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Welcome to the glossary

Our glossary offers brief explanations of important terms from our specialist area. Whether you are a newcomer or want to deepen your knowledge, you will find quick answers to your questions here. Have fun discovering!

Axial Resolution

The axial or depth resolution defines the smallest detectable height feature. According to the VDI/VDE 2655 guideline the axial resolution is determined from the areic root mean square height (sq-value) of the difference of two measurements. For a confocal microscope the achievable resolution is directly correlated to the FWHM of the measurement system. The lower the FWHM is, the better is the achievable resolution.

Confocal Principle

The confocal principle is based on a mechanical height scan with steps within the submicrometer range. During the mechanical scanning process, a stack of pictures is taken within a couple of seconds. The confocal microscope uses a spinning microlens disc for the areal scan. This innovative approach ensures a bright illumination, consequently reliable measurements can even be taken from strong absorbing or low reflecting objects.

Fully Focussed Image

With a confocal measurement system a fully focussed image representing the object relfectivity and structure is composed from the sharply imaged postions of the image stack acquired during a depth scan. This image is in contrast to conventional microscopic images completly focussed.

Field Size (F.N.)

Microscopic instruments usually have a limited field size when using off-the-shelf optics. Commonly lenses of one series have the same field number. The field number describes the maximal size of the intermediate image when using a 1x tube lens and it marks the area for which imaging errors are corrected. Common field numbers range between 25 mm and 30 mm. The actual field size of an image and accordingly the magnification are calculated from the nominal magnification of the front lens in combination with the focal length of the tube lens and the size of the image sensor.

FWHM (Full Width at Half Maximum)

The Full Width at Half Maximum value describes the 'width' of the confocal signal. It is influenced by the effective numerical aperture of the optical system, the wave length of the light and the quality of the optical system. With a decreasing numerical aperture the Full Width at Half Maximum (FWHM) value decreases and the achievable resolution improves. When measuring steep or very rough surfaces the effective NA decreases leading to an increased FWHM value, a reduced resolution and increased measurement uncertainty. From an analysis of the FWHM and the fully focussed image a quality map of the measurement can be obtained.

High Dynamic Range (HDR)

High Dynamic Range denotes digital images whose dynamic range is above the one of the camera used. This is achieved joining images with different exposure times. This technique allows the reliable measurement of objects with strongly varying reflectances.


Is the abbreviation of ITO measurement program, which is an open source measurement and automation program, that was developed by the institute for applied optics (Institut für Technische Optik) of the university of Stuttgart. You can find more information about itom in our services section.

Laterale Resolution

We have to distinguish between the pixel resolution and the optical resolution. For microscopic devices the optical resolution is usually considerably coarser than the pixel resolution.
The optical resolution describes the smallest distance two structures may have to be recognised separately, e.g. two lines of a black-white line strip pattern. The achievable resolution is influenced by the wavelength, the numerical aperture and the quality of the optics used. It can be determined numerically, by simulation or experimentally. Different criteria are used for the definition of a separate recognition of two objects, e.g. the Rayleigh criteria and the Sparrow criteria. As uncorrected image defects (aberrations) and mounting and fabrication tolerances greatly influence the achievable resolution an experimental determination is recommendable, e.g. using a USAF resolution target.


The magnification of a digital microscope is defined by the nominal magnification of the microscope lens, the tube lens and the pixel resolution of the imaging sensor. The usful magnification of a microscope is limited by the optical resolution and thus by the numerical aperture. Using magnifications above the useful magnification results analogue to digital zoom only in a larger representation of the object without the ability to resolve finer structures. This is also referred to as 'empty magnification', which ususally even leeds to a reduction of the transmitted information as the field size of standard microcope lenses is normally limited and reduces with a raising magnification. Therefore it is necessary to balance the magnification, the optical resolution and the resolution of the imaging sensor. Magnifications in the range of 20,000x are attained by relating the computer monitor size to the measurement area.

Measurement Reproducibility

Describes the deviation of the measurement results from the measurand doing repeated measurements under varying and not influencible conditions. It considers systematic and statistical influences of the measurements system and the user (see JCGM 200:2012, nr. 2.25)

Measurement Artefact

Artefacts are measurement errors resulting from an unexpected behaviour of the measurement devices causing a failure of the used signal modell. Using optical surface metrology devices this usually results from the indirect nature of their measurement results, i.e. measurands are calculated out of measured values using some sort of signal evaluation. A typical artefact is the so called 'Moiré-effect'. It can be observed when measuring periodical structures whilst the measurement device is offending the Nyquist-frequency. In this case the measurement result is overimposed with a low frequency beat component, determined by the sampling frequency and the structure period.


Describes the quality of a measurement system by determing the deviation of measurement results under the same conditions, with the same operator, the same measurement system and the same or a comparable sample. The absolute deviations of the measurement system are not considered (see JCGM 200:2012, nr. 2.21)

Measurement Uncertainty

Describes a deviation interval around the measured result, that includes the true value of the measurand. The measurement uncertainty includes statistically distributed components (noise) and uncorrected systematical errors (e.g. an erroneous calibration, see JCGM 200:2012, nr. 2.26).

Numerical Aperture (NA)

The numerical aperture (NA) is a measure for the acceptance angle of an optical system. The acceptance angle increases with an increasing NA and therewith the theoretical achievable resolution, whilst at the same time the depth of focus decreases.

Optical Measurement Principle

Different optical measurement principles were developed for topography measurements and surface metrology. The most common ones are: Focus variation (also including confocal systems), (laser-) triangulation (also pattern and fringe projection), time of flight and interferometry (also white light interferometry). Every principle has its specific advantages and disadvantages and therefore its favorable applications. E.g. white light interferometry is known for its high axial resolution whilst it is quite sensitive to vibrations. In contrast confocal microscopy often has a lower axial resolution but it is less senstitive to vibrations.


Is an abbreviation for 'twentieth of an inch point' and is a typographical measurement unit. It is defined as 1/1440 inch or 17.639 μm when derived from a PostScript point and as 1/1445.4 inch or 17.573 μm when derived from a printer's point.

Working Distance

The working distance is the distance between the front surface of an optical lens and its object plane. Its values varies extensively for microscopic lenses. Usually lenses with a higher numerical aperture have a shorter working distance.

2.5 D

Most areal measuring optical surface metrology devices are capturing a distance map between object and measurement device. This equals a projection of the three-dimensional object onto the imaging plane (e.g. focal plane). The bottom side of the object and undercuts cannot be measured without using additional kinematics (e.g. rotation stages). As the lateral position of the measured points is defined by the pixels of the imaging sensor and the measured data equals a topography map with a regular grid such a measurement data is called 2.5 dimensional.


We update the glossary regularly to ensure that you always have access to up-to-date and relevant information.

We will be happy to assist you and provide you with advice and support

Get in touch with us!

twip optical solutions GmbH
Gottlieb-Daimler-Straße 19
74385 Pleidelsheim

Phone: +49 7144 8970 495

Opening hours:
Mon.-Thurs. 08:30-17:00
Fri. 08:30-14:30
Additional appointments by request

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