The MSC Camera.
Depth of Field Enhancement for the Light Microscope.
A Proposal to Digital Camera OEMs.
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The MSC Camera.
The following document outlines a design concept for a multi - CCD digital camera (the MSC camera) which would enable greatly increased depth of focus if applied to the light microscope. It is adapted from a patent application drafted before it was decided to place the design concept in the public domain by publication on this website.
Patent searches have revealed that key elements of the MSC camera's function are contained within patents held by major camera manufacturers, who, so far as is known, have made no move to exploit their intellectual property with respect to the development of a real-time depth-of-focus enhanced camera.
A case is here made to persuade those companies, or other companies, or indeed anyone at all, to develop the MSC camera for the clear advantages it would represent to light microscopists.
In the 160 years since the invention of photography, a camera has consisted of some variant of a light-tight box with a film at one end and a lens at the other. Photographers took the one zone of focus available to them and represented a three dimensional object space by placing their focus at a point somewhere in that space and, depending on the lens aperture chosen, were reconciled to the fact that objects more or less distant from the focused point would be more or less out of focus.
Many fine images have been recorded using such a “single-axis” camera, and the camera techniques in use today are all variants of single-axis imagery.
Confocal microscopy has made the most determined attempt so far to escape the constraints of single-axis imagery, but the resultant representations of the subject, whilst remarkable, are not "real time" images.
If ever it is considered that the amount of information made possible by careful placement of a single zone of focus is insufficient to represent a chosen object space (notoriously the case with wide aperture telephoto lenses and the highest powers of the light microscope), what form could a solution take?
A single-axis camera could be used to acquire separate images of those planes in the object space which were of interest by focusing on each plane in turn and recording the image. The next step would be to remove the out-of-focus areas from each image and replace them where possible with the coresponding sharply focused areas from the other images, resulting in a composite image having more focused information than a normal camera image -- by a factor which increases with the number of images used in its composition.
(The preceding paragraph also summarises the substance of the Ricoh patent of 1990 -- See Part 5; Patents).
The final depth-enhanced image would be the end-product of segmenting and compositing processes well known to workers in confocal microscopy, and to digital special effects workers in the television and motion picture industries. Segmenting and Compositing are the S and C of MSC.
It will be seen that there are two essential requirements of images which are capable of being composited together.
1. The images must all be captured from the same point in space, along the same eyeline -- they must share a common axis. Images shot from different positions cannot be composited.
2. They must all be captured at the same moment in time, otherwise movement in the object space would make it impossible to composit the images.
Since the acquisition of images along the imaging axis by a single-axis camera must necessarily be a sequential process, requirement two above cannot be met by such a camera. In order for a camera making use of multiple focused zones in the object space to function in real time (form compositable images of moving objects), it must therefore be capable of simultaneously sampling the required number of axial images. The MSC camera is designed to enable simultaneous axial image sampling, and it achieves this by a process of optical multiplexing -- the M in MSC.
The Multiplexing - Segmenting - Compositing Camera.
Schematic diagram of a four axis MSC camera.
How it Works.
The MSC Camera’s operation can be visualised as occurring in four stages.
1. The optical hardware part of the camera design makes use of a single lens (or optical system) and an arrangement of beamsplitters (or other devices) to provide simultaneous image information on a number (n) of focused planes in the object space. This is the multiplexing stage.
(Strictly speaking, a multiplexing device produces identical copies of the original input data, which in this case is the three-dimensional image space produced by the prime lens. However, the MSC camera includes means by which n different images of the object space may be simultaneously bought to a focus. These n images have a common centre (share a common axis) and are identical to the images which would result if a single-axis camera were focused in turn upon the n selected planes in the object space).
2. The n focused images generated by the multiplexing stage are each projected onto a CCD array (or similar device), then input in digital form to the computer/software component of the camera, whose first function is to discriminate between the focused (sharp) and unfocused (unsharp) areas of the n images. This is the segmenting stage.
3. The second function of the computer/software is to combine the focused parts of the n images into a single image (still image or video frame) having enhanced depth of field characteristics. This is the compositing stage.
4. Finally, the composited image is output to the camera viewfinder, control monitor or storage devices. Alternatively or additionally, the image data from the MSC camera is used to generate a pair of quasi-stereoscopic image stacks which may be viewed and manipulated in an appropriate viewer or sent to storage devices.
In the following detailed description, the optical hardware and the computer/software components will be dealt with in two separate sections.