Achromatic Lenses

20 Jan 2022

When a single spherical lens is used with a polychromatic light source, different wavelengths do not focus to the same point, creating a colour distortion (colour fringes) within an image. This effect is known as chromatic aberration and can be corrected for two wavelengths (typically red and blue) using an Achromatic Doublet or Triplet Lens. 

Achromats are manufactured using two or three elements with high- or low-index materials with differing dispersive properties. The incident light will interact differently through each part with a result of two wavelengths sharing a common focal point, correcting what is known as primary chromatic aberration. Though this common focal point is close to that of green light (550nm), there is still a slight defocus (known as secondary chromatic aberration). 

Using Achromatic Aspheric Lenses, where one surface of an element is aspheric, can enhance performance with reduced chromatic aberration and corrected spherical aberration. Another benefit is the spot size at the point of focus, which is smaller than that of a standard Achromatic Lens. 

Finally, an Apochromatic Lens can be used to correct for three wavelengths, usually red, green, and blue, bringing their focal points to the same imaging plane, producing a clearer and superior image quality than that of an achromat. The material choices are essential, often with a combination of a crown, flint, and fluoride materials (including Calcium Fluoride).  

Knight Optical’s primary area of expertise is providing customers with custom-made optical components, including Achromatic Lenses, Aspheric Achromats and Apochromatic Lenses. What's more, we also offer Doublet and Triplet types for visible wavelengths, as well as Doublet Lenses manufactured from crystalline materials – such as Germanium and Zinc Selenide, for example – to provide colour correction in the infrared (IR) region. 

Below are our typical manufacturing capabilities:

We also have a stock catalogue of Achromatic Doublet Lenses, designed for use in the visible region, with focal lengths ranging from 5mm to 500mm.

Below are the worked tolerances:

In our state-of-the-art Metrology Laboratory and dedicated QA department, we test all optical components against their specification to ensure they meet every requirement. Equipment includes a Zygo Verifire for checking surface form error and radii, a Trioptics OptiSpheric for testing focal length, and an Agilent Cary 7000 spectrophotometer to verify any applied optical coatings are accurate. 

For more information on our services, please do not hesitate to contact our UK or USA sales team with your enquiries: usasales@knightoptical.com info@knightoptical.co.uk