Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography

The overall goal of this method is to use structured heating and highly resolved thermal imaging in a non-destructive and non-contact manner to locate subsurface defects perpendicularly oriented to a steel sample surface. This method can help answer key questions in the thermal imaging field. For instance, how small and how deep a defect can be in order to be detected.

The main advantage of this technique is that we can generate thermal wave fields that propagate in the plane of observation, making the approach highly sensitive to perpendicularly oriented defects. This laser-projected photothermal thermography system is arranged on a bench top breadboard. This system has undergone most of the preparatory steps required for use in an experiment.

At the head of the beam path is the laser source. This laser fiber is supported by a laser fiber mount. Next, a telescope reduces the beam diameter of the laser to an appropriate size for later in the beam line.

Behind the beam sampler, a 500 watt power meter head absorbs much of the beam energy to allow the laser to operate at full power. From beam sampler, the beam continues via a mirror to a projector development kit. This is a disassembled commercial projector with its light engine and lenses removed.

For the experiment, collimate the beam to enter the projector. After passing through the projector, the beam will encounter the sample which will be mounted on a computer control translation stage. To complete this setup, obtain a 100 millimeter focal length lens for the projector.

Attach the lens to the projector objective just before the translation stage. Next, use an LED flashlight as an input light source to the projector. Position a white sheet of paper in front of the objective and move it until there is a sharp illuminated rectangle on the sheet indicating the position of the image-plane.

At this point, obtain a sample for use in the experiment. Mount the sample in the beam path on the linear translation stage equipped with a lab jack. Raise the sample with the lab jack so its top is in line with the top of the projected rectangle.

Ensure that a defect is within the illuminated area in the image-plane. Next, arrange to do infrared photography by first obtaining a gold mirror on a post. The mirror will reflect the scattered beam to the camera.

Mount the mirror on a post holder near the projector. It should reflect the upper edge of the sample and be angled to see as much of the sample surface as possible. Light reflected from the mirror will enter an infrared camera that is mounted on a tripod.

Position it at the height of the projector objective so that it sees the projected white image via the gold mirror. Set up the camera to be controlled by computer, and let it warm up. After connecting the camera to its control software, obtain a steel ruler.

Hold the ruler at the surface of the sample and manually focus the camera on it. The temperature contrast to the steel ruler aids in focusing. Work to achieve the sharpest image.

One of the most critical steps is to achieve sufficient lateral resolution at the sample surface. This is important because the line of depletion must be resolved. Use the laser software to set the laser voltage to 10 volts and start the laser.

Work with the camera software to the relationship between the projector and the camera. Select Measure from the options along the top. Go to the Measure areas toolbar and choose the cross tool option.

When the laser is on there will be a thermal image. Use the tool to mark the corners of the image by left clicking on the frame and then note the coordinates. The camera control software must be configured for the experiment.

Begin by switching to the Camera panel. There, click the Remote button to open the remote control panel. There in the drop down menu, choose the option Process-IO.

Also, go on to click on the Sync In option and the Gate option. After this close the menu. From the Acquisitions parameters tab, open the Acquisition menu.

Choose External Sync from the drop down menu. Provide file and folder names in the Folder field. Then, move to the Count field and enter the previously computed number of frames and close the Acquisition menu.

Start camera data acquisition by choosing Record. At this point, move to the experiment control software. Click on Activate to active the motion controller.

Next, edit the Start and End Positions in millimeters to include the defect in the scan. After that enter the speed in millimeters per second. Click on Start Measurement.

Left click on the Choose Area Color field. In the color dialogue, select a color for the pattern area. Go to the drawing toolbar and choose the rectangled tool.

Move to the image area and use the tool to create a rectangle consistent with the previously found projector pixel domain. Continue by clicking define Area. The dialogue box allows setting the projected pattern properties.

In the Signal Type drop down menu choose Sine Wave. To define the sine wave, set the Phase Shift field to zero degrees. In addition set the Frequency in hertz.

Set the Amplitude to the maximum. Next, go to the Voltage field to enter the laser voltage in units of volts. In the field, Pictures per period field, enter a previously calculated value.

Click on Next. Follow analogous steps to create a second rectangle of a different color in phase shift of 180 degrees. Preview the sequence of images using them in a preview slider.

Then push Start to begin the experiment. The translation stage slowly moves the sample through the chosen range to expose different regions to the projected oscillating structured illumination. The total transit time for this experiment is 200 seconds.

As the sample moves, the thermal infrared camera acquires thermal images at 40 hertz. This sequence thermal images provides an example of the thermal wave fields generated by the illumination. Stop the experiment when all the frames have been acquired.

To perform necessary post-processing, load the data frames in the post-processing software. After the data is converted, insert the previously found projection point coordinates. Click Transform to put the data into the projector pixel domain.

To extract temperature information, define the depletion line by entering the coordinates for two points. Input the parameters for the speed in the start position of the sample during the experiment. Also enter the infrared camera's FrameRate and sine wave Frequency of the pattern.

Finally, ensure the data post-processing parameters are correct. When ready, click Evaluate. The crack position is shown in the highlighted field.

These data were collected from a test sample with a defect at an approximate depth of 1/4 of a millimeter. The sample was translated at 0.05 millimeters per second. The black curve represents the temperature as a function of time, which is along the top horizontal axis.

Time can also be translated to a position which is along the bottom axis. The solid red curve is a fit to not-oscillatory increase in temperature. The dashed red line indicates the position of the defect.

Here is the same data after additional post-processing. The blue curve is the Hilbert curve and the defect is at its minimum. These data were collected after doubling the scan speed to 0.1 millimeters per second.

In comparison with the first measurement, the elongation is the same but the oscillation frequency is reduced. Note that the sample was moved to a new position which is reflected in the measurements When the protocol is used with a defect one millimeter below the surface, its location can still be determined but with greater uncertainty. Both of these plots use data collected with a scan speed of 0.1 millimeters per second.

After its development, the technique paved the way for researchers in the field of nondestructive testing to explore the usage of structured illumination. Following this procedure, other and more complex illumination patterns can be used in order to find other defect types. Thus far only steel has been tested, but the method is very promising, especially for plastic, compound materials, and other very sensitive materials due to the low thermal stress that is applied.

The bottleneck of the current experimental setup is the thermal stress limit of the spatial light modulator. That's why we have to pay attention to the measurement time, which should be not more than two to three minutes. Up to now only two integral heat sources have been generated.

But in principle, using this setup it is possible to generate and control up to one million heat sources, which opens up another field of arbitrary normal wave shaping. After watching this video, you should have a good understanding of how to locate subsurface defects using laser projected, photothermal thermography. Don't forget that working with a class four high-power infrared laser can be extremely dangerous and that precautions such as wearing laser protection goggles should always be taken.