Effective focal length

As only moderate resolution is needed (because of the lock and key effect), focal lengths of 0.25-0.50 m and rulings of 600-3000 lines mm-i are commonly employed. Resolution in the region 0.2-0.02 nm are claimed. 

Another interesting application of the microfluidics and the optics is the use of variable-focus liquid lenses, which was developed by Philips Research Eindhoven [15]. The lenses which are composed of two immiscible liquids of different refractive indices can be manipulated by the eleclrowetting. By electrowetting, the meniscus curvature of the lenses can be changed and so does the effective focal length of the lens. 

FIGURE 6 A telephoto lens, with a long effective focal length, and a reverse telephoto lens, with a long working distance. 

It is possible to shorten the actual length of the optical transform whilst still keeping the effective focal length that is desired by including further lenses in a combination lens. One technique is to combine a positive lens with a negative lens to make a two lens composite. This gives a length compression of around /q/5 [28], which in the example above is still 2 m and impractical. Furthermore, the two lenses combine in aberrations, which leads to poor correlations due to poor optical quality. A second system is to use a... 

Figure 6.3. Typical parabola, with focal length /. For the off-axis section shown on this figure, the effective focal length is F.

Figure 6.4. Two off-axis parabolas, A and B, with effective focal lengths, Fa and Fb, respectively. The ratio of the diameter of the foci, dJdB, is equal to FpJF. ...

To reduce this light loss, the diameter of the beam must be reduced without loss of etendue. The way that this was accomplished before about 1990 was to mount a beam condenser in the sample compartment. Typical optics for a 6x beam condenser are shown in Figure 14.1. In this device, the effective focal length (EFL) of the mirrors located immediately before and after the sample is about 5 cm, whereas the EFL of the mirror that focuses the beam in the sample compartment of the spectrometer is about 30 cm. The ratio of these two EFLs gives the factor by which the diameter of the beam is reduced at the focus of the beam condenser (see Figure 6.4). For this example, the beam diameter is reduced by a factor of 6, and the area is reduced by a factor of 36. Thus, if the diameter of the beam in the sample... 

Effective focal length (EFL) Product of the aperture diameter and the focal ratio of the converging light beam at the focal position. For a single optic, the effective focal length and the focal length are the same. 

---