In one of the previous articles from the Microscopy and Imaging Channel, we took a look at ‘that other number’ which you’ll find on a microscope objective: namely numerical aperture. The information you’ll find on objectives doesn’t stop there though- another piece of information to be found on the barrel of the objective is the optical correction. There are a large number of these types of objectives available, but we’ll just take a look at the most common ones.
The most common objectives are the Achromatic objectives (abbreviated ‘Achro’ or ‘Achromat’). These objectives are corrected for an optical phenomenon called ‘axial chromatic abberation’. Now, you don’t need to know the physics behind this (whew!), but this arises when white light passes through a convex lens. The light is split into its components wavelengths of red, green and blue. This causes coloured blurring and coloured fringes around the edges of the image. The achromatic objectives are corrected for red and blue wavelengths bringing these to almost the same focal point as the green wavelength light.
Following on from achromatic lenses are the Plan-achromatic objectives (abbreviated ‘Achroplan’ or ‘Plan Achromat’). Further to the above correction for light scattering, these are corrected for ‘field curvature’. This is a natural phenomenon which occurs when light is focused through a curved lens. In a non-corrected objective, the user would be able to achieve sharp focus around the edges of the field of view, or the centre of the field of view, but not both together. For routine laboratory work, this isn’t a problem. However, if you are planning to produce images of your work for use in publications for example, then a corrected lens is needed. The correction for field curvature is also used in conjunction with other corrected objectives (see below).
The next level of correction you are likely to encounter is the Semi-Apochromatic, or Fluorite objectives (abbreviated as ‘Fl’, ‘Fluor’, ‘Fluar’ etc). These lenses were originally manufactured using the mineral fluorite (hence the name!), but are now mainly made using synthetic materials (although fluorspar is still used). The correction in these lenses is one step up from the achromatic objectives- these are also corrected for ‘spherical abberation’. This is similar to the above problems which arise when using curved lenses. The curvature of the lens does not focus the parallel light waves to a single point, instead they are spaced along a line. The fluorite objectives are spherically corrected for one or two colours (unlike the achromats which are only corrected for one). As with the achromatic objective, fluorite objective are available which are also corrected for field curvature. These are the Plan-fluorite objectives (abbreviated as ‘Plan Fluor’ or ‘Plan FL’).
The highest level of correction (and expense!) in objectives which you are likely to encounter is in the Apochromatic objectives (abbreviated as ‘Plan Apo’ or ‘Plan Apochromat’). These are chromatically corrected for three colours (red, green and blue) meaning all three wavelengths of light will converge at the same focal point. These lenses are also spherically corrected for two or three wavelengths. As the abbreviated name suggests, they are also routinely corrected for field curvature.
So, now you should know the meaning of (nearly) all of the information you find on an objective!