Backward / Forward designs : TFT as "Tilted surface"

  • 15 November 2021
  • 2 replies



I have got question regarding backward/forward designs and the control of TFT Tilts.

Here is the case :

  • In backward design, TFT has two tilts : X Tilt and Y Tilt.
  • In Forward design, TFT is usually compute as a “tilted” surface type. When TFT has only one tilt (Tilt X, we put as Y Tangent value tan(Tilt X )). Then backward and forward designs are the same (to check we do a superposition of both designs in Catia). However, when TFT has two tilts (Tilt X and Tilt Y), we put as Y Tangent value tan(Tilt X ) and as X Tangent value tan(-Tilt Y). Then backward and forward designs are differents (no matter the sign of X Tangent. I have tried to change the order of the coordinate break surface controlling the Tilt X and Tilt Y in the backward design, without success.

Do you have a solution in order to have the same exact design for backward and forward ?

Best regards,


2 replies

Userlevel 2

Hi Nicolas,

This raises a very good point. The Coordinate Break surface uses local coordinates, meaning that whether the order is normal (X then Y then Z) or reversed, it is always done with respect to the local coordinate system. Thus, after rotating about X, it then rotates about the new Y. The Tilted surface, however, takes a different formulation as seen by the formula z = Xtan(theta_x)+Ytan(theta_y), which is not fully local or global.

You could work out the differences in the rotation using matrices, but there is an easier solution, which is to let OpticStudio find the correct tilts for you. In the screenshot below, I have a tilted surface and a coordinate break, with the CB determining the desired angles. Then, using NORX, NORY, NORZ operands, I determine the surface normal in global coordinates of the CB and makes those numbers the targets for the Tilted surface. The variables are the inputs for the Tilted, and the system is optimized. As you can see, the result is similar, as expected for somewhat small angles, but not identical. You can see visually as well that the two are aligned. (The Object itself can’t be used in the operand, but the results are the same and can be put onto the object parameters.)



Hi Kevin,

Thank you for your answer. 

The thing is I do not want to have the OBJECT parrallel to the IMAGE. I want the OBJECT (=TFT) of forward design (from TFT to Virtual Image) to be in the same position as the IMAGE (=TFT) of backward design (from VI to TFT). Is this problem have a solution or maybe I do not understand well your point ?

Once again, thanks for your help


Best regards,