Our previous research has found that the main defects in digital cameras are “Hot Pixels” which increase at a nearly constant temporal rate. Defect rates have been shown to grow as a power law of the pixel size and ISO, potentially causing hundreds to thousands of defects per year in cameras with <2 micron pixels, thus making image correction crucial. This paper discusses a novel correction method that uses a weighted combination of two terms - traditional interpolation and hot pixel parameters correction. The weights are based on defect severity, ISO, exposure time and complexity of the image. For the hot pixel parameters component, we have studied the behavior of hot pixels under illumination and have created a new correction model that takes this behavior into account. We show that for an image with a slowly changing background, the classic interpolation performs well. However, for more complex scenes, the correction improves when a weighted combination of both components is used. To test our algorithm’s accuracy, we devised a novel laboratory experimental method for extracting the true value of the pixel that currently experiences a hot pixel defect. This method involves a simple translation of the imager based on the pixel size and other optical distances.
Optical imaging through biological tissue has the significant problems of scattering which degrades the image resolution
and quality. Research has shown that Angular Domain Imaging (ADI) improves image quality by filtering out the
scattered light in the biological tissue images based on the angular direction of photons. The advantage of this technique
is that it is independent of the wavelength, coherent, pulse, or duration compared to OCT or time domain. This allows us
to couple ADI with conventional fluorescence imaging technique. Previous work was creating test media by varying
Intralipid/water concentration to produce different scattering levels. This showed difficulties in producing a consistent
scattering medium in liquid states. Hence, ideally we want a reusable solid medium which has a stable scattering
characteristic. Our target is to investigate fluorescence ADI on skin with cancerous collagen tissue where healthy
collagen fluoresces while the cancerous collagen tissue does not. To mimic the characteristic of skin, a solid scattering
medium over a patterned fluorescence material with non-emitting structures is created. We used a solid agar medium, or
a transparent polymer, infused with Intralipid at different concentrations, as the scattering medium. The solid media with
similar scattering characteristic of skin (μs = 20cm<sup>-1</sup>, g = 0.85) is placed on top of a fluorescence plastic (415nm
excitation, ≈ 530nm emission) which is patterned by strips of non-emitting structures (200-400μm). Using small
apertures with acceptance angles of 0.171° a distance away from the solid scattering medium, these non-emitting
structures are detectable at shallow scattering tissue depth (1-2mm).
Image sensors are continuously subject to the development of in-field permanent defects in the form of hot pixels.
Based on measurements of defect rates in 23 DSLRs, 4 point and shoot cameras, and 11 cell phone cameras, we show in
this paper that the rate of these defects depends on the technology (APS or CCD) and on design parameters the like of
imager area, pixel size, and gain (ISO). Increasing the image sensitivity (ISO) (from 400 up to 25,600 ISO range) causes
the defects to be more noticeable, with some going into saturation, and at the same time increases the defect rate.
Partially stuck hot pixels, which have an offset independent of exposure time, make up more than 40% of the defects
and are particularly affected by ISO changes. Comparing different sensor sizes has shown that if the pixel size is nearly
constant, the defect rate scales with sensor area. Plotting imager defect/year/sq mm with different pixel sizes (from 7.5
to 1.5 microns) and fitting the result shows that defect rates grow rapidly as pixel size shrinks, with an empirical power
law of the pixel size to the -2.5. These defect rate trends result in interesting tradeoffs in imager design.