Thermalyze Operation

Window

Description

An emissivity table is a two-dimensional array of emissivity values that are applied to individual pixels in a thermal image.  Emissivity tables are used to correct thermal images of surfaces with non-uniform emissivity.  Circuit boards and semiconductor devices, for example, are often comprised of a variety of materials including metals, plastics, ceramics, and semiconductors, each having different emissivity.  The errors in measured temperature due to variations in emissivity can be significantly reduced by properly creating and applying emissivity tables.

How Emissivity Tables are Created

Emissivity tables are created by stabilizing a device at a known temperature that is at least 25°C higher than ambient background temperature.  This temperature difference is needed in order to create a difference between surface emitted energy and ambient reflected energy.  After inputting the device Control Temperature and ambient Background Temperature, Thermalyze can then calculate the emissivity at each pixel location.

Single and Double Temperature Tables

Thermalyze can calculate emissivity tables by measuring objects at a single control temperature or at two different control temperatures.  The accuracy of single temperature measurements depends on correctly setting the Background Temperature and can be affected by the spatial non-uniformity of background ambient temperature.  For these reasons, single temperature measurements are typically employed when ambient temperature is known and uniform (i.e., there are no distinct hot objects in the background whose emissions can reach the device).  Double temperature emissivity calculations, however, are independent of the Background Temperature setting and can be used when ambient temperature is unknown and/or non-uniform.

Device Heating Methods

Prior to calculating emissivity tables, devices must be heated to a stable and measurable temperature.  Furthermore, the device must be exposed to the ambient background emissions.  Given these constraints, one of the most effective methods of heating a device is by placing it on a heated plate.  Resistive element, thermoelectric, or fluid-heated plates can be used as long as their temperature can be precisely controlled.

Device Temperature Measurement

Do not assume that the temperature of the device and heated plate are equal unless the device is very thin or is attached to a heat sink which can be placed flat on the heated plate.  Alternatively, you can attach a contact temperature probe directly to the device (for example, a thermocouple, thermistor, or RTD) to measure the object’s temperature.  Make sure, however, that the probe is small enough so that it does not conduct heat rapidly away from the device.  Thermal pads or grease can be used to improve thermal conductance when mounting the device on the heated plate and when attaching a temperature probe to the device.

The Emissivity Tables window (see Figure 1) is used to create single and double-temperature emissivity tables.  To open the Emissivity Tables window, click the Emissivity Tables item under the Emissivity menu or press the  button in the Emissivity section of the Shortcuts toolbar.

Figure 1: Emissivity Tables window

File

Open Table	Select the binary emissivity table file (.oemi) to open.
Save Table	Emissivity table files are saved in binary format with a ".oemi" extension and preserve emissivity resolution of 0.001. When saving an emissivity table file, the file name that you provide is appended with ".oemi" and saved in the "Optotherm\Thermalyze\Emissivity Tables" folder unless you specify a different folder.
Export Window	The Emissivity Table window can be saved to file in the following formats: bmp, jpg, png, and tif.  Exported Emissivity Table windows are saved in the "Optotherm\Thermalyze\Export" folder unless you specify a different folder.
Print	Print the Emissivity Table window. Note: You must have a printer connected to your computer.
Print with Preview	Select this menu item to open the Print Preview dialog before printing.

Active Table

Choose the emissivity table to display in the Emissivity Tables window and to apply to the Main Image.  The active table is applied to the Main Image only when the Emissivity Tables window is open.  If no regions exist, the active table is applied to the entire Main Image.  If one or more regions exist, the active table is only applied to pixels that are enclosed by Regions.  Applying emissivity table correction to the entire Main Image consumes significant computer resources and therefore image capture rate may be reduced.  To maximize capture rate while applying emissivity tables, draw one or more Regions that enclose only the areas on the device that need to be analyzed.

Create Table

Control Temperature	Enter the temperature of the unpowered and stabilized device.  Tip: Larger differences between the control temperature and ambient temperature will improve the accuracy of the calculated emissivity table values.
Threshold	Enter the minimum emissivity value that can be added to the table.  Calculated values below this threshold will be ignored and set equal to 1.
Create Single 1	Click this button to create the Single Point 1 table.  The control temperature used when calculating the table is displayed to the right of the button.
Create Single 2	Press this button to create the Single Point 2 table.
Calculate Double	Press this button to calculate the Double Point table.  Tip: Before calculating the Double Point table, make sure that the control temperature for the Single Point 2 table is at least 10°C higher than the control temperature for the Single Point 1 table.
Ambient Correction	Click this button to correct for changes in ambient background characteristics that may have changed since creating the emissivity tables.
Active	Check this box to enable ambient correction.

Cursor Info

Emissivity	This field displays the emissivity value at the cursor location.
Location	This field displays the location of the cursor in pixel coordinates.

Region Tools

You can draw one Region of any shape or size on the emissivity table in order to view maximum, minimum, and mean emissivity of the pixels enclosed by the region.  To help in locating corresponding areas on the Main Image and emissivity table, the Region can be copied to/from the Main Image.

