What does FT mean on safety glasses?

F is the low-energy impact grade — the lens stops a 6 mm ball at 45 m/s. The T suffix means that impact performance was also proven at −5 °C and +55 °C. Markings without T only guarantee the impact grade at room temperature, which matters in cold stores, foundries and outdoor winter work.

What is the difference between S and F on eye protection?

S (increased robustness) is a drop test — a 43 g ball dropped from 1.3 metres. F is a genuine impact test — a projectile at 45 m/s, roughly 100 mph. Most industrial tasks that create flying particles need F as a minimum, and a lens marked only S is not sufficient for them.

Do the numbers 3, 4 and 5 appear on the lens or the frame?

The frame only. Under EN 166, protection against liquid splash (3), large dust (4) and gas/fine dust (5) is a property of the complete protector and is marked on the frame. A lens marking alone can never demonstrate splash or dust protection.

What if the frame and lens carry different ratings?

The complete protector takes the lower of the two. An F-rated frame with a B-rated lens is an F protector. Always read both markings and rate the pair by the weaker component.

What does 2C-1.2 mean on a lens?

It is a scale number: 2C designates a UV filter with good colour recognition under EN 170, and 1.2 is the lightest shade — effectively a clear lens that still blocks UV. It says nothing about impact; the impact grade is the separate S/F/B letter.

Is EN 166 being replaced?

ISO 16321-1:2021 is the emerging international successor. Existing EN 166 equipment and certificates remain valid; new assessments should still reference EN 166 today. Certus tracks the transition — see our EN 166 to ISO 16321 guide.

EN 166 Markings Explained: Interactive Decoder — Certus

Paste the marking from your safety glasses and decode every symbol — plus the rule most guides miss: frame and lens are rated separately, and the pair takes the lower grade.

Certus is a UK platform for managing prescription safety eyewear compliance to EN 166, covering hazard assessments, ordering, optician verification and certificate records. Operated by Certus Safety Ltd, registered in England and Wales, company number 17364362.