Material Science of Optical Mouse

BACKGROUND

For the first project in my Materials Science course in Fall 2008, my team needed to study a common consumer product and determine the materials science considerations (including choice of material type, processing, etc) required for an effective design.  We were encouraged to consider not only the technical aspects of the materials, but the economic and social perspectives as well (e.g. price, aesthetics, availability of materials).

SUMMARY


We chose to investigate the sensing system of a new (2008) General Electric optical mouse.  We dissected the LED and prism systems of the mouse to identify the materials used and attempt to explain why the particular material choices are suitable and advantageous to the design.

THE OPTICAL SENSING SYSTEM OVERVIEW


The optical system of the mouse consists of the LED, the prism, and the photo sensor. The light from the LED enters the prism, bounces off the surface below the mouse, and through a lens into the CMOS photo sensor chip. The sensor camera takes more than 1000 pictures every second, which are processed to determine the mouse’s position.


LED MATERIAL SCIENCE

LED Design Objectives:
  • High brightness
    • LED should illuminate as much of surface as possible for accurate sensor reading.
  • Red Spectrum emission
    • CMOS optical sensors (used in the mouse) are most sensitive to red light [3].
The primary semiconductor material used for modern high-brightness red LEDs is aluminum gallium indium phosphide (AlGaInP), which emits red light at about 630 nm [2]. Based on EDS and spectrograph tests, the semiconductor used in the optical mouse is likely AlGaInP.

AlGaInP satisfies both design objectives very well.  Compared to other red spectrum direct bandgap semiconductors, such as GaAs, AlGaInP achieves significantly better brightness performance (usually on the order of 20 lumen/W, which is over 2 times better than GaAs) [2].   This emission efficiency makes AlGaInP an ideal choice for high-brightness red spectrum emission.

The presence of tin in this semiconductor, as detected by EDS, is likely due to processing concerns, as indium layers in semiconductors are often obtained from indium tin oxide (ITO). ITO films have been shown to improve both brightness and lifespan of LEDs [4], which suggests that tin may play an important role in the function of the optical mouse.

PRISM MATERIAL SCIENCE


Prism Design Objectives:
  • Easy to mold and polish
  • High index of refraction
  • Does not absorb red light

Using Fourier transform infrared (FTIR) spectroscopy, we determined that the material is probably polycarbonate with bisphenol. This was consistent with our spectrophotometer analysis in both ultraviolet and visible spectra.

Polycarbonate is an inexpensive thermoplastic often used as a lens material. Polycarbonate parts can be easily molded or machined, and finished parts can be polished to a high gloss, such as the lens part of the prism. Therefore it is easy to mass produce cheaply. Since it is lightweight and does not shatter, polycarbonate are more suited to object that requires durability and complex shape than glass, such as the prism for our optical mouse.

Polycarbonate is also chosen for the prism because it is highly transparent to visible light and has better light transmission characteristics than many kinds of glass. While glasses have index of refraction of 1.4-1.5, Polycarbonate have index of refraction of 1.6. This high index allows total internal reflection (TIR) of most of the LED light within the prism to produce proper illumination under the CMOS sensor.  Although TIR could be accomplished by mirroring the surfaces that require reflection, that would be more expensive than using polycarbonate. 


FULL TEXT

Powerpoint version of our final poster document available for viewing and download below.

PDF version coming soon (hopefully).


REFERENCES

[1] Askeland, Donald R., and Pradeep P. Phul. Science and Engineering of Materials. Boston: Course Technology, 2005.

[2] Schubert, E. F. Light Emitting Diodes: Device Physics, Fabrication, and Applications. Cambridge University Press.  2003.

[3] Turchetta, Renatto, Kenneth R. Spring, and Michael W. Davidson. "Introduction to CMOS Image Sensors." Digital Imaging in Optical Microscopy. Ed. Michael W. Davidson. 16 July 2004. National High Magnetic Field Laboratory. Florida State University. 12 Oct. 2008 <http://www.microscopy.fsu.edu/primer/digitalimaging/cmosimagesensors.html>.

[4] Wu, Meng-Chyi, Jyh-Feng Lin, Ming-Jiuun Jou, Chuan-Ming Chang, and Biing-Jye Lee. "High Reliability of AlGaInP LED’s with Efficient Transparent Contacts for Spatially Uniform Light Emission." Electron Device Letters, IEEE 16 (1995): 482-84.

[5] "How do optical mice work?."  08 May 2001.  HowStuffWorks.com. <http://computer.howstuffworks.com/question631.htm>  12 October 2008.



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Mike Hughes,
Mar 4, 2009, 4:28 AM
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