Select	Select this button to position, resize, and rotate the Region.
Point	Select this button and then click on the table to create a Point Region.
Line	Select this button and then click and drag on the table to create a Line Region.
Rectangle	Select this button and then click and drag on the table to create a Rectangle Region.
Ellipse	Select this button and then click and drag on the table to create an Ellipse Region.
Polyline	Select this button and then click on the table to locate the first vertex of the Polyline Region.  Click to locate each vertex and then right-click on the last vertex to complete the polyline.
Polygon	Select this button and then click on the table to locate the first vertex of the Polygon Region.  Click to locate each vertex and then right-click on the last vertex to complete the polygon.
Invert Region Color	The default color of an unselected Region is white.  If the Region is difficult to distinguish from lite background colors, press this button to set the unselected Region color to black.
Delete Region	Click this button to delete the Region.
Max, Min, Mean	These fields display emissivity statistics within the Region or the entire table if no Region exists.
Send	Click this button to copy the Region to the Main Image.
Get	Click this button to copy the Region on the Main Image to the Emissivity Tables window. Note: If there are multiple Regions on the Main Image, the first Region is copied.

Create

Region Setup

1. Draw a Region around the area of interest on the device in the Main Image and set the Region's Emissivity setting to 1.00.

2. Set Image Averaging equal to a minimum of 32 images to produce an image with low temporal noise.

Create Single Point 1 Table

1. Control the temperature of the unpowered device to a known temperature that is at least 20°C higher than the Background Temperature and allow the device temperature to stabilize.

2. Measure the temperature of the device and set Control Temperature to the measured value.

3. Open the Emissivity Settings window and set Background Temperature to the current ambient temperature or check the Use Camera Temp box if a microscopic lens is being used.

4. Click the Create Single 1 button.

Create Single Point 2 Table

Single Point 2 table should be created only if you intent to create another single point table for more accurate measurements at higher device temperatures or if you intend to create the double point table.

1. Control the temperature of the unpowered device to a known temperature that is at least 20°C higher than the control temperature used to create the Single Point 1 table and allow the device temperature to stabilize.

2. Set the Enable Image Averaging button to  to disable image averaging.

3. Using the micrometer handles on the xy positioning stage, adjust the position of the device to align its image with the Single Point 1 table.

4. Set the Enable Image Averaging button to  to enable image averaging.

5. Measure the temperature of the device and set Control Temp to the measured value.

6. If the ambient temperature has changed since creating the Single Point 1 table and the Use Camera Temp box is not checked, set Background Temperature to the current ambient temperature.

7. Click the Create Single 2 button.

Calculate Double Point Table

The double point table is created using the image data collected when the Single Point 1 and 2 tables were created.  No additional information or data is needed to create the Double Point table.  After the Single Point 1 and 2 tables have been created, simply press the Calculate Double button.  For best results, the difference between the control temperatures for the Single Point 1 and 2 tables should be at least 20°C.

1. Create Single Point 1 and 2 tables as described above.

2. Click the Calculate Double button.

Ambient Correction Prior to Device Testing

After an emissivity table has been created, there may be a period of time before it is used.  In that time, ambient background characteristics may change including room temperature and the location and temperature of any heat sources such as light fixtures, windows, and heating vents.  These changes will reduce the accuracy of emissivity table correction.  Ambient correction can improve accuracy by compensating for changes in ambient conditions.  Ambient correction is typically performed immediately prior to testing a powered device.

1. Control the temperature of the unpowered device to the desired test temperature that is at least 20°C higher than the Background Temperature and allow the device temperature to stabilize.

2. Set the Enable Image Averaging button to  to disable image averaging.

3. Using the micrometer handles on the xy positioning stage, adjust the position of the device to align its image with the active emissivity table.

4. Set the Enable Image Averaging button to  to enable image averaging.

5. Measure the temperature of the device and set Control Temp to the measured value.

6. Click the Ambient Correction button.

7. Check the Active box to enable ambient correction.

8. Power the device.

Apply

Single or Double Point Correction

For most applications, single point emissivity tables provide acceptable compensation for emissivity variations.  Although double point tables have the ability to correct for non-uniform background emission, their use can lead to larger measurement errors than single point tables if background emission changes during the measurement process.  Also, the requirement to control devices at two difference temperatures when creating double point tables can lead to device movement caused by thermal expansion.  Furthermore, the elapsed time between changing device temperature when creating double point tables increases the possibility for device movement and background emission changes.

Device Control Temperature

When creating an emissivity table, more accurate pixel emissivity values can be calculated with larger difference between the device and ambient Background Temperature.  For most applications, it is suggested to control the unpowered device to a temperature that is at least 30°C higher than the Background Temperature.

Using Image Sequences

To obtain more accurate results when using emissivity tables, it is helpful to minimize the time between creating an emissivity table and applying it to thermal images.  This will result in minimum device movement and background emission changes.  Image Sequences can be used to record thermal images of an unpower device and as it is being powered.  The images recorded of the unpowered device at the beginning of the sequence can then be used to create a single point table.  The emissivity table is then applied to the images of the powered device that were recorded later in the sequence.

The following procedure describes using an image sequence to create and apply an emissivity table.

Region Setup

1. Draw a Region around the area of interest on the device and set the Region's Emissivity setting to 1.00.

2. Set Image Averaging equal to at least 32 images to produce an image with low temporal noise.

Record Sequence

1. Control the temperature of the unpowered device to at least 20°C higher than the Background Temperature and allow the device temperature to stabilize.

2. Measure the temperature of the device and set Control Temp to the measured value.

3. Begin recording an image sequence.

4. Power the device.

5. Stop sequence recording when device power is removed.

6. Stop capturing images.

Create Emissivity Table

1. Locate and display the sequence image immediately prior to any temperature increases due to device powering.

2. Set Background Temperature to the temperature displayed in the Background Temp field at the bottom of the Sequence panel.

3. Click the Create Single 1 button.

Apply Emissivity Table

1. Locate and display the sequence images recorded after power was applied.

2. To toggle the emissivity table off and on, toggle between the Active Table None and Single Point 1 options.

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