Confocal microscopy

Confocal microscopy is an optical method for obtaining three-dimensional images of surface textures and objects. With a reflected-light method, the roughness of a surface can be accurately measured without contact. The technically similar method of laser scanning fluorescence microscopy is used mainly in microbiology.

Basic technical principle

With a confocal microscope, a point light source is generated with the help of a pinhole aperture. The light returning from the object passes through a second pinhole aperture, or the same aperture a second time, and impinges on the detector, as a rule a video camera. An image of the illuminated object area is formed with a very low depth of field, i.e. the light intensity reduces sharply above and below the focal plane so that only this plane is displayed.

A rotating aperture disk with holes arranged in the form of a spiral (Nipkow disk) is used to detect the whole surface of the object rather than just one point. Modern instruments use a micro-lens disk instead of or in combination with pinhole apertures. This increases the light yield, thus also enabling poorly reflecting or even transparent objects to be displayed. For three-dimensional scanning, the distance of the lens from the object is adjusted in tiny increments, thus building up a spatial image from the sharply displayed planes.

Optical setup of a confocal microscope

Roughness measurement

Confocal microscopy is described in Norm ISO 25718 as a contactless method for measuring roughness. The optical cross sections of the sample can be superimposed upon one another with a resolution of down to 10 nm. This enables the finest roughnesses to be accurately measured, and standard parameters such as the roughness coefficient R’ and the mean roughness Ra to be determined using confocal technology.

Rough cylinder bore surface and height analysis using a confocal microscope

Rough cylinder bore surface and height analysis using a confocal microscope