical limit because of optical aberrations.  Therefore a key design and manufacturing goal is to get as close as possible theoretical limit by correcting for optical aberrations.

High cost of the optical materials for infrared optics

The cost of the optical materials intended for creation infrared optical systems is rather high and can be main cost driver of a designed devices. To decrease final price optical engineers optimize system in such a manner to use minimal amount of expensive optical materials. The first step is to try to decrease number of optical elements, then they make them as thin as possible. One approach to decrease number of optical elements is to use aspherical surfaces and this is common in infrared optics.

Sometimes diffractive optical elements (DOE) also are used for this purposes. As a rule diffractive optical element is a phase optical surface which is affect on a phase of transmitted light in the required way. Usually DOE is implemented in the form of circled zones on the surface of optical substrate like it is implemented in Fresnel lenses. The difference between DOE and Fresnel lens is that the DOE holds strict phase difference 2π between adjacent zones. One of the most useful advantages of DOE consists in its dispersion features. Dispersion of DOE has opposite sign in comparison with dispersion of refractive optical element. This makes it possible correction of chromatic aberrations without using of additional elements and decreases axial thickness of the optical elements that makes cost of the optical system lower. Moreover DOE helps to correct of the higher-order optical aberrations.

Using of the DOE for correction of the chromatic aberrations

Using of the DOE for correction of the chromatic aberrations

High absorption coefficient of the infrared optical materials

Very often IR optical materials have a high absorption coefficient- for example Germanium which is widely used for producing of the infrared optics. This results in the decreasing of the efficiency of the optical system. It may result in reducing the amount of light incident on sensor and decrease signal-to noise ratio. Also small amount of the light on sensor may lead to misrepresentation of some data. This is one more reason why minimal number of optical elements must be used during design of the infrared optics. 

To know more about selection of the materials for IR optics you see our previous post here.

High refraction index of some infrared optical materials

Some optical materials used for production of the infrared optics have high refraction coefficient, for example, refraction coefficient of the Germanium is about 4. This means the difference between air refraction index and optical material refraction index is rather high. This leads to decreasing of the amount of light which propagates through interface of optical material and air. As result, amount of useful light is decreased. Reflected light may cause appearance of glare and background noise.

Comparison of two focusing lenses produced from two different materials for using with different wavelengths. Seen that number of reflected rays is higher for germanium lens.
N-BK7 lens - 91.5% of the light is focused.
Germanium lens - 38.1% of the light is focused.
Simulation was conducted in OpticStudio software.

To minimize this effect the usage of the high quality optical coating is required. This reduces impact of the difference between refractive indexes of the optical material and air. Another approach is to design the optical system in such a manner to make angle of incidence of the light on the interface of two environments became less. The also decrease amount of reflected light.

Spectral range of thermal radiation of metals

Spectral range of thermal radiation (longwave IR-radiation) is 8-15 µm. Maximum intensity of the thermal radiations of the metals in normal conditions is about 8-12 µm.  In case any part of the device made from metal is in the field of view of the sensor (not optics) then radiation that it emits can reach sensor sensitive layer. As result level of stray radiation may be very high. This can lead to losses of the image contrast and corruption of the obtained data. The situation is aggravated by the fact that thermo-optical devices require using of the highly sensitive sensors.

This means care must be taken during optomechanical design to exclude chance of placement of the metal parts of the construction in the field of sensor view.

Narcissus effect

Narcissus effect consists in existence of the "cold" (dark) background spot on the image. The reason of this spot is in difference between luminosity of the sensor and other parts of the device. In fact dark spot is image of the sensor.

Original image owned by Photonics Spectra
Dark (cold) central circle and ring around it are are result of the Narcissus effect.

Sensor sensitive surface of any thermo-optical device effectively absorb IR radiation and almost doesn't emit it. Sometimes if optical scheme of the device was designed wrong distribution of background light noise is very non uniform. In center of image can be seen dark spot. This happens when image of the sensor is reflected by some surface and focused on image plane. As result we can see image of the sensor in the form of dark spot (because of its low radiating capacity). A correctly designed optical system excludes existence of the echo image of the sensor.

The temperature coefficient of refractive index

The value of the refractive index of some optical materials which are actively used during optical IR devices production strongly depends on temperature variations. This can lead to degradation of optical quality and distortion of  obtained data. Temperature coefficient of the refractive index indicates how strong variation of temperature effect on refractive index of the optical material. The lower this coefficient, the lower impact of temperature.

To increase thermal stability of the IR optical device optimization with consideration of temperature coefficient is required.

Conclusion

Design of the IR-optical system requires specialized knowledge and skills in optical design. These issues need to be resolved during design to get a good quality optical system.