What Are Achromatic Lens Used For
Introduction
In the world of optics, image clarity and color fidelity are crucial. Whether in imaging and visual systems, microscope objectives, or lasers, the ability to focus light accurately and consistently across the visible spectrum can make the difference between blurry, distorted images and sharp, true-to-life results. One of the most effective optical solutions to achieve this is the Achromatic Lens.
An Achromatic Lens, often called an Achromat, is a lens consisting of more than one carefully engineered lens element combined (often cemented) together, used to minimize chromatic aberration, a common optical problem where light of different wavelengths does not converge at the same ideal focal point. This phenomenon leads to colored fringes around images and a general loss of sharpness. In addition, due to the compound structure, achromatic lenses can also partially correct spherical aberration.
In this article, we will explain the fundamentals of achromatic lenses, what achromatic lenses are used for, and the designs of achromats.
History
C.M. Hall first designed and successfully manufactured the achromatic lens in 1729, but he did not publish the invention at that time. In 1758, London optical instrument merchant J. Dorland independently manufactured the achromatic lens and used it in telescopes. In 1761, J. Dorland's son P. Dorland successfully used the achromatic lens in microscopes.
What is Chromatic Aberration
Before diving into the topic of achromatic lenses, it's essential to understand the problem that achromatic lenses solve: chromatic aberration. This occurs because of the dispersive nature of the lenses (i.e., lenses refract light with different refractive indices based on its wavelength). Shorter wavelengths (like blue or violet light) bend more sharply than longer wavelengths (like red light). A simple lens, made from a single type of glass, cannot bring all wavelengths of light to the same focal point.
Chromatic aberration manifests in two forms:
Longitudinal chromatic aberration: different colors focus at different distances along the optical axis.
Lateral chromatic aberration: different colors focus at different positions in the image plane, often resulting in color fringing at the edges of objects. Lateral chromatic aberration tends to be far more visible than longitudinal CA.
Click here to learn more about chromatic aberration.
What are Achromatic Lenses used for
As mentioned above, in a conventional singlet, a broadband light source will have a varied refractive index as the wavelength changes, resulting in inconsistent focus both in the transverse and longitudinal directions. An achromatic lens is used to somehow make the focal length more consistent within a certain spectrum. Achromatic lens is typically made by combining two individual lenses (in this case, the achromatic lens is called an achromatic doublet) with different dispersion properties. An achromatic lens is used to correct these two lenses are:
- A positive (convex) lens made from crown glass, which has a low refractive index and low dispersion.
- A negative (concave) lens made from flint glass, which has a higher refractive index and greater dispersion.
Figure 1. shows how an achromatic doublet lens composed of a positive crown glass lens and negative flint glass lens brings the red light and the blue light to the same focal point and correct chromatic aberrations
Two lens elements are combined so that the chromatic aberration of one is offset by the other. When the optical power of the crown glass positive lens is not cancelled by the flint glass negative lens, a positive achromatic doublet is formed, and the combination of the two lens elements can focus different wavelengths of light together at a single focal length. Achromatic lenses can focus two different wavelengths of light on the same location, bringing them to a single point.
By cautiously selecting the lens curvature and substrate glass types, the lens pair is designed so that two specific wavelengths of light—commonly red light (around 656.3nm, called the C-light) and blue light (around 486.1nm, called the F-light)—come to the same focal point. The result is that the combined lens minimizes chromatic aberration, particularly within the visible spectrum.
Compared to a single lens, the spread of focal points across wavelengths is narrowed, and the circle of least confusion becomes smaller and more concentrated. So in essence, using an achromatic doublet doesn’t eliminate all chromatic aberration, but it significantly reduces it, and with it, the size of the circle of least confusion. This leads to a tighter focus, better image quality in situations using white light, especially where high resolution and clarity are essential.
Some advanced achromatic lenses may also correct for green (around 550 nm) and offer even better performance, but true correction for three wavelengths typically requires apochromatic lenses, a more complex and expensive solution. Correction of 4 wavelengths may be realized using a super-achromatic lens.
Design Types of Achromatic Lenses
Achromatic lenses come in several forms depending on their configuration:
Achromatic doublets: The most common type, consisting of two elements cemented or air-spaced.
Triplets: Include a third lens element for better correction.
Cemented or air-spaced designs: Cemented doublets are compact and easier to manufacture, while air-spaced doublets offer more flexibility in optical design.
