Which type of glass is primarily used for lenses that require high optical quality?

Flint glass is primarily used for lenses that require high optical quality due to its special composition that allows for better light transmission and reduced distortion. This type of glass contains a higher lead oxide content, which enhances its refractive index, making it ideal for applications such as optical instruments and fine glassware. The high optical quality is essential in lenses where clarity and precision are paramount, such as eyeglasses, camera lenses, and microscopes.

In contrast, window glass is designed for structural integrity and does not possess the necessary optical clarity. Green glass, commonly used for bottles and containers, offers aesthetic qualities rather than optical performance. Fused quartz, while excellent for specific applications due to its resistance to thermal shock and transparency in the UV range, does not match the optical properties of flint glass for general lens-making purposes. Therefore, flint glass stands out as the preferred choice for high-quality optical lenses.