The Interactive Advertising Bureau has made public its guidelines for measuring rich media ad units.

The Bureau is also asking for public comments before making them final.

The new IAB guidelines seek to make clear just when a rich media ad unit can be said to have generated an impression by the visitor to a website. That definition is tricky, since rich media as a category contains many different online ad formats.

A primary component of the guidelines dictates that an impression is not generated until as deep into the page's visit as possible. That allows for whatever creative the unit might contain to load and be viewed.

The IAB allows publishers, advertisers and other stake-holders to comment on the proposed guidelines for 30 days, at which point it will review suggestions and make its final statement.