Depending on the application's needs, achromatic lenses may be designed for either converging or collimating light.
The most common achromatic lens shape is a convex-concave pair, with a positive (convex) crown glass lens in front and a negative (concave) flint glass lens behind it. The outer surfaces are usually plano-convex or biconvex (front) and plano-concave or biconcave (rear), depending on the specific application.
Applications of Achromatic Lenses
Achromatic lenses are used in a broad range of optical systems. Their ability to minimize chromatic aberration makes them essential in any application where image quality is a concern.
1. Microscopy
In biological and materials science research, compound microscopes often employ achromatic objective lenses to ensure that images of specimens are sharp and color-accurate. Without achromatic correction, microscopes would display blurred edges and colored halos around the specimen, severely limiting their usefulness.
2. Telescopes
Astronomers rely on achromatic refractor telescopes to observe celestial bodies without distracting color fringes. Achromatic telescopes, while not as optically perfect as apochromatic ones, are more affordable and sufficient for many amateur astronomy needs. They allow stargazers to see planets, the moon, and stars with reasonable clarity and minimal chromatic distortion.
3. Laser Systems
In laser optics, achromatic lenses are used to collimate or focus laser beams, particularly when the system uses broadband light or multiple wavelengths. For instance, in laser-induced fluorescence or spectroscopy, achromatic lenses help maintain beam shape and focus accuracy over a range of wavelengths.
4. Medical Imaging and Diagnostic Equipment
Medical devices like endoscopes, ophthalmoscopes, and spectrometers often incorporate achromatic lenses to ensure that diagnostic images or measurements are not distorted by chromatic effects. This contributes to more accurate diagnoses and better patient outcomes.
5. Machine Vision and Industrial Inspection
In automated manufacturing, machine vision systems use cameras to inspect products for defects. Achromatic lenses ensure that these images are sharp and true to color, enabling high-speed, high-precision inspections critical to quality control.
6. Spectroscopy and Scientific Instrumentation
Spectroscopy instruments often need to focus light from a wide range of wavelengths. Achromatic lenses help to avoid misalignment of focal points, ensuring that the spectral data collected is accurate and reliable.
Advantages of Achromatic Lenses
Achromatic lenses offer several key benefits that make them the optical element of choice in many applications:
- Reduction in chromatic aberration: Significantly improves image sharpness and color fidelity.
- Cost-effective solution: More affordable than apochromatic lenses, yet sufficient for most visible-light applications.
- Compact design: Especially in cemented doublets, achromatic lenses correct without excessive complexity or size.
- Versatility: Suitable for both imaging and beam-shaping tasks across various industries.
Limitations and Considerations of Achromatic Lenses
While highly effective, achromatic lenses are not perfect:
Limited to two wavelengths: Standard achromats correct only two wavelengths precisely. Intermediate wavelengths may still show minor aberration.
Residual aberrations: Some degree of spherical aberration or coma may still exist, especially off-axis.
Temperature sensitivity: The refractive index of glass changes with temperature, which may affect precision applications unless thermally compensated designs are used.
Achromatic lenses are an essential component in optical engineering, offering a practical and effective solution for minimizing chromatic aberration. By combining lenses made of different types of glass, these doublets (or triplets) bring multiple wavelengths of light into a common focus, vastly improving the clarity and color fidelity of images.
From microscopes and telescopes to laser systems and machine vision, achromatic lenses are at the heart of many technologies we rely on daily. Though not as optically perfect as apochromatic lenses, their affordability, versatility, and performance make them the go-to choice for a wide range of optical applications.
For high-quality achromatic lenses, both standard and custom-designed, Hangzhou Shalom EO provides reliable solutions backed by precision manufacturing and optical engineering expertise. Their products support diverse sectors including biomedical optics, industrial automation, and scientific instrumentation.
Hangzhou Shalom EO supplies a wide range of off-the-shelf and custom achromatic lenses manufactured with high precision. Our standard achromatic lenses are cemented achromatic doublets consisting of flint glass lenses and crown glass lenses, the glass substrates are equivalent to SCHOTT. Examples include BAK4+SF5, BAFN10+SFL6, LAKN22+SFL6, BK7+SF2, offering durable performance. The lens substrates are deposited with various standard AR coating options from visible to NIR (350-1580nm) or other custom coating options. We can also offer achromatic triplets.
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