achromat selection guide    achromat selection guide    achromat selection guide

Image credit: Thorlabs, Inc. | Universe Kogaku Inc. | Knight Optical Ltd.

 

Achromats are optical lenses designed to minimize chromatic aberration. Achromats are also known as achromatic lenses.

 

Achromat Basics

 

Chromatic Aberration

 

Chromatic aberration is a type of optical distortion which occurs when a lens fails to focus all colors to the same convergence point. This distortion is often recognizable as "color fringing," in which the image blurs at high contrast edges (or dark-light borders) of the image. The image below shows simulated effects similar to those caused by severe chromatic aberration. The blurring and yellow tint of the dark sections on white background are classic examples of color fringing.

 

achromat selection guide

Chromatic aberration. Image credit: Noise Industries

 

Chromatic aberration comes in two types:

 

  • Axial, or longitudinal, aberration occurs when different wavelengths are focused at different points on the optical axis (focus shift). It occurs throughout the image.
  • Transverse, or lateral, aberration is caused when different wavelengths are focused at different points on the focal plane. Transverse aberration does not occur in the image center and increases in intensity as toward the edges of the image.

 

achromats selection guide

An example of chromatic aberration. Image credit: ARRI

 

 

Correcting Aberration

 

Achromats use at least two individual lenses to correct chromatic aberration. Lens assemblies consisting of two lenses are known achromatic doublets, while those using three lenses are known as triplets. Doublets consist of a concave lens with high dispersion attached to a convex lens with a lower dispersion. In positive doublets, the most common type, the optical power of the convex lens is greater than that of the concave one; the two together form a weak positive lens that brings two different wavelengths of light into common focus.

 

achromats selection guide

An achromatic doublet. Image credit: Vik Dhillion / University of Sheffield

 

Achromatic triplets, also known as apochromatic lenses, use three lenses to correct aberration even further. Whereas doublets are able to focus two distinct wavelengths, apochromats can focus three wavelengths to achieve greater image clarity. Apochromats are typically more expensive than doublets due to their use of low dispersion lenses constructed of fluorite, flint glass, and optically transparent fluids. The image below shows apochromat operation.

 

achromats selection guide

Image credit: Tutorial9

 

Specifications

 

Wavelength Range

Achromats can be designed and manufactured to operate within one or several wavelength ranges. Buyers of achromats should determine the wavelength range based on the lens's intended application. The three common wavelength regions are:

 

  • Infrared describes electromagnetic radiation between 750 and 2500 nm.
  • The visible spectrum consists of wavelengths between 380 and 750 nm.
  • Ultraviolet radiation consists of the 4 to 380 nm wavelength range.

 

achromats selection guide

Image credit: Mato48

 

Optical Surface Quality

Surface quality is an important specification to consider when selecting achromats. The optics industry rates products based on the MIL-0-131830A standard first published in 1963. The standard specifies two different types of surface defects:

 

  • Scratches are defects with lengths many times their widths.
  • Digs are defects with nearly equal lengths and width.

 

Surface quality ratings consist of two numbers separated by a hyphen, as in x-y. The x in this formula refers to the maximum allowable width of a scratch — expressed in tenths of a micron — while the y refers to the maximum width of a dig, expressed in hundredths of a millimeter. For example, a 20-10 scratch/dig rating specifies that any scratches have a maximum width of .002 millimeters (2 microns), while any digs must not exceed .10 mm in width. For both scratch and dig specifications, smaller numbers are desirable.

 

Flatness

Manufacturers often specify optical surface flatness using a "peak to valley" (P-V) measurement. This value is the difference between the relative highest and lowest points of the optical surface. P-V is expressed as a ratio of a set wavelength, as shown in the table below. When expressed as a fraction, a higher denominator indicates better quality.

 

 

Surface flatness

Relative quality

Description

λ/2

Very low

Lowest quality; suitable for noncritical applications.

λ/4

Low

Typically used for beam splitters; not suited to high power applications.

λ/10

Good

Suitable for many laser and scientific applications.

λ/20

Very good

Most precise quality; suitable for critical wavefront control applications.

 

 

References

 

Georgia State University - Chromatic aberration 

 

Optical Technologies - What is an optic surface figure?

 


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