Figure 9.2-2: Light fall-off in aperture R of the Emilio Camera. The difference between the darkest pixel and the lightest is about 140 (of a possible 255).
Figure 9.2-3: Light fall-off in aperture G of the Emilio Camera. The difference between the darkest pixel and the lightest is about 170 (of a possible 255).
Figure 9.2-4: CAD model of a sensor shifting setup to test for light fall-off and verify output fromECG designer. The lens is mounted on a plate with the appropriate mount which is placed at the correct register distance with respect to the sensor which sits on a linear stage mounted perpendicular to the axis of the lens.
Table 9.2-2: Basic effects of camera parameters.
When increased... When decreased...
“L” Increases mappable region size, decreases sensitivity and resolution, and decreases sensor offset; requires longer f or smaller sensor size to maintain mappable region and largersij to maintain sensitivity.
Decreases mappable region size, increases sensitivity and resolution, and increases sensor offset; requires shorter f or larger sensor size to maintain mappable region and shortersij to control sensor offset.
“sij” Decreases mappable region depth, in- creases sensitivity, and increases sensor offset and light fall-off.
Increases mappable region depth, de- creases sensitivity, and decreases sensor offset and light fall-off.
“h”,
“w”
Increases mappable region size and in- creases maximum sensor diagonal and light fall-off; requires longer f to main- tain mappable region; probably decreases framerate.
Decreases mappable region size and de- creases maximum sensor diagonal and light fall-off; requires shorter f to main- tain mappable region; probably increases framerate.
“pmm” Increases resolution and decreases f- number needed for diffraction blurring.
Decreases resolution and increases f- number needed for diffraction blurring.
“f” Decreases mappable region size, increases sensitivity and resolution; requires longer L to maintain mappable region.
Increases mappable region size, decreases sensitivity and resolution, increases [bar- rel] distortion; requires shorterLto main- tain mappable region.
9.3.2 The Lens
Aside from having the desired focal length, the lens construction has to be considered. Any minute movement of a lens or its components will render a multi-plane dewarping set useless. As rigid as a focusing mechanism may seem to the touch, there will be several pixels of play. So the lens should be locked in place as rigidly as possible. In the modern cameras this is done by cleaning the focusing threads off the lenses (which are chosen purposefully to be of the manual-focus variety) and the assembly is glued with liquid epoxy once the lenses are focused during assembly. Although it could possibly be easier to simply remove the lens unit from the barrel and build a custom holder, this has the advantage that it is easy to focus the lenses during assembly.
Manual focus lenses have the advantage that the assembly is relatively simple and the lens moves as a unit while focusing. Autofocus lenses, on the other hand, can have quite complex focusing movements. The Micro-Nikkor 105mm autofocus lens has a complex, four-layer system of curved tracks for focusing in which one lens group stays stationary (relative to the camera), the next moves in two different directions through the focusing track, and the remaining two move in the same direction but at different rates. Fixing such a lens would be impossible, unless careful measurements of the inter-group distances were taken and a custom, rigid barrel was made. The probability of this being successful is close to zero, as the only hope of doing so would require the entire mechanism to
be modeled in a CAD program since there is no access to the elements themselves while the lens is still in a state where the focusing mechanism is functional.
Figure 9.3-1: The Micro-Nikkor 105mm autofocus lens with two of the four layers of curved tracks that comprise its focusing mechanism exposed.
9.3.3 The Sensor
To remove the hysteresis of the heat movement of the sensors there are two alternatives: the Revelle Camera approach or the Ian Camera approach.
The Revelle Camera method may be used, whereby the chips are mounted to custom-made aluminum frames. The faceplate is assembled with lenses in place and the sensors are aligned to the faceplate by external stages, verifying the alignment by continuously imaging a target. The faceplate should have “receptacles” in which the frames sit with a gap between the frames and the walls of the receptacles. Once the sensors are in place the gap is filled with epoxy.
