To be seen or not to be seen

We tend to think of LEDs today as a visible light source, but what we often forget is that while the infrared spectral area is not visible to us mere humans, it can be exploited through LEDs to communicate perfectly with machines.  Lance Hemmings, Global Product Group Manager at RS Components, explains.

Infrared is typically segmented into three zones, each a primary target for LEDs.  Near infrared, 700 to 1400nm, is typically used for remote control devices, light sources for surveillance cameras, industrial machine vision, and increasingly in touch panels, gesture recognition and tilt sensors for smartphones and tablets.  The mid-range from 1400nm facilitates heat sensing, widely used for military and civilian night vision equipment, for hazard (fire) detection and search and rescue.  The third, far infrared area, with wavelengths beyond 3000nm, is used for thermal imaging.  This article will focus on LED applications in the near infrared sector, the most popular wavelengths being 850nm and 940nm.

 

Emerging applications

One of the most exciting emerging applications for invisible infrared LEDs is in the entertainment sector, ironically, to create dramatic visible colour lighting effects across a crowd.  It has been seen at high profile events such as the London Olympic Games opening and closing ceremonies, the Superbowl in the US and increasingly at music concerts.

Figure 1: UK technology company Xyloband’s wristbands in use at a Coldplay concert recently.

Either spectators wear LED wristbands or hats, or seating is fitted with LED arrays.  Typically, the LEDs emit specific colours and intensities in response to signals from infrared LEDs in a central control console.  The effects can include moving waves of colour, or patterns that pulse in time with the music, creating an immersive experience for the crowd and an impressive performance for TV or video viewers.

In the automotive world, infrared emitters and sensors are important components in the growing range of automatic driver assistance systems (ADAS) for features such as parking aids, detecting pedestrians or obstacles in the road under varying environmental conditions.  Other automotive applications include driver eye movement monitoring and seat occupancy detection as well as automatic number plate recognition and traffic management systems.

Infrared and near infrared LEDs have been found to accelerate wound healing and treat muscle and bone atrophy, generating a range of new applications in specialist medical as well as consumer level light therapy products.

The latest prototype smartphone from Google demonstrates 3D mapping, as part of Project Tango, designed to build a detailed depth map of a space around the user.  Infrared sensors and emitters allow the phone to make hundreds of thousands of 3D measurements each second, updating its position and orientation in real time, and combining the data into a single 3D model.

 

Light touch

Touch screens for smartphones, tablets and other digital consumer device displays have become hugely popular in recent years.  High efficiency infrared LEDs are an enabling technology, replacing traditional resistive and capacitive methods.   The LEDs create a light grid over the display area, such that an object touching the display either casts a shadow or causes light to be reflected.

The key is that infrared LEDs are now small and powerful enough, yet low cost, to be used around any size of screen.  Devices such as Osram Opto Semiconductors’ SFH 4053 (RS 735-0585) measures only 0.5 x 1 x 0.45mm, one of the lowest profile available, and ideal for use in touch screens, even in mobile phones.  Yet it is powerful enough (260mW at 700mA in pulse mode), with an angle of half intensity of 70o, to flood a display from just two corners, making it suitable for use with camera chips and larger screens.

Gesture recognition, increasingly features in smart phones, tablets, games machines and other portable digital devices.  High power, wide beam angle infrared LEDs are the order of the day here, to provide an even illumination over a near field area, so that the on board camera can reliably and accurately detect movement and proximity.

 

 

Figure 2: Osram’s Oslon Black SFH 4716S 850nm infrared LED can be used for a range of applications, including gesture recognition as well as for surveillance cameras.

A typical solution for this type of application is the 850nm SFH 4716S (RS 778-1444), a member of Osram’s Oslon black range, pictured above.  Features include an optical output of 1030mW at an operating current of 1A, providing a radiant intensity of 225mW/sr.  Beam angle is an impressive 150o.  Yet the device can also be combined with external reflectors to create a narrow beam angle, providing high radiant intensity and a longer range.  This makes it suitable for surveillance cameras and monitoring systems, such as automatic number plate recognition.

