Dichroic filter conformed to optical surface

A method for forming an optical element, the method comprising:

• a) forming a first multilayer thin film coating on a first surface of a first transparent glass substrate that is substantially flat, wherein the first coating has a filter characteristic that transmits light of a first wavelength band and reflects light of a second wavelength band, wherein the first surface has a first surface form Z1, and wherein the first substrate has a second surface opposite the first surface and having a second surface form Z2;
• and
• b) bonding the second surface of the first substrate to a third surface of a second transparent glass substrate, wherein the third surface has a third surface form Z3, to form the optical element having a fourth surface form Z1′,
• wherein each surface form Z1, Z2, Z3, and Z1′ has a corresponding sagitta deviation value and wherein the magnitude of the difference between sagitta deviation values of the fourth surface form Z1′ and the third surface form Z3 is less than the magnitude of the difference between the sagitta deviation values of the first surface form Z1 and the third surface form Z3.

A method for forming a dichroic beam splitter, the method comprising:

• a) depositing a first multilayer thin film coating to form a first coated surface on a first transparent glass substrate that is substantially flat, wherein, for incident light at an angle of incidence exceeding 5 degrees, the first multilayer thin film coating has a filter characteristic that transmits light of a first wavelength band and reflects light of a second wavelength band, wherein the first coated surface has a first surface form, and wherein the first substrate further has a second surface that is opposite the first coated surface, wherein the second surface has a second surface form after the coating is deposited;
• and
• b) bonding the second surface of the first substrate to a third surface of a second transparent glass substrate using adhesive-free bonding to form the dichroic beam splitter,
• wherein the third surface has a third surface form prior to bonding and wherein the first coated surface of the beam splitter has a fourth surface form after bonding,
• wherein the difference between the fourth surface form and the third surface form is less than the difference between the first surface form and the third surface form.