What Does ISO 12312-2 Mean on Eclipse Glasses and Solar Filters?
Why solar equipment is a specification question
Observing the Sun is the one area of amateur astronomy where equipment specification is everything, and it is exactly the area where this site defers entirely to the professional standard. The authoritative reference is the American Astronomical Society's eclipse eye-safety guidance, which explains the international standard for solar viewers and maintains information on reputable filter suppliers. This page explains the terminology so that guidance is easier to read; for any actual decision about what to look through, follow the AAS directly.
What the standard is
Safe direct solar viewers — the cardboard "eclipse glasses" and handheld viewers sold around eclipses — are specified by an international standard, ISO 12312-2, which defines how much visible, ultraviolet, and infrared light a filter may transmit. A compliant filter passes only a minuscule fraction of sunlight, which is why the Sun appears as a sharp-edged disk through one and everything else appears black. The AAS explains the standard and how to interpret compliance claims, including the important caveat that a printed ISO label alone is not proof of compliance — counterfeit markings have appeared on the market, which is why the AAS points buyers toward suppliers it has vetted.
Glasses are not telescope filters
A second piece of terminology trips people up: eclipse glasses are specified for naked-eye viewing only. Telescopes, binoculars, and camera lenses concentrate sunlight, and they require full-aperture solar filters — filters engineered to mount securely over the front of the instrument, filtering the light before it enters the optics. The AAS guidance covers safe practices for optics as well; the design principle to understand is that filtering must happen at the front aperture, at the specification the standard requires, with a filter made for that purpose. Improvised materials are not part of any specification and have no place in solar observing.
Indirect methods exist too
Not all solar observing looks through a filter. Pinhole projection — letting sunlight pass through a small hole and viewing the projected image on a surface — is a long-standing indirect technique the AAS also describes, and NASA's skywatching resources cover eclipse events and viewing approaches when one is coming up. Indirect methods are how many classrooms handle eclipse days.
The practical checklist for eclipse season
- Read the AAS eye-safety page before buying anything, and buy from suppliers it identifies as reputable.
- Treat "ISO 12312-2" as a specification to verify through the AAS's guidance, not a magic phrase on a package.
- Keep the categories straight: eclipse glasses for eyes only; purpose-made front-aperture filters for any optics; projection when in doubt.
- Inspect filters per the manufacturer's and AAS's instructions, and retire anything damaged.
Local astronomy clubs run public solar-observing sessions with properly equipped telescopes, and watching how experienced members handle filters is worth more than any article. The Night Sky Network lists such events, and eclipse days are when clubs are most visible in their communities.