Scantron profilometer

The (S-Type) Chromatic Technique

Scantron Proscan 1000

Surface digitisation using chromatic aberration technique.

Light from a white light source is reflected by a beam splitter and transmitted along a fibre optic cable to a measurement head. The measurement head focuses the white light through a lens with spectral aberration. This lens causes the different wavelengths of light to be focused at different positions in the measurement range. As the light falls onto the object to be measured only one wavelength is in focus for a given position on the object. The light returning from the object passes through the spectral aberration lens and back along the fibre optic cable. This time it passes through the beam splitter and is projected onto an optical pin hole. This has the effect of only allowing the wavelength in focus to pass through. To detect the wavelength of receiving light a spectrometer grating deflects the incoming light onto a CCD sensor (the different wavelengths are deflected by different amounts). So from the position of the image on the CCD sensor and the wavelength of the light, the position of the object can be measured.




Chosing the right sensor for an application

The capability of the sensor to function for a particular application is dependant on the following points and consideration should be given when making a sensor selection:
  • Measuring Range. The interval between the lower and upper measuring limits of the sensor and the maximum deviation on a surface that the sensor will measure.
  • Stand off. The distance set between the sensor and the middle of the measuring range. This distance should allow enough space so as to avoid any physical aspects that sit higher than the area to be measured.
  • Resolution. The smallest quantity that can be measured. The sensor selected needs to have a resolution high enough to measure the smallest feature.
  • Linearity. The actual error of the sensor over the entire measuring range.
  • Spot Size. The diameter of the circle formed by the cross section of the field of view of an optical instrument at a given distance. The size of the spot has a direct relation to the lateral features being measured. If for example the surface has a very fine level of surface roughness, a sensor with a very small spot size would need to be used. As a general rule, features smaller than the specified spot size can be measured due to the technology employed in the sensors.

Sensors available with our instrument

Sensor ModelResolution (μm)Linearity
(+/-% of range)
Measuring Range (mm)Stand off(mm)Spot size (μm) nominalTechnology
S5/03 0.01 0.1 0.3 5 4 Chromatic
S38/3 0.1 0.1 3.0 38 10 Chromatic