What is Infrared Optics? An Introduction to Infrared Optics.

Author: Yana Abiso – Product Engineer

Editor: Bryan Ng – Marketing Manager

Editor: Preethi – Technical Support Engineer

Editor: Ng Ci Xuan – R&D Intern

1. What is Infrared Optics?

Infrared optics are used to collect, focus or collimate light in the near-infrared (NIR), short-wave infrared (SWIR), mid-wave infrared (MWIR) or long-wave infrared (LWIR) spectra. The infrared spectrum falls in 700 – 16000nm wavelengths and each respective spectra are ranged as:

• NIR 700 – 900nm
• SWIR 900 – 2300nm
• MWIR 3000 – 5000nm
• LWIR 8000 – 14000nm

2. Short-Wave Infrared (SWIR)

SWIR system covers from 0.9μm-3μm spectrum range. The optics material needs to transmit visible and infrared light and does need a light source such as the sun, moon, stars, etc.

It is being utilized in numerous applications such as spectroscopy for sorting, moisture detection, thermal imaging (used for glass and plastic objects), and imaging – night vision and imaging lasers.

3. Mid-Wave Infrared (MWIR)

MWIR system covers a 3-5μm spectrum range and is typically a cooled system. Thus, it is less affected by humidity compare to the LWIR system for most target ranges, which is suitable for applications like coastal surveillance, vessel traffic surveillance, or harbor protection.

Since the primary goal is to obtain high-quality images rather than focusing on temperature measurements and mobility.

The images below indicate that MWIR image is sharper and has high thermal contrast compared to LWIR image.

4. Long-Wave Infrared (LWIR)

LWIR system operates from a 7-14μm spectrum range. However, most LWIR camera effectively covers from 8-12μm.

LWIR system is commonly known as “thermal imaging” since it detects heat signatures emitting from an object and does not require illumination to form an image.

Yes, our range of LWIR lenses also plays an important role during the COVID-19 pandemic.

The thermal imaging capabilities of the LWIR system have made it an attractive key component for a growing number of military, security, surveillance, scientific and industrial applications.

5. Functionality Categorization

Generally, infrared optics are categorized into SWIR, MWIR, and LWIR in terms of the wavelength spectrum. However, it is also sub-categorized into how it is being designed corresponding to its functionality such as:

• Athermal Lens
  An optical system is athermalized if its critical performance parameters such as modulation transfer function, back focal length and effective focal length, etc. do not change appreciably over the operating temperature range.
• Zoom Lens
  An optical system that covers from narrow and wide FOV continuously.
• Dual FOV Lens
  An optical system that provides an option to switch from wide to narrow FOV distinctively.
• Dual-Band Lens
  An optical system that covers both MWIR and LWIR spectrum range.

6. Infrared Optics Specifications

To give you a better visualization, we’ve included our SWIR lens table and its specifications as above. Here are some infrared optics specifications generally used in the industry:

• Focal Length
• F#
• Spectrum Range
• FOV (HFOV / VFOV)
• BWD
• Detector Size – Refer to the resolution
• Athermal (working temperature)
• Cooled or Uncooled, if cooled then ask for
  • Cold shield position
  • Cold shield height
• Lens Performance Indicator
  • MTF
  • Distortion
  • Relative Illumination
• Focus Type
• Mount
• Sealing

Bear in mind that you need to understand the infrared detector or sensor specifications before you choose which lens to be paired.

7. What is an Infrared Detector or Sensor?

An infrared detector/sensor is a transducer of radiant energy, converting radiant energy in the infrared band into a measurable form.

There are many detector materials with response curves that fit within the mentioned infrared spectrum.

Let’s look at its resolution and pixel size you may be familiar with:

•     VGA (Video Graphics Array) – 640 x 480
•     QVGA (Quarter Video Graphics Array) – 320 x 240
•     XGA (Extended Graphics Array) – 1024 x 768
•     HD (High Definition) – 1280 x 1024
•     The trend is getting smaller from 30um down to 12um pixel size

Infrared detectors are classified into thermal types, which have no wavelength dependence, and quantum types which are wavelength dependent.

7.1 Thermal/Non-Quantum Type

The thermal/non-quantum type is a detector/sensor which changes temperature depending on the impacting radiation.

The temperature change creates a voltage change in the thermopile and a change in resistance in the bolometer, which can then be measured and related to the amount of incident radiation.

This includes thermocouples, thermopiles, bolometers, and pyroelectric detectors. One of the most attractive characteristics of thermal detectors is the equal response to all wavelengths.

This contributes to the stability of a system that must operate over a wide temperature range. Another significant factor is that thermal detectors do not require cooling. 

The most common thermal/non-quantum type detector is VOX microbolometer.

7.2 Quantum Type

The quantum type is a detector/sensor that operates based on an intrinsic photoelectric effect and interacts directly with impacting photons.

These materials respond to infrared radiation by absorbing photons that elevate the material’s electrons to a higher energy state, causing a change in conductivity, voltage, or current.

There is a need to cool down to cryogenic temperatures to increase infrared detection efficiency/sensitivity. Cooling methods include Stirling cycle engines, liquid nitrogen, and thermoelectric cooling.

Cooled thermal imaging cameras are the most sensitive to small differences in scene temperature. 

Quantum detector materials include – InSb, InGaAs, PbS, PbSe, HgCdTe (MCT)

8. Conclusion

In summary, infrared optics applications are used in the NIR to LWIR spectra from 700 – 16000nm wavelengths. They’re also categorized into their functionality such as athermal lens, zoom lens, dual FOV lens, or dual-band lens. Now you know the basics of infrared optics and its detector types, why not check out our full range of infrared lenses?

Enquire Now