What ICAO 9303 actually covers
ICAO Document 9303 is the global specification for machine-readable travel documents. It is a multi-part document that defines the data page of a passport, the chip layout, the optical machine-readable zone, and the portrait image stored on both the printed page and the embedded chip. The portrait rules sit in Part 9 and are written to make the photo usable by automated border control gates that perform 1:1 face matching against the chip image.
The portrait requirements in plain language
ICAO 9303 expects a neutral, front-facing portrait taken under even lighting against a plain background. The head must be straight and centered, both eyes open and looking at the camera, and the expression neutral with the mouth closed. Hair must not cover the eyes. The minimum image resolution is targeted at producing roughly 90 pixels between the eye centres, which is the practical threshold modern matchers need to work reliably. Headwear is only permitted for documented religious or medical reasons, and even then the full facial oval must be visible.
How it influences national rules
National passport agencies layer their own constraints on top of ICAO 9303 - print size, exact background colour, glasses policy, age-based smile allowances - but the underlying geometry almost always follows the same template. That is why a US passport photo, a Schengen visa photo, and an Indian biometric photo share the same head-up, plain-background look. When a country tightens its rules (the UK banning all glasses, the US banning all glasses, several Schengen states going further on uniforms or jewelry), it is narrowing the ICAO baseline rather than replacing it.
What the standard does not specify
ICAO 9303 deliberately leaves room for national decisions. It does not mandate a specific print size, file format on submission, or background colour code. It does not pick between white, off-white, or light blue backgrounds. It does not state a fixed head-height percentage in millimetres - that is national policy. Software that targets ICAO 9303 alone, without applying the per-country overlay, will produce a photo that border systems can read but that a passport clerk may still refuse.
Where to read the source
The current edition of ICAO 9303 is published openly on the ICAO website. Part 9 is the most useful section for anyone working on photo rules. The companion biometric image specification is ISO/IEC 19794-5, which goes deeper on pixel geometry and quality scoring; you can think of 9303 as the policy layer and 19794-5 as the engineering layer underneath it.