How do Infrared (IR) Sensing Optics Work?

Infrared (IR) sensing uses infrared-sensitive optical components to detect light in the IR region of the electromagnetic spectrum. IR sensing is now widely used in motion detection, machine vision and industrial automation, agriculture, and medical applications1 as it allows for the visualization of complementary information to visible imaging approaches. For example, infrared machine vision applications can be used for fault inspection in photovoltaics and quality control in manufacturing.2

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This blog post will explore what IR radiation is, how IR sensing works and the typical optical components used, and an outlook on how Iridian’s product range can help you with your IR sensing applications.  

What is Infrared (IR) light?

The IR region of the electromagnetic spectrum is the wavelength region longer than the visible but shorter than radiowaves. The IR region is typically subdivided into near-infrared (~ 0.75 – 1.4 µm), the short-, mid-, and long-wavelength regions, and the far infrared (~ 15 – µm). These sub-classifications are useful as different optical technologies are required within the different wavelength ranges. 

One of the most useful elements in IR sensing is that any object with a finite temperature will emit IR radiation. Whereas with visible light, the detection of an object with a camera requires sufficient illumination and direct line of light, IR images can be used to ‘see through’ certain objects as well as recover temperature information on the object. 

How do IR Sensing Optics work?

A completely passive IR sensor will consist only of an IR-sensitive detector. Excitation of the sample of interest with a radiation source is not required due to the object’s thermal emission of IR light. Typical sensor types include pyroelectric sensors, but some progress is being made on the development of IR-sensitive phosphor-based sources.2 The sensor type will need to correspond to the particular wavelength region in which the device will be operating.

Often a series of optical filters will be used in an IR sensor. This includes longpass, shortpass, bandpass, and interference filters. Filters are used for a number of purposes in IR sensing devices. Often, this ensures transmission of only the spectral region of interest so the device does not become saturated by any unwanted wavelengths. Filters can also be used for the attenuation of strong signals or to improve signal-to-noise ratios and signal contrast. 

Midinfrared filters are commonly used in astronomical and remote sensing equipment. A series of filter types may be integrated into telecommunications devices to avoid cross-talk between communications on different frequency ranges. 

Longpass filters: Longpass filters are normally defined by their cut-on edge, which is the wavelength below which all other wavelengths of light will be blocked. 

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Shortpass filters: Shortpass filters are the inverse of longpass filters and are a type of edge filter that allows shorter wavelengths to be transmitted while all longer wavelengths are blocked.

Bandpass filters: Bandpass filters are defined by their central wavelength, which determines which region of the spectrum they allow to pass through. Bandpass filters have a full-width half maximum which determines how broad the transmitted spectral region is. 

Interference filters: Interference filters are sometimes called dichroic filters, and they allow for the reflection of one or more spectral bands and the transmission of all other wavelength regions. Unlike most filters that absorb all wavelengths that are not transmitted, interference filters instead separate spectral mixtures through reflection and transmission. An interference filter may also have longpass, shortpass, or bandpass characteristics. 

Some important properties of filters are the wavelengths of the cut-on and cut-off points, the contrast between the transmitted and blocked regions, and the steepness of the blocked region. 

In conclusion, when designing IR sensing, it is essential to consider the properties of the optical components, particularly if the sensitivity to weak signals is important, so any unwanted spectral contributions must be filtered out. High-quality filter design can improve device performance and allow IR imaging to be performed over selected wavelength regions. 

Iridian Optical Filters 

Contact Iridian today to find out how their extensive expertise in the development of custom IR filters or off-the-shelf solutions can benefit your application. Enhance the signal-to-noise and improve acquisition times with the right filter collections and enjoy the maximum benefits that IR sensing technologies offer, whether for medical diagnosis or process automation. 

References and Further Reading

IR illumination has many applications, ranging from basic CCTV surveillance cameras to very sophisticated machine vision and automation systems. Infrared has many advantages over visible light – in surveillance applications it keeps lighting discreet and helps to conceal the camera’s viewing direction. In machine vision and automation applications, longer wavelengths of IR have a higher penetration rate through smoke and fumes.

IR emitters have many advantages over other IR illumination sources. They are smaller in size, have lower electric consumption, are durable and insensitive to moderate vibration. Most of LEDiL’s standard secondary optics are optically compatible with IR emitters. However, LEDiL also has developed lenses specifically for Osram IR emitters. The light distribution of these lenses is optimized for use with the specific IR emitters.

EXAMPLES OF LEDiL’s IR-OPTIMIZED SECONDARY OPTICS

 

IRENE IR-OPTIC FAMILY

• Specifically designed for CCTV surveillance
• Rectangular beam designed to match the rectangular field of view of camera sensor
• Beam widths are optimized for different focal length camera optics
• IRENE has same footprint as 21.6 mm diameter LEILA family of optics

LISA2 OPTIC FAMILY

• Small footprint suitable for tight PCB layout
• Diverse selection of beam angles ranging from real spot to wide including oval pattern

IRIS SPOT OPTIC

• Tight real spot lens for longer range
• FHWM 11° with Ostar IR emitter
• High intensity peak

TYPICAL APPLICATIONS

• Surveillance of large areas and spaces
• Military applications
• Night vision systems

DEMONSTRATION OF HOW DIFFERENT BEAM ANGLES INFLUENCE IMAGE IN CCTV SURVEILLANCE SYSTEMS

• The illumination level is inversely proportional to the square of the distance from the light source – Light focusing optics provide more reach with less power used
• To be most effective, light distribution should be adjusted to fit the camera’s field of view
• LED in use: OSRAM SFH S IR

LED Only – Visibility approx. 2.8 m With wide beam optics (EVA-W) – Visibility approx. 7 m With medium beam optics (LXP-M) – Visibility approx. 14 m With spot beam optics (IRIS-IR) – Back wall of the corridor is still brightly illuminated as well as the subject. Secondary spot optics are best suited for outdoor areas where long distance illumination is mandatory.

• Light that is too wide for camera’s field of view means wasted light and reduced illumination range.
• Light that is too narrow produces glare and white-out in the center of the area, with corners not correctly illuminated.

CHOOSING RIGHT OPTICS FOR RIGHT APPLICATION MEANS BETTER QUALITY AND BEST POSSIBLE EFFICACY

Find all our optics for IR lighting

The information contained herein is the property of LEDiL Oy, Joensuunkatu 7, FI- SALO, Finland and is subject to change without notice. Please visit www.ledil.com for additional information, such as the latest photometric files, 3D mechanical models, and application notes relating to handling, gluing and taping.

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