The New Cartesian Sign Convention takes distances measured against the direction of incident light as:

1. The convention places the object on the left of the mirror. 2. Light travels left to right; this is the positive direction along the principal axis. 3. Distances to the left of the pole (against light direction) are taken as negative. _Source: syllabus Science, Ch 9 "Light Reflection and Refraction", §9.2.3_