Face images in ePassports: how high resolution are they?
Chips in ePassports contain the digitally-signed high-resolution face image of the owner of the ePassport. This is a very important feature of ePassports, since it makes it possible to check if the holder of the identity document is also the owner. This facial matching can be done by a human, e.g., look-a-like detection by a border guard, or by software, e.g., by comparing it with a selfie. For both humans and software: a better picture means trustworthier facial recognition. The data page of an ePassport only contains a low quality photo, typically with some form of watermark printed on it to hinder falsification. Doing automated facial recognition from the face image on the data page is therefore in our experience not a good idea (see, e.g., this research). Fortunately, the face image from the chip is of much better quality. We first discuss what the standard requires from the face image stored on the chip and focus on what sizes of face images we encounter every day.
The eMRTD (Machine Readable Travel Documents, ICAO 9303) standard mandates that the face image is a high-resolution photo, suitable for automated facial recognition. This means, among others, that the picture is taken in a uniform manner: face forward, no smiling, good lighting, etc. (see also ISO/IEC 19794-5 on Face Image Data). We noticed, however, that the face images quality differs significantly between countries. Not only on how strict the requirements on, e.g., the lighting are enforced, but also on the size of the image. Face images that are too small will significantly reduce facial recognition performance.
The ICAO 9303 specification (part 9) does not specify the exact size of the image, so countries have a choice here. This is not a trivial choice, since the amount of storage on a ePassport chip is limited and the face image typically takes up most of this. The ICAO standard considers 12kB a minimum for reliable facial recognition, and 15-20kB as optimum.
We looked at the meta information on face images from ePassports from 13 countries, at least 45 ePassports per country. The graph below depicts the smallest, the average (in purple) and largest image sizes we encountered. The overall average of our samples was 15.5 kB. But as you can see below, the differences are significant between countries, and also within countries. A possible explanation for the, in our opinion somewhat remarkable, differences within a country is that we probably had different generations of ePassports in our sample. We did not analyze this in more detail yet. Note that some ePassports in our sample were below the 12 kB minimum that is in the ICAO spec, but by far most are above.
Please be ware that there are actually two different file formats allowed: JPEG and JPEG 2000. There are also different ways to compress pictures. Not only image size, but also the format and the compression will influence how ‘good’ a face image is. ReadID’s API allow access to the original face image as well as converted and resized versions of the original face image.
We plan to share more data on ePassports in future blogpost, let us know on firstname.lastname@example.org if you have a suggestion for this.