Alps Alpine’s
Achromatic Lenses

Alps Alpine has been producing lenses for many years and notably enjoys a sizeable share of the market for aspherical lenses for optical communication, but we also develop aspherical lenses for other markets, too. This article provides an introduction to the industry’s smallest achromatic lenses.

Principles of Achromatic Lenses

As you know, light is made up of colors like blue, green and red. Each color has its own unique wavelength and different refractive index (see Fig. 1). This is why colors can be split using a prism. The color red, for example, has a long wavelength and is not refracted (does not bend) much, whereas blue, with a short wavelength, bends a lot. Because of this, when light is passed through a normal convex lens, the focal length is not the same, as shown in Figure 2. This is called “chromatic aberration.”

Figure 1
Figure 2

To correct this chromatic aberration, an achromatic lens – combining a concave lens and a convex lens – is used (Fig. 3).

Figure 3

*Chromatic aberration is either axial chromatic aberration, as above, or transverse chromatic aberration. There are also five types of monochromatic aberration, including spherical aberration.

Alps Alpine’s Achromatic Lenses

The use of achromatic lenses to overcome chromatic aberration is an already-known method. Alps Alpine created a compact 2mm-diameter aspherical achromatic lens using high-precision glass molding and glass bonding technology built up over many years.

⌀2mm + ⌀2mm Achromatic Lens

Approximate Specifications (Part Number: FLKWMSMD2A)
Supported wavelengths 440-660nm
Effective diameter (beam diameter) 1.4mm on collimated side (at effective NA)
Effective NA 0.172 (full angle approx. 19.8°; half angle approx. 9.9°)
Lens diameter 1.5mm
Focal length 4mm (@587.6nm)
Working distance 2.85mm (adjustment expected)
Dimensions ⌀2mm × t 1.57mm

Aspherical surface bonding allows correction of not just chromatic aberration, but also spherical aberration.

Comparing an image with and without correction, we see the right-hand side, without an achromatic lens, is red-tinged.

We are also currently in the process of developing a 1mm-diameter + 1mm-squared achromatic lens – the industry’s smallest class. In this way, Alps Alpine’s glass bonding and alignment technology enables both size reductions and bonding of various shapes, allowing flexibility in accommodating customer requirements for such parameters as shape, light source height, optical path length and package dimensions.

⌀1mm + □1mm Achromatic Lens

Target Specifications
Supported wavelengths 440-660nm
Effective diameter (beam diameter) 0.8mm on collimated side (at effective NA)
Effective NA 0.316 (full angle approx. 36.8°; half angle approx. 18.4°)
Lens diameter 0.83mm
Focal length 1.2mm (@520nm)
Working distance 0.546mm (adjustment expected)
Dimensions □1.00mm × t 0.98mm

This image gives you an idea of the relative sizes of 2mm-diameter and 1mm-squared achromatic lenses alongside the tip of a 0.5mm-diameter mechanical pencil.

Alps Alpine's Achromatic Lens Usage Sample

Smaller, lighter lenses enable use in a variety of applications. One example is augmented reality (AR) glasses. Alps Alpine achromatic lenses or collimator lenses could be fitted into an ultra-compact RGB laser light source to enable viewing of video through AR glasses.

Other possible applications where compact size and light weight are required include compact projectors for mobile devices.

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