Working distance (microscopy)

As a working distance , English free working distance (FWD), is referred to in microscopy the distance between the object and the outer lens limitation in scharfgestelltem image.

Light microscopy

In light microscopes , the working distance is a property of the lens used . The calculation differs depending on whether the objective is intended for use with or without a cover glass .

For objectives that are used without a cover glass, the working distance is the distance between the focal plane and the front lens of the objective.

When using a cover slip, the working distance corresponds to the distance from the objective to the top edge of the cover slip if the focus is on a point in the specimen directly on the bottom edge of the cover slip. This is the same distance that the plane of focus is from the lower edge of the cover slip when the front of the objective touches the cover slip. In both cases it is assumed that the cover slip has the correct thickness, usually 0.17 mm. The thickness of the cover slip to be used is indicated on quality lenses.

Working distance and opening angle. The front of the lens is shown here (blue), half the opening angle α and a red point in the plane of focus. The further away the focal plane is from the lens, the smaller the opening angle, which also reduces the achievable resolution of the lens.

The working distance thus determines how deep into a specimen can be focused: The maximum depth is reached when the objective hits either the specimen or the cover slip. As a rule, the greater the magnification , the smaller the working distance : Lenses with a higher magnification should also achieve a better resolution . For a higher resolution, a larger opening angle of the objective is in turn required in order to be able to collect more strongly diffracted light from smaller structures. Instead of the opening angle, the sine of half the opening angle, the numerical aperture, is specified on objectives .

Since immersion lenses often have a particularly high resolution, they usually have a smaller working distance than those that work without immersion (so-called dry or air lenses).

Typical values for simple objectives (type achromat ) are:

enlargement	numerical aperture	Working distance in mm
4x	0.10	30.00
10x	0.25	6.10
20x	0.40	2.10
40x	0.65	0.65
60x	0.80	0.30
100x	1.25	0.18 (oil immersion objective)

Special objectives offer a working distance of more than 10 mm at 50x magnification and a numerical aperture of 0.4–0.5 or a working distance of 2 mm at 20x magnification and a numerical aperture around 1 for water immersion.

Electron microscopy

Scanning electron microscopes have a large working distance. At 10,000 times magnification, it is larger than 5 mm. In contrast, an optical imaging transmission electron microscope has a working distance of 0 mm: This is where the sample lies in the objective.

Individual evidence

^ Friedrich Lottspeich, Joachim W. Engels (Ed.): Bioanalytik . 3. Edition. Springer Spectrum, Berlin / Heidelberg 2012, ISBN 978-3-8274-2942-1 .

[1] Friedrich Lottspeich, Joachim W. Engels (Ed.): Bioanalytik . 3. Edition. Springer Spectrum, Berlin / Heidelberg 2012, ISBN 978-3-8274-2942-1 .