Plastic Optical Materials
Sheng-Huei Derek Lu
College of Optical Sciences
University of Arizona
Apple iPhone 4 Camera Lens
• Apple iPhone 4 Camera Specs: 5.0 Mpixels (2592 x 1936) 3.85 mm lens focal length f/2.8 lens aperture
• Equivalent 35mm film camera 30 mm f/22
Sample image is 2592x1936 pixels
(5.0 MP, 4.02:3)
• Don't throw away your FourThird, APS-C or full frame SLR
Cost is Everything!!
iPhone 4 Camera Module
• Glass cell phone lenses cost $20-$40 in the market.• Plastic cost: $3.55 only
• Manufacturer: Largan Precision Co., LTD, Taiwan Product#: 9457F Composition ： 4 Plastic Elements
• Very high production volumes Injection Molding: 8-Holes Method 8 cavities at the same time
Monthly volume = 207,306 parts
Outlines
• Plastic optical material application• Advantages• Disadvantages• Issues: Structural Stability Thermal Stability Moisture Absorption Injection Molding
• Wavefont error budget• Reference
What are optical elements made of plastic?
• Cellphone/ NB/ Car/ Camera Lens• Bar code scanners in CD players• Prisms• LED collimator• Diffractive Optical Elements
Main Advantages
• Cost• Production volume• Weight reduction• Aspheric and diffractive surfaces• Integral features – can have mounting features or multifunction
parts
Disadvantages
• Lower service temperature than glass - max service 80-130 ºC
• Low structural stability
- soft, larger deflection while applying force, lower resonant frequency than glass
• Poor thermal properties
– high dn/dT, high CTE
• Moisture Absorption• Limited material selection
Material Properties of Plastic Optical Element
Plastic Element Structural Stability
• ρ = 1 g/cm3, E = 3 Gpa (glass: ρ=2.5 g/cm3, E=80 Gpa)
• Resonant frequency
• Plastic with 1/3 resonant frequency than glass• Deflection per thickness
• Plastic with 10 times deflection than glass
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Plastic Element Thermal Stability
• dn/dT = -8.5 ppm/ºC, CTE = 72 ppm/ºC, λ = 0.21 W/mK (glass: dn/dT = 1.86 ppm/ºC, CTE = 7 ppm/ºC, λ = 1 W/mK)
• Defocus due to one degree change
• β = 89 ppm/ºC for plastic, β = 3.28 ppm/ºC for glass• Linear thermal distortion of a constant heat flux Φ
• Plastic with 50 times thermal deflection than glass
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Moisture Absorption
• Water absorption: change of volume, change of refractive index- The coefficient of moisture expansion (CME) for PMMA is about 0.5%
- The change of refractive index for PMMA is about 900 ppm
• Cycloolefin Polymer (COP) and Cycloolefin Copolymer (COC) are known as low moisture absorption.
Injection Molding
• Surface Imperfections - Melt temperature is too high to cause gas evolution: bubbles
- Pressure is too low to cause incomplete filling of mold.
• Shrinkage - If the part thickness is too thin, part will
shrink while cooling.
- Applying holding pressure is maintained
to compensate for material shrinkage
Tolerance Analysis: wavefont error budget for the plastic optical component
1 mold costs $200,000!
Reference
• Selected plastic optical material characteristics are listed in my tutorial: “Tutorial for Plastic Optical Materials”
• Great reference book: Stefan Bȁumer etc. “Hand Book of Plastic Optics”, 2005 WILEY
• Formulas: Prof. Burge, Opti 521/421 class note
