What is Infrared Optics? An Introduction to Infrared Optics.

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 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 object), 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 compare 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.

Our range of LWIR lenses also plays an important role during COVID-19 pandemic, learn more here.

The thermal imaging capabilities of the LWIR system have made it attractive key components 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

Part Number	Wavelength (µm)	Focal Length (mm)	Focus Type	F#	BWD (mm)	Mount	Detector
Infra-SW122.5-15	1.5 - 5.0	12	Manual	2.5	33.1	Bayonet	640 x 512, 15µm
Infra-SW252.5-15	1.5 - 5.0	25	Manual	2.5	33.1	Bayonet	640 x 512, 15µm
Infra-SW253.0-17	1.5 - 5.0	25	Manual	3.0	33.1	Bayonet	1024 x 768, 17µm
Infra-SW502.5-15	1.5 - 5.0	50	Manual	2.5	33.1	Bayonet	640 x 512, 15µm
Infra-SW502.3-17	1.5 - 5.0	50	Manual	2.3	39.4	Bayonet	1024 x 768, 17µm
Infra-SW1002.3-17	1.5 - 5.0	100	Manual	2.3	33.1	Bayonet	1024 x 768, 17µm
Infra-SW1002.5-15	1.5 - 5.0	100	Manual	2.5	33.1	Bayonet	640 x 512, 15µm
Infra-SW2002.5-15	1.5 - 5.0	200	Manual	2.5	33.1	Bayonet	640 x 512, 15µm
Infra-SW252.5-30	0.9 - 2.5	25	Manual	2.5	13.5	C-mount	320 x 256, 30µm
Infra-SW352.0-30	0.9 - 2.5	35	Manual	2.0	13.4	C-mount	320 x 256, 30µm
Infra-SW502.0-30	0.9 - 2.5	50	Manual	2.0	13.5	C-mount	320 x 256, 30µm
Infra-SW752.0-30	0.9 - 2.5	75	Manual	2.0	13.5	C-mount	320 x 256, 30µm
Infra-SW1002.0-30	0.9 - 2.5	100	Manual	2.0	13.5	C-mount	320 x 256, 30µm
Infra-SW2002.0-30	0.9 - 2.5	200	Manual	2.0	13.5	C-mount	320 x 256, 30µm

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 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?

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, that have no wavelength dependence, and quantum types that are wavelength dependent.

7.1 Thermal / Non-Quantum Type

The thermal/non-quantum type is a detector/sensor which changes temperature depending upon 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 thermocouple, thermopile, bolometer, 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 interact 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. Infrared optics are also categorized into its 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 infrared optics?