[shailesh]

The equipment in the optometrist's office uses Shack-Hartmann equations, while EyeNetra uses its inverse of that. Therefore, it is gives similarly accurate results, based on which, a Doctor can prescribe the glasses. The reason the optometrist's equipments are expensive is because they are fighting against human eye, which is very efficient in absorbing light. EyeNetra is really an optical hack, but quite an elegant one at that.

Edit: clarity.