7.4 Camera History—Concept 5
7.4.9 The “Ian Camera”
Table 7.4-9: Important parameters of the Ian Camera.
Original sensors ImperX 2M30L
Probe volume (a) 100 mm
Distance to reference plane (L) 551.5 mm
Type of lens Photographic objective (Tokina SL28) Focal length of lens (f) 28 mm
f-number of lens f/16
Aperture triangle side (sij) 220 mm
Angle of lens 0°
Sensitivity coefficient (B¯ij) 11.77 mm Planar resolution (R) 7.23 pixels/mm
Figure 7.4-23: 3D pinhole model of the Ian Camera.
The Ian Camera provided a push in defocusing camera development. Several years had been spent with the Silver Camera, the Black Camera, and the Revelle Camera, and much had been learned.
This camera featured key information engraved into the back plate and two laser diodes used for aiming which would cross more-or-less at the reference plane.
The highest resolution sensors affordable were targeted, but they were physically larger than any other sensor (except the JAI M2-CL, which had the same chip) and image quality looked to be an issue. Up until this time, the lenses were tilted so that the distance between the axis of the lens and the corner of the image was minimized, thus reducing aberrations and light fall-off. Even then, with such large sensors the sensitivity would be limited as even on-axis an achromatic doublet does not provide a satisfactory image at the corners of this chip.
Figure 7.4-24: A photograph of the Ian Camera during assembly.
So 35-mm-format photographic objectives were chosen since they should provide ample coverage when used straight-on for the image size plus the required offset. This camera had by far the largest sensitivity because of this. Its construction was concurrent with the development of multi-plane dewarping, which was essential in the operation of this camera, as discussed in section6.3.
The sensors were taken apart and assembled directly onto the faceplate so that they could be bolted and glued to the same contiguous piece of aluminum in hopes that this would replicate the hysteresis-less Revelle Camera arrangement (in which the chips were bolted and glued to an aluminum frame which was then glued to the faceplate). The assembly was a success, and moreover proved that the Kodak chips have well-aligned pixel planes.
Dealing with photographic objectives proved a challenge at first as if the camera was to hold its calibration (by solving the heat-movement problem) then the lenses had to be fixed somehow.
Originally the lens housings were designed to hold the lenses in space via padded set screws, but that method became questionable during assembly because it would link the lenses to any impact on the outside. The decision was made then to take the lenses completely apart, clean the grease off the focusing mechanism, and glue every moving part together with liquid epoxy.
Once the lenses were cleaned, they were reassembled and mounted to the camera. The camera was turned on and left to warm up. The lenses were focused by checking the magnification—not image sharpness—by using the software to measure the sub-pixel distance between four dots on a target at the reference plane. Once they were focused the glue was injected with a syringe into every accessible thread, and it was left to set with the camera on. The assembly was then tested over several power cycles with single-plane dewarping and showed to hold. Impact tests were conducted
which the camera failed—this included hitting the camera with a hammer (with considerable force) and dropping it from a four inch height onto a wooden surface. It was thought that the aperture diaphragm was moving, so all visible joints on that were glued but the problem persisted6.
The final reference plane engraved on the back of the camera for reference was taken as the distance at which the images of the three sensors best matched—so it is slightly different than the design distance.
This camera produced what is by far the largest, most precise, and most impressive data set of all. In homage to the original propeller experiments it was used to map the flow around a propeller in a water tunnel seeded with bubbles. The resulting data set, a phase-average of 200 pairs every 5 degrees of rotation, contains velocity and bubble population information for a half-rotation of the two-blade propeller with data points every 1 mm in each direction.
This camera was the first of the third-generation cameras. Appendix B of Graff [2007a] is a step-by-step report of the assembly of this camera.
Figure 7.4-25: One phase station of the propeller data set taken with the Ian Camera. The blue blobs are population concentrations of bubbles, clearly showing the tip vortex and a second vortex which seems to come off the trailing end of the propeller hub. The colored tubes are instantaneous streamlines of velocity color-coded by speed.
6In these lenses it was impossible to remove (or glue) the aperture leaves themselves so it is thought that this is the source of the problem.