Wednesday, May 12, 2021

Depth from Chromatic Aberrations

OSA Optics Express publishes KAIST paper "Compact and fast depth sensor based on a liquid lens using chromatic aberration to improve accuracy" by Gyu Suk Jung and Yong Hyub Won.

"Depth from defocus (DFD) obtains depth information using two defocused images, making it possible to obtain a depth map with high resolution equal to that of the RGB image. However, it is difficult to change the focus mechanically in real-time applications, and the depth range is narrow because it is inversely proportional to the depth accuracy. This paper presents a compact DFD system based on a liquid lens that uses chromatic aberration for real-time application and depth accuracy improvement. The electrical focus changing of a liquid lens greatly shortens the image-capturing time, making it suitable for real-time applications as well as helping with compact lens design. Depth accuracy can be improved by dividing the depth range into three channels using chromatic aberration. This work demonstrated the improvement of depth accuracy through theory and simulation and verified it through DFD system design and depth measurement experiments of real 3D objects. Our depth measurement system showed a root mean square error (RMSE) of 0.7 mm to 4.98 mm compared to 2.275 mm to 12.3 mm in the conventional method, for the depth measurement range of 30 cm to 70 cm. Only three lenses are required in the total optical system. The response time of changing focus by the liquid lens is 10 ms, so two defocused images for DFD can be acquired within a single frame period of real-time operations."

4 comments:

  1. I remember seeing proposals for this sort of thing in the mid 2000's, back when EDOF was cool.

    Most lenses have CA minimize. The residual error is usually a weird function, not the simple single-lens CA in the diagram here.

    On the other hand, deliberately designing a lens to have large CA for the purposes of depth perception leads to predictably bad RGB images.

    I'm sure this works fine in the lab and simulations, but runs into a lot of problems outside.

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    Replies
    1. It depends. The original 3-tube prism assembly used in broadcast color cameras was specifically designed to have a total offset of ten microns in the focal planes for the three channels. The lenses lenses were designed to match. The offset provided some benefits in assembly and channel alignment.

      Sensors with stacked photodiodes (Foveon) could also use offests in the focal plane position but the total offset would only be about 3 microns.

      The natural CA offsets in old lenses provided another interesting result. It made it possible to extract some color information from old thick monochrome film emulsions.

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    2. Albert Theuwissen - Harvest ImagingMay 13, 2021 at 3:58 PM

      Hi Dave, very interesting comments. Especially the last one, thanks for sharing. Albert.

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    3. Albert Theuwissen - Harvest ImagingMay 13, 2021 at 3:58 PM

      If I remember well, DxO did interesting work in this field where they used the chromatic aberration to extend the depth of focus.

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