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Apparatus and methods for measuring the optical thickness and index of refraction of thin, optical membranes

  • US 4,453,828 A
  • Filed: 12/02/1981
  • Issued: 06/12/1984
  • Est. Priority Date: 12/02/1981
  • Status: Expired due to Fees
First Claim
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1. A method comprising directing light onto a thin, optical membrane at one or more known angles of incidence, said membrane having a thickness in the range of about 0.5 to about 10 micrometers, said membrane being capable of being edge-supported;

  • finding two consecutive null angles of incidence for said membrane;

    calculating the thickness t of said membrane from the expression;

    t√

    (sin I2)2 -(sin I1)2 =(λ

    /2)√

    k-1, where I2 and I1, are said two consecutive null angles of incidence for said membrane, λ

    is the wavelength of the incident light, and k is an assumed, even integer value, in quarter wavelengths of light, for the optical thickness of said membrane;

    then calculating the index of refraction N of said membrane from the calculated value of t and the assumed value of k in the expression;

    Nt=k(λ

    /4), where N is equal to the square root of the expression N2 -(sin I)2, I is I1 or I2, and N is the index of refraction for said membrane.

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