The MSC Camera.
Depth of Field Enhancement for the Light Microscope.
A Proposal to Digital Camera OEMs.
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Potential Applications of the MSC Camera.
The High Power Light Microscope.
The HPLM is the optical instrument most in need of depth of field enhancement. Light microscopists have bemoaned their instrument's lack of depth perception since its invention, but have always accepted the fact as an inescapable property of the optical system.
Of all optical instruments it is the one which works at the highest aperture, but is the most demanding of intense illumination, even for visual use. The reason for this is the high magnifications achieved. The most highly corrected oil-immersion objective will have an N.A. of 1.4, which is equivalent to a camera lens aperture of f 0.36. This objective, with its associated eyepiece, can produce an image of over 1000 magnifications, which when projected onto 35mm film has a photographic aperture of around f 100.
It is also the instrument in which the resolution loss that must be traded for depth of focus is least acceptable. The depth of focus of a 1.4 NA oil immersion objective is about 0.2 micrometers. To increase this to 0.4 micrometers, still very little, the resolution will be halved -- giving the equivalent performance of a x40 power objective at 0.65 NA., and negating the use of a high quality OI objective in the first place. Aperture reduction in the HPLM is therefore not an option.
The optical qualities which make the HPLM problematical for single-axis photomicrography are the very qualities which are an advantage to the MSC camera. The optical sectioning effect associated with the extremely small depth of focus would, with brightfield illumination, provide the MSC software with an almost ideally clean separation of image planes in the object space. This would provide the optimum conditions for both the compositing of a single DoF enhanced image and for the generation of quasi-stereoscopic image stacks.
Given this, there will be a light reduction at each CCD of the MSC camera in direct proportion to the number of image axes generated; so in the case of the HPLM, and for any given CCD sensitivity, an increased light demand must be accepted as the necessary sacrifice for a gain in depth of focus.
Depth perception in the HPLM is not simply a matter of increased depth of focus. Even a tenfold increase in depth of focus would still necessitate constant use of the fine focus adjustment in order to build up an impression of depth over time -- the practice adopted by microscopists in all fields.
True depth perception in microscopy is only found in the stereo binocular microscopes, which are restricted to the lower powers. Some stereo depth perception in the HPLM has been possible in the past by a process of bisecting the illuminating aperture of the instrument into left and right halves, and by various means, delivering the image forming rays from the right half of the aperture to the right eye, and those from the left half to the left eye. In the original device, this was achieved by the use of crossed polarizing filters in the substage and in each eyepiece, and today synchronously switched liquid crystal shutters in substage and eyepieces achieve the same end.
This method achieves a true stereo effect at the expense of halving the horizontal resolution of the instrument.
An MSC camera, set for contiguous focus, can be used to generate a quasi-stereoscopic effect by taking the vertical stack of images obtained and using software to offset the images according to some predetermined perspective geometry into a left stack and a right stack for presentation to the left and right eyes by one of the existing methods of viewing stereoscopic images. This would give a quasi-stereoscopic image of enhanced DoF at the full resolution of the microscope.
If this approach were combined with the above aperture bisection method, the resulting stereo image stacks would have halved horizontal resolution, but would be genuinely stereoscopic, with the option for software controlled image offset to augment or otherwise modify the effect.
The MSC camera would then be offering a user-controllable 3D depth perception for the first time in the 400 year history of the microscope.
Long Focal Length Telephoto Lenses.
This another area in which the MSC camera could be used to advantage. The rapid action of TV sports coverage of varous kinds would especially benefit from the autofocus speed gains mentioned under Design Possibilities (see below).
The depth-enhanced composite image made possible by the MSC camera would also be an advantage in surveillance situations involving the use of extremely long focal length lenses.
In the region of 1:1 imaging, common in biological, medical and natural history photography/video, it would be possible, using a nine-axis MSC camera, to devote say seven axes to ensuring complete focus on a small insect, whilst the remaining two axes could be used to present the middle background and distant background in whatever degree of focus the operator felt was appropriate to the subject.
Other MSC Design Possibilities.
One of the problems encountered in using autofocus on a single-axis camera is that the camera has no way of knowing in which direction the image has gone out of focus. The lens must focus back and forth to establish this, then overshoot slightly before returning to the point of sharpest focus. A good deal of the time spent in achieving focus can be wasted in this hunting process.
If autofocus is used in conjunction with an MSC camera, there is sufficient information available from the multiple focused zones in the object space for the computer to know the direction in which the focus has shifted. It is then possible for the software, by comparing the sharpness of consecutive zones, to eliminate hunting for direction, and make an interpolation enabling the lens to move directly to the optimum focus (or very close to it).
Even without going to a fully-fledged MSC camera, these gains in autofocus speed could be obtained by multiplexing to provide another two (or even one) image axes, thus providing the autofocus computer with the necessary image data to eliminate hunting.
A Dynamic Focus Buffer.
The contiguous image stacks generated by an MSC camera can be seen as a kind of "focus buffer" from which the relevant images can be extracted later if the image data is stored at the time of shooting.
The use of stored image data from the MSC camera in this way would relieve the autofocus mechanism of the responsibility to maintain optimum focus at all times, as long as the focus zone of interest was kept within the contiguous focus zone of the MSC camera.
Storage of Raw Image Data for Later Processing.
Present computer processor speeds may not allow some of the more demanding of the MSC software operations to be carried out in real time. In this case, some means of storing the image data from the multiple imaging axes of the camera must be provided which records and preserves the relationships between the images on one axis and the images on the other axes representing the same moment in time. This will allow the opportunity for various forms of image selection, enhancement and analysis in post-production which could not be performed at the time of shooting.
Such storage methods might include timebase locked computer hard drives or digital tape drives.