Optical Sensor Application Notes: Point Source Emitter

LEDs (Light Emitting Diodes), semiconductor light sources, have been introduced and developed for several decades. LEDs are applied in many devices as indicators and general illumination products such as lighting components. As a “green” light source, LEDs can provide a long life time and high efficiency light for many applications. However, for some special applications, standard LEDs are not always the perfect choice. Point Source Emitters (PSEs) offer a great alternative in applications needing a precise beam of light such as encoders, machine vision and medical fiber.

A PSE is a semiconductor diode similar in structure to a standard LED, however, the light is emitted through a well-defined circular area, typically 25 μm - 200 μm in diameter. The light produced appears as a “spot.” The output light produces very narrow, almost parallel viewing angles. These two characteristics are well suited for applications that require a near parallel light source and lower power, as compared with laser diodes.

Figure 1 shows side view structure of standard LED and PSE. The first difference in these two structures is emitting light direction. Standard LED output light is directed to the side. In order to refocus the light direction, standard LEDs normally need a reflective cavity to force the light from the side to the top. This can cause light output loss, power dissipation, and variations in final output light beam and viewing angle. However, PSEs emit light to the upper surface through an aperture / window on top of the structure. The second difference in these two structures is the position of the cathode contact. The cathode contact pad of a standard LED is typically located in the center of the structure, which can obstruct light output due to the top wire bond. SEs can easily solve this problem by locating the cathode contact wire bond to the side of the aperture window, eliminating any obstructions and dark spots (represented in Figure 2). The light emitted from the standard LED (left in Figure 2) has several dark spots due to the bonding pad, obstruction from the wire bond as well as the reflector cup (shown in Figure 3). A PSE (right in Figure 2) has a much more narrow, defined, and precise beam with no dark spots.

Figure 1: Side view structure of standard LED and Point Source Emitter. a) Standard LED Structure (b) Point Source Emitter Structure.

Figure 2: Lighting comparison of standard LED and Point Source Emitter. Left is standard LED light output, and right is point source emitter.

Figure 3: Schematic of standard LED.

Aperture size, a key parameter of PSEs, can affect power output. A smaller aperture will typically result in a lower power output; however, it will also increase the resolution capability. PSEs are offered in a variety of wavelengths and aperture sizes to satisfy individual application requirements. Currently, PSEs are available in wavelengths from yellow light (595 nm) to infrared (IR) light (850 nm). More details are presented in Table 1.

Emitting light color	Wavelength	Aperture window size
Yellow	595 nm	150μm
Red	650nm	25μm
Red	650nm	80μm
Red	650nm	150μm
Infrared (IR)	850nm	50μm
Infrared (IR)	850nm	150μm

Table 1: Standard aperture window size ratings.

Spectrum bandwidth for point source emitters

Standard LED emitting light is neither monochromatic like a laser nor broadband like a tungsten lamp, but rather lies in between these two. Hence, output light bandwidth is another significant consideration for different applications. In general, the spectrum bandwidth of LEDs is in the 30 nm to 100 nm range, where PSEs offer a much narrower 20 nm - 50 nm bandwidth. However, some special applications need an even narrower bandwidth. For example, the image system in some microscopy applications require narrow band emissions from the emitter as well as less stray light in the background of the image of the test sample.

Figure 4 shows the spectrum of an 880 nm standard LED and an 880 nm PSE. The blue line in this figure represents the 880 nm standard LED spectrum with a 60 nm bandwidth; the red line shows the spectrum of the 880 nm PSE. The typical bandwidth of an 880 nm PSE is 40 nm, 20 nm less than standard LEDs. This demonstrates that PSEs emit a much tighter monochromatic beam compared to LEDs.

Figure 5 shows the spectrum of the 850 nm standard and narrow bandwidth PSEs. The green line in this figure is the 850 nm standard PSE spectrum with a 40 nm bandwidth; the orange line symbolizes the spectrum of an 850 nm PSE with narrow bandwidth. The typical bandwidth of an 850 nm narrow bandwidth PSE can reach 25 nm; even lower depending on the customer’s requirement.

Figure 4: Spectrum of 880 nm Standard LED and Point Source Emitter.

Figure 5: Spectrum of 850 nm standard point source emitter and narrow bandwidth point source emitter.

Viewing angle for point source emitters

LED viewing angle is primarily a function of chip type, package and optics. Standard LEDs have a viewing angle of 15° - 120°. PSEs can be offered with viewing angles as narrow as 4° using special optics. There are several technologies that may be utilized to optimize viewing angle including using an external optical glass lens or internal ball lens. These lenses not only reduce light loss and optimize the light trace, but also enhance product reliability. Standard LEDs are typically encased in an epoxy or plastic materials. Some of these encapsulants can chemically react with the die causing premature light output degradation. Many PSEs are packaged in hermetically sealed housings with either air or nitrogen inside. This can significantly improve the life time and output degradation of the PSE. Figure 6 shows an example of an external glass optical lens being applied.

Figure 6: Standard point source emitter.

A variety of high reliability package options, including hermetically sealed TO-18 and TO-46 metal cans, coaxial metal can and ceramic surface mount packages can be obtained. All these packages can have varying package heights and lens options, providing the designer with optimum flexibility in sophisticated design efforts. Lens options includes flat glass window, dome lens, etc. Figure 7 shows some standard PSE package styles. Custom package solutions are also available.



Figure 7: Point source emitter package styles.

Advantages and applications

There are several advantages associated with use of PSEs over standard LEDs including:

• Unobstructed Light Output
• Well Defined Beam for High Accuracy
• Flexible Emission Area (25 μm - 150 μm)
• Narrow Spectral Bandwidth
• High Reliability
• High Current Capability
• High Temperature Capability
• No Side Light Emissions
• Low ESD Sensitivity
• Fiber Optic Cable can be placed close to the die

This list of advantages make the use of PSEs better suited for these applications:

• Encoders
• Linear Positioning
• Line Sensing
• Medical Sensing
• Short Haul Fiber
• Instrumentation
• Machine Vision
• Edge Sensing
• Optical Switches
• Distance and Range Finding Indication
• COB Emitter Module
• Food Inspection

Please visit the Marktech Optoelectronic page on the DigiKey site for a complete listing of Marktech Optoelectronics product offerings through DigiKey.