Hardware Overview

With the development of multiple camera technology, comes the ability of image based rendering techniques to capture images of scenes from multiple viewpoints. While this novel method was implemented, camera hardware and polariscope sys- tem technology is also advancing. For semiconductor based techniques, the cost and power are constantly decreasing. The advent of𝐶𝑀𝑂𝑆 provides image sen- sors that are inexpensive and easy to use, because of their interface. Moreover, the processing power of a 𝐶𝑀𝑂𝑆 sensor can be added to the sensor itself for future advanced work.

four-lens camera

In the work described by this thesis, we describe a four-lens array based on one image sensor, and a 𝐶𝑀𝑂𝑆 image sensor array, called the ”Full Angle Synchro- nised Phase-Shifting Polariscope Camera”. The device is designed to record a synchronised phase shifting at different angle conditions with much visibility and control over the cameras. The four-lens system uses a high speed𝐶𝑀𝑂𝑆 camera based on the 𝑅𝐽45 interface with a 𝐺𝑃 𝐼𝑂 connector. The multi-camera array system is a modular embedded device based on the Ethernet 𝑅𝐽45 high speed

𝐺𝑖𝑔𝐸 bus, which is faster than the 𝐼𝐸𝐸𝐸1394𝑏 and 𝑈𝑆𝐵2.0 techniques.

𝐶𝑀𝑂𝑆 image sensors normally have a worse noise performance than𝐶𝐶𝐷sensor, but the former is suitable for this experiment for many reasons. 𝐶𝑀𝑂𝑆 sensors do not require uncommon voltage supplies or clock signals, or extra circuitry, com- pared with 𝐶𝐶𝐷 sensors. Furthermore, they are easy to use and control because

𝐶𝑀𝑂𝑆 sensors contain some functions of digital processing and interfaces within. Furthermore, characteristics such as exposure times and auto-exposure algorithms, gamma correction, white balance, color gain, etc., can be easily configured in the processing of combined data from a multi-camera.

Camera Configuration

It is a standard circular polariscope, except for two groups of four quarter-wave plates and analysers, which are mounted with the lens in front of sensors in the camera.

x y

Polarise Filter

Light Source

1st Quarter Wave Plate

Specimen

2nd Quarter Wave Plate Group

Group of Analyser

Multi-lenses

Camera

z

Specimen

Figure 3.38 Schematic Diagram of the Synchronised Phase Shifting Polariscope System for Dynamic Real-Time Event.

The operation of the system is as follows. A collimated light beam passes through the polariser and quarter-wave plate, as we discussed in previous chapters and cal- culations, so that the circularly polarised light beam can be achieved. It transmits a birefringent specimen, then captured by the image sensor. When it arrives at the 4-lens camera, the optical energy of the beam is split into four paths along the beam axis in the same direction. The different configurable quarter-wave plates and analysers are inserted with each lens respectively to generate a phase shifting image. The phase shifting image is captured by a single 𝐶𝑀𝑂𝑆 sensor. The instrument set up is shown in the Fig. 3.39.

Apart from its simplicity, this system has several advantages. Firstly, the instru- ment can be directly integrated with the configurations of the quarter-wave plates and analysers. The size and position of images can be adjusted easily, and the phase changes of each image can be adjusted separately. Another advantage of the design allows synchronised triggering for the image data, which is important for dynamic range analysis.

Lens Frame

Quarter Wave Plate Group Group of Analyser

(transparent water clear filter) (transparent gray filter)

Group of Quarter Wave Plate and Analyser are Mounted in front of Image Sensor with Multi-lens

Replaced Normal Lens

Image Sensor

Figure 3.39 Synchronised Phase Shifting Polariscope System Camera.

Experiment

The demonstration of the proposed system is assessed using a ring model and ”U” shape model, which is shown in Fig. 3.40. The forces are applied from the top and

F F F F T ens ion C om pr es si on Neutral Axis

Figure 3.40 Schematic of specimens under vertical compression (a) Ring shape model

(b) 𝑈 shape model.

bottom at the same time, both on the ring shape model and on the ”U” shape model. The ring model gives a simple demonstration which is very similar to that of the classical round disk. The ”U” shape model shows that the stress distribution in

the area of the corner is subject to combined compression and bending and it serves as a good evaluation of this system since, for beam bending, the stresses on either side of the neutral axis have opposite signs. The configuration of the polariscope system is the same setting as in the previous four-step algorithm. The light source is monochromatic 𝐿𝐸𝐷, of which the wavelength is 530nm. A 𝐶𝑀𝑂𝑆 sensor camera which has 1024_×1024 pixels is used for capturing the image. The four phase shifting images are captured as shown in Fig. 3.41.

Figure 3.41 Sample under vertical compression images (a) Ring shape model (b) ”U_”

shape model.

With the calibration work, the whole (four-sum) images can be separated into four individual images of the same size, with no relative shifting between them. The calibaration images are shown below.

The four sub-images are cropped into four images separately, and the same pixels in each of the four separated sub-images represent the same point in the specimen. The reference is defined as the first position of the specimen at zero degrees. The complicated pattern distribution on the ”U” shape sample causes the fringe on the surface to disturb the calibration work, which can be seen in Fig.3.43, and so the images are first calibrated using the sample without compression.

corrected top left quadrant corrected top right quadrant

corrected bottom left quadrant corrected bottom right quadrant

100 200 300 400 500 100 200 300 400 500 100 200 300 400 500 100 200 300 400 500 100 200 300 400 500 100 200 300 400 500

difference between (ref) and (I) difference between (ref) and (II)

difference between (ref) and (III)

Figure 3.42 Ring shape sub-images (a) corrected image (b)difference between the first angle as reference.

images which captured from other three lens are compared with it respectively using edge detection techniques. This calibration determines the position of the camera and the sample, and to get a exactly fitting, as shown in Fig.3.44. Then the four images with the force load can be used for analysis. The ring shape sample is limited by the size and resolution of the camera; the fringe pattern has no more disturbance during calibration, therefore we can obtain the results as shown in Fig. 3.42.

The wrapped map and unwrapped map are shown in Fig. 3.45.

The contour map and surface map are shown in Fig.3.46 and 3.47.

The ambiguity in the isoclinic pattern and its effect on the isochromatic pattern can be seen in the figure. The discontinuities, which can be observed in the isoclinic map are a common problem with the phase shifting technique. These are caused by the non-deterministic isoclinic angle at points where the four light intensities are all equal to zero. It follows that discontinuities can generally be alleviated through interpolation if necessary. With the corrected isoclinic pattern, the isochromatic can be properly ordered as shown in Fig.3.48 which can be suitably unwrapped to give the fringe order distribution. Fig.3.49 and Fig.3.50 show the contour and surf map of the ”𝑈” shape specimen.

corrected top left quadrant corrected top right quadrant

corrected bottom left quadrant corrected bottom right quadrant

difference between (ref) and (I)

100 200 300 400 500 100 200 300 400 500

difference between (ref) and (II)

100 200 300 400 500 100 200 300 400 500

difference between (ref) and (III)

100 200 300 400 500 100 200 300 400 500

Figure 3.43 ”𝑈” shape sub-images (a) corrected image (b)difference between the first

angle as reference.

The contour map and surface map are shown in Fig.3.46.

It can be seen from the results from the”𝑈”shape specimen analysis, except for the noise and sample damaged part, that there is a phase jump occurring at the lines shown in Fig. 3.51. The fringe orders along a line 𝐴𝐴′

are also shown. The combined bending and compression effect is obvious from the different maximum fringe orders as is the shift of the neutral axis from the centroidal axis of the beam.