 

Watching brief

Most security lighting applications, and especially those incorporating CCTV pictures, rely on infrared emitters and sensors to enable good quality pictures during darkness, without causing light pollution.  Visible light can be a source of annoyance and can cause distraction to road users at night.

Most cameras are designed to be sensitive to infrared light in the 840 to 860nm wavelength region, switching to monochrome images when there is no or little visible light.  The cameras operate by flooding an area with infrared light, and detecting the infrared light reflected back from objects.   Most systems incorporate arrays of LEDs, and the quality of the image and the range of the camera will depend on how much infrared light is available, as well as the camera lens and the supporting circuitry, of course.

Packing large numbers of infrared LEDs into a compact surveillance system requires small size and low cost.  Careful selection and internal design is required to address heat problems, and to ensure infrared light is not reflected back into the lens, which can generate a hazy image.

Typical surveillance cameras might use a bank of 150 high intensity 840nm LEDs to cover a range up to 80m.  Radiant intensity is the key characteristic, measured in watts per steradian, to indicate the light output within a solid angle segment.  For a longer range, to 100 or 150m, LEDs with a more focused beam are needed, at 12 to 40o, for example.

 

 

Figure 3: The ABUS TVIP71501 outdoor network dome camera in action on a service station forecourt.  It incorporates 24 infrared LEDs with an operating range of 15m.

Ideal for security surveillance over large distances is the 850nm infrared Dragon Dome (SFH 4783 – RS 784-5787) LED from Osram.  It has a focused beam of +/-12o, which from a current of 1A achieves a radiant intensity of 2.3W/steradian, giving it a potential range of more than 100m, depending on the sensitivity of the field of view of the camera.

The device is compact, measuring just 11.2 x 6.08 x 5.9mm, and does not need an external lens.  It is compatible with other Dragon designs, is available on a reel for automated production lines, and is suitable for reflow soldering processes.

 

Covert operation

An interesting aspect of security lighting is that although infrared light at 850nm is not visible to the human eye, the LEDs can take on a visible glow when in operation.  This means that hidden security cameras may be discernible, particularly when it is totally dark.  For true covert operation, cameras compatible with infrared LEDs in the 940nm region are used.

The Oslon black SFH 4725S (RS 778-1383) LED provides 940nm light with a radiant intensity of 450mW/sr at an emission angle of 90o.  The black package ensures the component is completely concealed behind the camera lens.

 

Trade-offs

In most applications, there will be trade-offs to be made in commercial (cost), technical (power, performance) and physical (size, orientation).  Designers may have to choose between lower driving currents, for low power (longer battery life) operation, or high intensity to increase the range (transmission distance).

Viewing angle is another design consideration.  Narrow angle devices tend to be more popular for touch panels and proximity sensors, not only because they are higher intensity, but also because packages tend to be smaller.   Wider viewing angles, meanwhile, are better for remote control devices, so that the controller does not need to be pointed to accurately at the receiver.

While top view packages are suitable for proximity sensor and remote control applications, side view LEDs are preferred where devices are incorporated into the edges and corners of touch display panels.

Finally, it is worth noting that infrared LEDs are supplied in industry standard packages, can be easily incorporated with visible LEDs when required, controlled using the same drivers, and they are compatible with allied, industry standard circuitry for wireless communications, and lighting and video network systems.

When it comes to selection, there are hundreds of infrared LED models available from dozens of manufacturers, providing a wide mix of features and options.  It is useful therefore, to find a vendor website that provides the ability to narrow down the choice according to the design constraints of the application, whether it be package size, radiant intensity, peak wavelength or viewing angle!

 

http://www.AU.rs-online.com/

 

Sources of Artwork:

Figure 1: www.xylobands.com

Figure 2: www.osram.com

Figure 3: www.abus.com