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14.1 Objectives
We apply the knowledge gained from the preceding sections by analyzing six different objectives for the MWIR spectrum with the same focal length (50 mm), the same fâ# (1.5), and the same square field coverage (4 degÃ4 deg). We start with the simplest form of a lens and reach (in five steps) an objective that is frequently found in thermal imaging systems.
The six configurations are:
1. plano-convex singlet
2. best shaped singlet
3. aspheric singlet
4. aspheric hybrid
5. Petzval objective with one aspheric element
6. Petzval objective with one aspheric hybrid
The plot in Fig. 14.1 is based on using square sensor elements (pixels) that contain 80% of the received energy from a point source at infinity. It is clear that the simple singlets are not suitable for IR cameras, but are sufficient for radiometers (pyrometers) if the target is large enough to resolve the projected sensor element. The single aspheric hybrid with a 70-μm pixel size over a linear field of view of 3.5 mm in the horizontal and vertical directions may begin to get interesting for thermal imaging. The total number of pixels amounts to (3.5â0.07) 2 =2500, and the angular resolution is 0.07â50=1.4mmrad. At the end of the list is the frequently employed Petzval objective, which was discussed in detail in Sec. 7.4. With a 20-μm pixel size, the focal plane array contains more than 30,000 sensing elements. The angular resolution is 0.4 mrad.
Remarks
1. For the off-axis calculations, the field was increased by the factor 2 √ to cover the pixels located at the extreme of the field's diagonal.
2. All lenses are made from silicon with the exception of the aspheric hybrid, which is made from GASIR-1, a material that lends itself to molding. Its refractive index is 2.5116 at 4 μm, and its Abbe number in the MWIR region is 197.3.
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