The figure depicts a proposed wide-angle lens that would be especially well suited for an electronic camera in which the focal plane is occupied by an image sensor that has small pixels. The design of the lens is intended to satisfy requirements for compactness, high image quality, and reasonably low cost, while addressing issues peculiar to the operation of small-pixel image sensors. Hence, this design is expected to enable the development of a new generation of compact, high-performance electronic cameras. The lens example shown has a 60° field of view and a relative aperture (f-number) of 3.2.

This Optical Layout shows the main features of the lens design. The total length of the lens from aperture stop to the image plane is less than two times its focal length.
The main issues affecting the design are the following:

  • The response of a small-pixel image sensor is sensitive to the angle of incidence of the light. At large angles of incidence, the response includes excessive crosstalk among pixels.
  • When a lens of typical prior design images a wide field, rays from the edge of the field are typically incident on the image sensor at large angles. This effect can be mitigated by use of a so-called image-space telecentric lens, for which the angle of incidence is constant. However, such a lens is typically much larger than is a comparable non-telecentric lens.
  • In the original intended application, in which the lens would be used to focus light on a back-side-illuminated image sensor, there are requirements to minimize the size of the lens while making its optical behavior nearly telecentric, to obtain nearly diffraction-limited image quality while limiting distortion. The following are some key characteristics of the lens design:
  • The lens would include an element that would function like an immersion lens. The image sensor would be mounted in direct contact with this element. The incorporation of this element would enable maximization of the degree of telecentricity by bending rays from the edge of the field proportionately more than those from the middle, while otherwise exerting little effect on performance.
  • A first doublet element, comprising two subelements made of glasses characterized by a large difference between their indices of refraction, would be placed immediately after an aperture stop. This doublet would control the field curvature and the color correction.
  • A second doublet element made from two glasses that have similar, high indices of refraction but very different dispersion values. This element would control the chromatic correction and provide most of the positive lens power necessary for imaging.
  • An “air lens” between a third doublet element and a meniscus element would be used to balance the positive power while affording some correction for aberrations.
  • The aperture stop would be located at the front of the lens.
  • All of the lens elements and subelements are designed to have spherical surfaces and to be made of commonly used glasses. Hence, the lens could likely be produced at lower cost than would be possible if aspherical shapes or unusual glasses were required.

This work was done by Pantazis Mouroulis and Edward Blazejewski of Caltech for NASA’s Jet Propulsion Laboratory. NPO-44404

Topics:
Imaging and visualization Optics Photonics Physical Sciences Sensors and actuators
This Brief includes a Technical Support Package (TSP).
Document cover
Miniature Wide-Angle Lens for Small- Pixel Electronic Camera

(reference NPO-44404) is currently available for download from the TSP library.

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    Photonics Tech Briefs Magazine

    This article first appeared in the May, 2009 issue of Photonics Tech Briefs Magazine (Vol. 33 No. 5).

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    Overview

    The document discusses NASA's Technical Support Package for the Miniature Wide-Angle Lens designed for small-pixel electronic cameras, identified as NPO-44404. This lens addresses a significant challenge faced by small-pixel CMOS sensors, which are sensitive to the angle of incidence of light. At large angles, excessive crosstalk can occur, leading to degraded image quality. Traditional wide-angle lenses often exacerbate this issue, as rays from the edges of the field strike the sensor at steep angles.

    To mitigate these problems, the document outlines a novel lens design that aims to produce a nearly telecentric image while minimizing the lens size. Telecentric lenses maintain a constant angle of incidence, which is crucial for high-quality imaging in small-pixel sensors. However, conventional telecentric lenses are typically larger than the sensors they serve, which poses a design challenge.

    The innovative solution presented includes several key design elements:

    1. Immersion Lens Element: The imaging chip is mounted directly onto an immersion lens, which enhances telecentricity by bending rays from the edge of the field more than those from the center, thus reducing the angle of incidence.

    2. Negative Doublet: Positioned immediately after the stop, this component consists of two glasses with a significant difference in refractive index. It helps control field curvature and color correction.

    3. Positive Doublet: This element, made from two similar high-index glasses with different dispersion, is placed after the negative doublet. It primarily manages chromatic correction and provides the necessary positive power for imaging.

    4. Air Lens: Formed between the surfaces of a doublet and a meniscus element, this component balances positive power while allowing for aberration correction.

    The design allows for the lens to be fabricated using all spherical surfaces and preferred glass types, which helps reduce manufacturing costs. The result is a compact lens that achieves high-quality, near-telecentric imaging without exceeding the diameter of the sensor array.

    Overall, this lens design represents a significant advancement in optical engineering, providing a solution that meets the stringent requirements of small-pixel CMOS sensors while maintaining a small form factor. The document emphasizes the potential applications of this technology beyond aerospace, highlighting its relevance in various technological and commercial fields.