Chromatic Aberration

A reading glass is a low power magnifier that uses a

biconvex lens: this is a clear disc made of either plastic

or glass with a convex surface on both sides.

If you take a biconvex lens and place it close to an

object so that it lies within its front focal point, you

generate an image of the subject from light reflected

from its surface. This is seen when looking through the

opposite side of the lens. When viewed in this

way, the subject appears enlarged and right side up.

Even at this low power, a critical observer can see optical

artefacts. One such is chromatic (colour) aberration,

which is when a border displays colour fringes, a

defect caused by the lens acting like a prism. White

light is a composite of all the colours of the rainbow,

where each colour corresponds to a specific wavelength

of light. When white light passes through a

lens or a prism, it separates to its component colours

as the different wavelengths are bent to differing

degrees. Thus, a lens will bring the red light rays to a

more distant focus point than the blue rays, and this

creates a colour fringe at the borders of the subject.

This defect can be corrected by using a lens that

consists of two types of glass with different optical

properties. Such a lens is described as being ‘achromatic’.

In the case of the lower-powered reading glass

the colour fringes resulting from chromatic aberration

are not sufficiently large to disturb most viewers.

However, at higher magnification, the subject’s fine

borders will show these colour fringes and will detract

from the appearance of the image.

An example of an achromatic loupe is the Hastings

Triplet Magnifier. Superficially this looks like a single

lens, but it is really a composite lens made up of three

elements cemented together. In this manner, the

magnifier corrects for chromatic aberration and

improves the optical quality of the image by rendering

the specimen’s borders without the colour fringes.

High quality magnifiers, such as the Hastings Triplet,

are a worthwhile addition to any photomicrographer’s

arsenal. They will be used to evaluate specimens before

using higher power examination with the microscope.

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