Large field of view, high numerical aperture compound objective lens with two pairs of identical elements and near IR spectrometer containing two such compound lenses
An apparatus consisting of two pairs of identical lenses which is suitable as a compound objective lens for high numerical aperture imaging is disclosed. The compound lens also has a wide field of view as a fraction of the compound lens focal length. By suitable choice of lens materials, well corrected near infrared imaging can be achieved. When two such compound lenses together with a diffraction grating are assembled into a spectrometer, excellent wavelength resolution in the near infrared can also be obtained.
1. A compound lens comprising:
exactly four lenses, wherein a shape of the first lens is essentially identical to a shape of the fourth lens, and a shape of the second lens is essentially identical to a shape of the third lens;
wherein the compound lens has a numerical aperture greater than 0.25;
wherein the compound lens has a field of view of at least 25% of the compound lens focal length; and
wherein an angular frequency at which a modulation transfer function exceeds 0.5 is at least 0.3 cycles per milliradian.
2. The compound lens of claim 1 , wherein the first and fourth lenses are polystyrene and the second and third lenses are acrylic.
3. The compound lens of claim 1 , wherein one surface of the second lens and one surface of the third lens are nominally planar.
4. The compound lens of claim 1 , wherein all non-planar surfaces are aspherical.
5. The compound lens of claim 1 , wherein the first lens and fourth lens are meniscus lenses.
6. A spectrometer comprising:
a first compound lens comprising exactly four lenses, wherein a shape of the first lens is essentially identical to a shape of the fourth lens, and a shape of the second lens is essentially identical to a shape of the third lens;
a diffraction grating positioned to receive light from an optical input that passes through the first compound lens; and
a second compound lens comprising exactly four lenses, wherein a shape of the first lens is essentially identical to a shape of the fourth lens, and a shape of the second lens is essentially identical to a shape of the third lens, the second compound lens positioned to receive light diffracted from the diffraction grating,
wherein the first and second compound lenses have numerical apertures greater than 0.25;
wherein the first and second compound lenses have field of views of at least 25% of the compound lens focal length; and
wherein the first and second compound lenses comprise a measure of resolution defined by a product of a minimum spatial frequency at which a modulation transfer function exceeds 0.5 and the respective compound lens focal length exceeds 400, for everywhere in the respective compound lens field of view.
7. The spectrometer of claim 6 , wherein the first and second compound lenses are essentially identical.
8. The spectrometer of claim 6 , wherein the optical input has a wavelength of 800-1000 nm.
9. The spectrometer of claim 6 , wherein the diffraction grating is a volume phase hologram.
10. The spectrometer of claim 6 , where the diffraction grating has a frequency of greater than 1000 mm −11 .
11. The spectrometer of claim 6 , wherein the first and fourth lenses of each of the first and second compound lenses are polystyrene, and the second and third lenses of each of the first and second compound lenses are acrylic.
12. The spectrometer of claim 6 , wherein one surface of the second lens and one surface of the third lens of each of the first and second compound lenses are nominally planar.
13. The spectrometer of claim 6 , wherein all non-planar surfaces are aspherical.
14. The spectrometer of claim 6 , wherein the first lens and fourth lens of each of the first and second compound lenses are meniscus lenses.