How to provide controled sm30 access to a field in a customised table??

Hi,

I recieved a request from our users to create a role which will give sm30 access to a particular field in a customised table.

I know we can restrict access to atable by using S_TABU_NAM authorization object, but not sure about how to restrict access to a particular field.

I tried S_TABU_LIN which is allowing to restrict to a whole cloumn in a table, but I am looking for single field.

Please post your suggestions and recommendations.

Thanks

Hari.