What is photopenia involving femoral head?

Less dense: Likely that you are looking at a report of an X-ray. There is a standard density that bone has. On an X-ray anything with an appearance more dense than that would be called sclerotic and anything less dense could be called lucency or osteopenia or photopenia. An X-ray is like a photo, it can be under or over penetrated; so a more true measure of bone density would be to get a dexa scan.