Design
The first-order design of an achromat involves choosing the overall power of the doublet and the two glasses to use. The choice of glass gives the mean refractive index, often written as (for the refractive index at the Fraunhofer "d" spectral line wavelength), and the Abbe number (for the reciprocal of the glass dispersion). To make the linear dispersion of the system zero, the system must satisfy the equations
where the lens power is for a lens with focal length . Solving these two equations for and gives
Since, and the Abbe numbers are positive-valued, the power of the second element in the doublet is negative when the first element is positive.
Read more about this topic: Achromatic Lens
Famous quotes containing the word design:
“Nowadays the host does not admit you to his hearth, but has got the mason to build one for yourself somewhere in his alley, and hospitality is the art of keeping you at the greatest distance. There is as much secrecy about the cooking as if he had a design to poison you.”
—Henry David Thoreau (1817–1862)
“Teaching is the perpetual end and office of all things. Teaching, instruction is the main design that shines through the sky and earth.”
—Ralph Waldo Emerson (1803–1882)
“Westerners inherit
A design for living
Deeper into matter—
Not without due patter
Of a great misgiving.”
—Robert Frost (1874–1963)