The magnification obtained by an infinity corrected objective in an imaging microscope is the ratio between the focal length of the Tube Lens (TL) and the Objective. The most common Tube Length focal length used by most vendors is F = 200mm. The optical resolution delivered by the optical column is related to the numerical aperture (NA) of the objective. The Airyz Spot size is
1) FAiry = 0.61l/NA
Equation (1) shows clearly that a high NA Objective will yield a smaller diffraction limited spot. Additionally, knowing the focal length of the Objective and the NA allows computation of the Objective Exit Pupil Diameter:
2) FEx. Pup. = 2NA*F
Table 1 shows that for low magnifications (values between x1 to x10) the Objective pupil diameter is between 10.0 to 11.2mm.
Omek Optics has introduced a Tube Lens with 80mm focal length instead of 200mm which is the industry standard. The Omek Mic401ET.80 provides a means to image at low magnifications yet high resolutions due to the use of higher NA Objectives. The resolution is increased by a factor of 2.5 which is the 200/80 TL ratio.
z) Airy, G. B., “On the Diffraction of an Object-glass with Circular Aperture,” Transactions of the Cambridge Philosophical Society, Vol. 5, 1835, p. 283-291.
Figure 1 is a summary Tables 1 & 2 showing the advantage of the Short Tube Lens.
The main advantage of using the 80mm tube lens is the possibility to grab an area larger by a factor of 4 to 6 as compared to standard microscopes delivering the same resolution. Applications which take advantage of this gain are PCB Inspection, Semiconductor Defect Inspection and all Inspection Microscopes.
Figure 2 shows the superior resolution and image quality of a wafer image obtained with the Mic401E.80 Omek Microscope (right image) as compared to the industry standard solution (left image). Notice that the “Omek” image is also much brighter due to the higher NA employed.