Although the alignment of the sensor is overkill, especially with the relatively well-aligned as- sembly of the Kodak chips3, by mounting the chips to a small frame the warm-up time is reduced
3At least the KAI 2001’s in the Ian Camera have pixel planes well aligned to the package.
if the epoxy is not a good heat conductor. The Revelle Camera has a massive faceplate but only 4 minutes elapse before the movement in the sensors has stabilized.
The Ian Camera approach entails duplicating the part onto which the chips are mounted in the original sensors into the back of the faceplate and then mounting the chips directly to the faceplate.
A bit of epoxy was used in the corners of the chip for safety in the Ian Camera but this was omitted in the Emilio Camera. The direct-mounting approach has the added advantage that no external stages are necessary, but since the entire faceplate is now the heat sink of the sensors, warm-up times can be considerably longer. The warm-up time on the Ian Camera is conservatively estimated at 30 minutes.
Many sensors today include heat spreaders in their design—usually a simple plate that extends along the back of the chip and bolts to the sensor body somehow. Some sensor manufacturers use this as the structural mount for the package, that is, the package is bolted to the heat spreader which is then bolted to the sensor faceplate. In the modern defocusing cameras the opposite approach is used: the package is bolted to the faceplate and the heat spreader is only connected to the package via thermal transfer grease (no rigid mechanical link). It has been proposed to machine the heat spreader as part of the faceplate (all one single piece) and mount the sensor to this thin strip faceplate, however, it is not clear what thermo-mechanical effects this would have.
Placement of the protective glass on chips can be quite arbitrary and this reduces further the available front contact area; of the three KAI 2001’s in the Ian Camera, one had the glass well over 10° off horizontal. The specifications for the KAI 40XX, on the other hand, call for much more precise glass placement (because the glass is bigger relative to the package). The faceplate must be designed with these tolerances in mind unless the intention is to remove the glass altogether or reposition it with better alignment.
It should be emphasized here that the image orientation on the sensor should be checked prior to design. In the case of the KAI-40XX, the image is upright when the long end of the package is vertical, that is, the chip is “sideways”. This is indicated in the full specification of the chip but not in the mechanical drawings, where the chip is shown with its long side on the horizontal like the drawing for all other Kodak chips.
Tolerances on mechanical aspects of the ceramic package can be surprisingly loose, especially for the dowel holes meant for alignment during assembly. These must be treated carefully in the design of the camera. One of the alignment holes is usually a slot so concentricity is not a big problem, but diameter can be. The alignment pins should be manufactured separately andnot “built into” the faceplate (as was done with the Ian Camera) or made to be “press fit” so they can be replaced in the case that they do not fit a specific chip. In the case of the Emilio Camera, the dowels that were made are exact replicas of the ones made for ImperX.
It is best to design the camera such that the sensor body (minus the front, which is replaced by
the faceplate) can be fully assembled and fully encloses the sensors so that even without the back part of the camera assembly the sensors remain independently housed units.
9.3.4 Optical Channel
In professional photography cameras, glare from light that is off the image can cause a loss of contrast in the images. Many manufacturers adopt devices such as lens hoods or baffles in the lens-film space to reduce or completely block stray light. In custom machining these may be expensive.
Both the Ian Camera and the Emilio Camera have simple cavities aft of the lenses. The aluminum faceplate is anodized black but no other precautions are taken. The Emilio Camera was bead-blasted before the anodization so that a more matte finish was achieved, but whereas the Ian Camera has no glare problems, the Emilio Camera has slight spots of glare when the probe volume is illuminated laterally (as is done normally). Simple masks could be made to mount in the lens cover window that simply block all light except that in the field-of-view cone of each sensor, but these would be tedious to align and the glare spots are small enough to be of minimal concern.
The glare spots can be seen as hazy spikes of brightness, clearly visible in figures9.2-2and9.2-3.
Chapter 10
Software
10.1 Introduction
Completing the system is a custom software suite written to process raw images into three-dimensional particle and vector fields.
The suite can be divided into two parts: calibration software, comprised of gridfind and dewarpC, and the processing software, calledDDPIV, written by Francisco Pereira and Emilio Graff.
Extensive detail of the operation of the processing software necessary to operate a DDPIV camera can be found in partIIofGraff and Pereira[2007].