Achromatic Doublet Glass Selector
Pick the two glasses for a cemented achromatic doublet. Enter focal length, F-number, waveband and operating temperature; the tool evaluates every crown–flint pair in the CDGM and NHG catalogs and ranks them by secondary spectrum, spherical aberration and coma, thermal defocus, cement stress from thermal-expansion mismatch, and relative glass cost. Each pair comes with a starting prescription you can paste into Zemax or CODE V.
Loading the glass selector…
If this text stays, the page editor removed the <script> code. Paste the code into a Custom Liquid section instead.
Need achromatic doublets built to your prescription?
Request a quoteFrequently asked questions
How do I choose glasses for a cemented achromatic doublet?
Pair a low-dispersion crown (high Abbe number) for the positive element with a high-dispersion flint for the negative element. A larger Abbe-number difference lowers the element powers and curvatures. A smaller difference in relative partial dispersion lowers the secondary spectrum. For a cemented design the refractive-index difference must also be large enough to correct spherical aberration, and the two glasses should expand at similar rates with temperature.
What is secondary spectrum and how can I reduce it?
Once two wavelengths (for example the F and C lines) share a focus, the other wavelengths still focus at slightly different distances. That residual is the secondary spectrum. With ordinary glass pairs it is about 1/2000 of the focal length across F–C and larger when the g-line is included. Crowns with anomalous partial dispersion, such as fluor crowns (FK) and phosphate crowns (PK), paired with suitable flints reduce it, usually at the cost of higher thermal expansion and price.
Why does thermal-expansion mismatch matter in a cemented doublet?
Optical cement cures at room temperature. When the doublet is cooled or heated, two glasses with different expansion coefficients strain against each other through the thin cement layer. This causes shear stress in the cement, stress birefringence in the glass and slight bending of the lens. Large mismatches over wide temperature ranges can make the cement fail. A low-modulus cement, a thicker bond line or an air-spaced design reduces the risk.
How does temperature shift the focus?
The refractive index changes with temperature (dn/dT), and the lens and its barrel expand. A doublet is passively athermal when the temperature coefficient of its optical power cancels the barrel expansion. For every glass pair the tool reports the defocus at the temperature extremes and the barrel thermal-expansion coefficient that would make it athermal.
Which glass catalogs are included?
The current CDGM and Hubei New Huaguang (NHG) catalogs in Zemax AGF format, about 600 glasses. Visitors can also import other AGF catalogs; they are processed only in the visitor’s own browser.
Is the result a finished lens design?
No. The tool ranks glass pairs and gives a starting prescription. A final design still needs full optimization, tolerancing and, for demanding environments, finite-element stress analysis.