Optical Lenses and Glass Properties

High Degree of Chemical and Physical Stability

• An achromatic doublet combines crown glass and flint glass.

Homogeneity

• Non-homogeneity results in:
  • Striae: streaks or lines in the glass caused by uneven mixing
  • Bubbles: also called seeds, air-bells or boil
  • Inclusions: stones & crystallites; undissolved particles
  • Cloudiness: caused by precipitated colloidal material during the cooling period

Specific Gravity

• Defined as the ratio of the weight of a substance to the weight of water with the same volume

Three Main Varieties of Optical Glass

• Crown Glass:
  • 70% Silica
  • 14-16% Soda (Sodium Oxide)
  • 11-13% Lime (Calcium Oxide)
• Flint Glass:
  • 45-65% Lead Oxide
  • 25-45% Silica
  • 10% Soda & Potassium Oxide
  • Used for bifocal segments for some fused bifocals
• Barium Glass:
  • 25-40% Barium Oxide

Optical Glass Materials

• Optical Glass Materials Properties:
  • Refractive Index
  • Abbe Value (Nu)
  • Specific Gravity
• Examples of Optical Glass Materials:
  • Crown: Refractive Index 1.523, Abbe Value 58.9, Specific Gravity 2.54
  • Light Barium: Refractive Index 1.573, Abbe Value 57.4, Specific Gravity 3.21
  • Dense Barium: Refractive Index 1.616, Abbe Value 55.1, Specific Gravity 3.36
  • Dense Flint: Refractive Index 1.649, Abbe Value 33.8, Specific Gravity 3.90

Glassmaking Process

• Batch Process
• Manufacturing of Optical Glass: 1944 B&L
• Steps in Glassmaking:
  • Reheated & cast into molds
  • Final Annealing

Abbe Value and Refractive Indices

• Abbe value (ν) is a measure of dispersive power, calculated using refractive indices at specific wavelengths: 𝑛" $% = (𝑛' - 𝑛") / (𝑛" - 1).
• The refractive indices used are nd, nF, and nC, corresponding to the Fraunhofer d, F, and C spectral lines (579.56 nm, 486.1 nm, and 656.3 nm, respectively).

Fraunhofer Lines

• Fraunhofer lines are absorption lines in the solar spectrum, discovered by Joseph Fraunhofer.
• Over 25,000 Fraunhofer lines exist in the solar spectrum, between 2,950 and 10,000 angstroms.
• Some common Fraunhofer lines include:
  • i (365.01 nm, UV, Mercury)
  • h (404.66 nm, Violet, Mercury)
  • g (435.84 nm, Blue, Mercury)
  • F' (479.99 nm, Blue, Cadmium)
  • F (486.13 nm, Blue, Hydrogen)
  • e (546.07 nm, Green, Mercury)
  • d (587.56 nm, Yellow, Helium)
  • D (589.3 nm, Yellow, Sodium)
  • C' (643.85 nm, Red, Cadmium)
  • C (656.27 nm, Red, Hydrogen)
  • r (706.52 nm, Red, Helium)

Dispersive Power

• No chromatic dispersion occurs if the refractive index of a substance is the same for all wavelengths.
• To achieve correct dispersive power (ν value), the refractive indices for the D, C, and F lines must be controlled.