hands on

Q: What is an explanation of polynomial aspheric design and deformed conicoid aspherics, and what are the differences?

A: A general equation for the sag of a conic section is: Sag = x^2 * R / (1 + SQRT(1 – p * x^2 R^2))

Where: x is the semi-diameter; R is the central radius of the lens surface; and p is the eccentricity of the conic surface. When p > 1 you have an oblate ellipse. When p = 1 you have a circle (sphere). When 1 > p > 0 you have a prolate ellipse. When
p = 0 you have a parabola. And when p < 0 you have a hyperbola.

A general equation for the sag of an aspheric surface expressed as a polynomial is:

Sag = A * x^2 + B * x^4 + C * x^6 + D * x^8

Where: x is the semi-diameter; A, B, C, and D are aspheric coefficients.

Either of these equations can be used to describe an aspheric surface. Additionally, the sagittal curve is derived from the first derivative and the tangential curve derived from the second derivative.

The differences between the two equations in describing an aspheric surface are, at the limit, insignificant. In other words, for radically symmetric aspherics, one equation does not have advantage over the other. The quality of the aspheric is determined by the design parameters of the aspheric, and not the equation used to describe it.

— Dan Torgersen, vice president IS and special projects, Walman Optical, Minneapolis, Minn.

CHANGING TERMINOLOGY

Q: What is the proper term for lenses that activate light to dark and back, photochromic or photochromatic?

A: The International Union of Pure and Applied Chemistry (IUPAC) is considered the authoritative source for terminology in this area. The IUPAC defines photochromism, from the Greek words phos (light) and chroma (color), as a light-induced reversible change of color. Photochromic is used to describe material that has this ability, such as a photochromic lenses. The IUPAC does not use the term photo-chromatic, and we don't consider it the preferred terminology.

— John Ligas, director of product leadership, Transitions Optical, Inc., Pinellas Park, Fla.

If you have a question you'd like to have answered in Ask the Labs, send it to Karlen McLean. E-mail: mcleank@lwwvisioncare.com. Fax: 215-643-1705. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebusiness.com.

FREE thinker

Q: What is freeform? Is it a lens product or is it the process (manufacturing) of how a lens is made?