Patent ID: 6913356

Claim:
A method of fitting a holding block ( 6 ) to a semifinished ophthalmic lens blank ( 1 ) intended to have a predetermined prism, which method includes the following steps: positioning the blank ( 1 ) on a fixed base ( 19 ), in a centered and angularly defined manner, so that the finished face ( 2 ) of the blank ( 1 ) bears conjointly on a plurality of bearing points (S 1 , S 2 , S 3 ) of said base ( 19 ), defining an orientation of the holding block ( 6 ) relative to the blank ( 1 ), orienting the holding block ( 6 ) in the defined manner, and fixing the holding block ( 6 ) to the finished face ( 2 ) while maintaining orientation, characterized in that the step of defining the orientation of the holding block ( 6 ) includes the following steps: taking account of the three-dimensional shape of the finished face ( 2 ) and the position of said bearing points (S 1 , S 2 , S 3 ) deducing therefrom the orientation of the finished face ( 2 ) when the blank ( 1 ) is positioned on the base ( 19 ), taking account of a predetermined prism, and deducing from the orientation of the finished face ( 2 ) and the predetermined prism the orientation of the holding block ( 6 ) relative to the finished face, characterized in that, to orient the finished face ( 2 ) when the blank ( 1 ) is positioned on the base ( 19 ), a positioning prism resulting from tilting of the blank ( 1 ) when it is placed on the base is calculated, and characterized in that, to define the orientation of the holding block ( 6 ), two angles γ and φ are calculated defined by the following equations: γ = Arc ⁢ ⁢ cos ⁡ ( tan ⁡ ( AngV ) × sin ⁡ ( AngV 0 ) + cos ⁡ ( AngV 0 ) 1 + tan 2 ⁡ ( AngH ) + tan 2 ⁡ ( AngV ) ) ϕ = Arc ⁢ ⁢ tan ⁡ ( sin ⁡ ( AngV - AngV 0 ) sin ⁡ ( AngH ) ) ⁢ in which: AngH and AngV are defined as follows: AngH = Arc ⁢ ⁢ tan ( ( ∂ f N ∂ x ) x = 0 , y = 0 L ) AngV = Arc ⁢ ⁢ tan ( ( ∂ f N ∂ y ) x = 0 , y = 0 L ) where ƒ N is a function of the type z=ƒ N (x,y) defining the shape of the finished face ( 2 ) in a system of axes XYZ fixed relative to the base ( 19 ) and x,y,z are the cartesian coordinates linked respectively to the axes X, Y and Z of said fixed system of axes, L being defined by the following formula: L = 1 + ( ∂ f N ∂ x ) x = 0 , y = 0 2 + ( ∂ f N ∂ y ) x = 0 , y = 0 2 AngV 0 is defined as follows: AngV 0 = Arc ⁢ ⁢ tan ⁡ ( Pr ⁢ ⁢ V 0 100 ) n - 1 PrV 0 being defined as follows: PrV 0 =K ×add where add is the power addition of the ophthalmic lens to be obtained and K is an index of proportionality preferably equal 2 3 ⁢ .