A novel optoelectronic setup based on a quasi confocal, z-axis extended field, proprietary design has been developed for High Resolution Non Contact 3D Surface Metrology including roughness characterization and surface flaw detection. The z-axis field extension is obtained by stretching the axial chromatism generally encountered in a classical confocal dioptric setup when working with a polychromatic point source. One can then get a continuum of monochromatic diffraction limited images corresponding to the extent of the spectral composition of the light source. It then appears that the very specific optical sectioning property of the classical monochromatic confocal setup - which basically can be considered as being a "single point" viewing system (figure 1) - is perfectly preserved for each wavelength and consequently the chromatic confocal setup turns to be a "single (axial) segment" viewing system with a univocal color coding (figure 2).

Furthermore, the chromatic confocal setup exhibits the unique property of Perfect Focus Depth of Field (Figure 3), since at any given point of the chromatic axial field of view there is only one wavelength perfectly focused, all the other wavelengths being inactive.

This appears to be a particularly interesting method for overcoming the basic trade-off of lateral resolution against depth of field which in fact is a basic severe limitation of optical microscopy, since, for obtaining a large depth of field (an acceptable defocusing) one has to choose a microscope objective with a low NA and consequently to accept to have a larger spot size onto the scanned object. On the opposite the novel chromatic confocal setup can be implemented with NA's as high as 0.75 and chromatic PERFECT FOCUS Depth of Field of several tens of microns. Practically, the chromatic confocal setup definitely avoids the z-axis scanning of the now classical Confocal Scanning Optical Microscope. It also avoids the time consuming computer reconstruction of the object image since at any given point of the scanned field the entire chromatic depth of field is "seen" at the same time. From the 3D Surface Metrology point of view, the z-axis chromatic confocal setup can be considered as generating a highly resolved spectral coding capable of providing a highly accurate height information when combined with an adequate spectral decoding performed after the pinhole spatial filter (figure 4).

As a consequence of the spatio-chromatic filtering performed by the chromatic confocal setup, only a quasi monochromatic light beam comes to focus onto the filtering pinhole which acts also as the entrance port of a spectrometer (figure 4). The central wavelength of this monochromatic light beam corresponds to the exact height of the measured object point. By scanning the object surface in the (x, y) direction, one gets the microtopographic structure of any type of surface, rough as well as polished ones, for any types of material, glossy or mat, transparent or opaque. The chromatic confocal setup is a very powerful mean for obtaining simultaneously the Extended Depth of Field Confocal Scanning Optical Microscopy representation and the 3D surface topography of the scanned area. When considering (partially) transparent layers the chromatic confocal setup has the very specific property of being able to perform simultaneously dual surface roughness and mapping (figure 5).

Absolute 3D Surface Mapping (figure 6) can be performed when designing a specific sensor comprising a parallel glass plate with its base surface located inside the chromatic depth of field. The glass plate is firmly fixed to the object in order to have an absolute reference in situ during the electromechanical scanning thus avoiding any error which could be due to roll, pitch and yaw.

The innovative setup of chromatic confocal imaging opens new ways for simultaneous extended field confocal scanning optical microscopy and 3D Surface Metrology with an absolute reference surface in situ.