Thoriated glass

How it works

Thorium raised the refractive index while keeping dispersion low, which let designers correct aberrations with fewer elements. It was a genuine optical advantage before rare-earth glasses replaced it.

Radiation gradually creates colour centres in the glass, giving it a yellow to amber cast. This is a change in the glass itself, not a coating fault or haze, and it shifts white balance and cuts blue transmission.

The yellowing is reversible. Prolonged exposure to strong ultraviolet light, whether sunlight over days or a UV lamp over hours, bleaches the colour centres and returns the glass close to neutral. The radioactivity is unaffected by this and is very low: measured at the rear element it is a small multiple of natural background and it drops off sharply with distance.

Why it matters when buying

A yellow example is usually cheaper and can be corrected for free with UV exposure, which makes thoriated lenses good value. Do not store them against your body for long periods, and do not grind or break the glass.