Let your finger do the (optical) navigating
It is important to consider the technological differences among current OFN implementations on the market today. Although all OFN modules offer the same functionality, manufacturers utilize unique technologies to arrive at the same goal. There are two main technologies: image correlation and spatial frequency detection.
Image correlation technology uses a light emitting diode to shine light onto a surface (a finger, in this case). The light reflects off microscopic features in the area and a system of lenses collects the reflected light to form an image on a sensor.
An illustration of a typical setup for image correlation systems can be seen in Figure 1 below. As the user’s finger moves, new images are formed on the sensor and the set of images are compared to determine the movement in the horizontal and vertical directions. The movement values are then sent to the processor in the mobile device which reflects a movement of the cursor on the screen.
Figure 1: Image Correlation Technology
Spatial frequency detection, on the other hand, is a laser based technology. Coherent light from a Vertical Cavity Surface Emitting Laser (VCSEL) is reflected off microscopic textural features on the user’s finger. The reflected light creates different spatial frequencies which is a signature of the surface texture and motion. A 2-D comb array sensor detects the spatial frequency and processes the motion into horizontal and vertical movements which are communicated to the mobile device’s main processor. Figure 2 below provides an illustration of spatial frequency technology.
Figure 2: Spatial frequency detection technology
A closer investigation into the two technological approaches to OFN reveals some advantages and trade-offs. The high-level ideas are very similar in that both approaches shine a light source on a surface and then analyze, compare, and process the reflection into horizontal and vertical movements that can be sent to a processor.
One of the major differences in the technologies is that image correlation requires a complex system of lenses to gather and focus the light reflection so that is can be processed. In contrast, spatial frequency detection requires no additional parts or lenses beyond the silicon die itself.
The additional lenses required in image correlation technologies complicate assembly, add process control and quality issues during manufacturing, and increase the bill of materials. Furthermore, image correlation utilizes a light emitting diode as its light source which can be affected by ambient light sources and result in tracking errors.
Because spatial frequency detection use a laser light source which is not part of the visible light spectrum, ambient light is not a factor and does not impact navigation reliability. In addition, because ambient light has no effect on performance, spatial frequency detection is able to offer external lighting features to provide halo or glowing effects to OFN modules for esthetic appeal and to improve visibility in darkness.
Finally, because of the comparison techniques required for image correlation, spatial frequency is able to operate with lower overall power consumption, a critical factor for mobile devices relying on a battery source. This being said, image correlation systems have the flexibility to reduce the resolution of tracking by reducing the amount of information captured in each frame in order to lower the power consumption below that of spatial frequency detection.
Spatial frequency detection does not have the flexibility. However, it is important to remember that while reducing resolution reduces power consumption in image correlation, it also reduces tracking performance. Table 2 below shows a technology comparison between image correlation and spatial frequency detection.
Table 2: Technology comparison summary
With ever increasing advances in smartphone and mobile device development, the challenge of integrating navigation tools to meet consumer demands has become a significant issue. OFN modules are becoming one of the most popular interfaces but in order to reap the benefits of both worlds, some smart phone manufacturers are turning to designs that incorporate OFN as well as touch screen functionality.
It is crucial for developers of smartphones to understand the technology behind OFN and that not all designs are equal. Spatial frequency detection designs offer significant advantages over image correlation solutions that include higher reliability, simple assembly, and less power consumption.
The applications for OFN are continually expanding. They can be seen today in top smartphone designs by RIM, Samsung, and HTC. They are also beginning to reveal themselves in cutting edge netbooks and ultra-mobile PCs. Consumers will begin to see OFN in a growing number of mobile electronic products and it is clear that this new technology is here to stay.
Mike McCauley is currently working as a Product Manager at Cypress Semiconductor. Prior to this, he worked in the mobile consumer electronics and semiconductor industries.
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