Today we conclude our three part series, “Sensors for Temperature Measurement and Their Application”. We’ll be talking about the last three sensor types: Infrared or Radiation, Fluorescent Detector and Liquid Crystal. To catch up on the series, you can click to Part 1 of our series or click over to Part 2 of our series.
4. Infrared Thermography
Infrared thermography works on the basis of the IR waves emitted from a heated surface. The infrared system captures the waves, and based on internal calibration converts them into temperature. The following are required for IR-based measurements:
- Infrared imaging system — The market offers a broad range, but a worthy system starts at around $30-70k. For IR microscopy (down to 5mm only-lower limit of IR wave-length), the system starts at $180k.
- Signal processing equipment.
- Knowledge of emissivity if the test specimen must be coated with a known emissivity material.
Figures 5a and 5b show a typical IR image of heat-emitting surfaces:
5a shows and IR image of a PCB in forced convection air flow
5b shows and IR image of a PCB in forced natural convection air flow
The following points are noteworthy when using an IR camera for temperature measurement:
- Application accuracy is a function of emissivity.
- The measurement situation must duplicate the actual environment, as far as air velocity, temperature and air flow distribution are concerned.
- The IR camera is sensitive to reflected radiation.
- Carbon dioxide and water vapor absorb significant energy and may cause significant error.
- In an electronics application, surfaces typically have different emissivities thus, one must make the emissivity uniform before measurement of known emissivity (black paint or powder).
- In most IR equipment, the temperature readout is the average over an area. Therefore, temperature peaks may be ignored as the result of integration. To remedy the situation, better IR optics must be used to reduce the area where integration occurs.
5. Optical Probes
Optical sensors are light-emitting devices that illuminate the test body with source radiation, and can detect reflected radiation, or simulated radiation such as fluorescence. Although not broadly used, optical probes are used at the die or component level. Figure 6 shows one such a probe.
Figure 6. Optical Probe For Surface Temperature Measurement The Probe Either Touches The Surface Or Captures The Reflected light From A Fluorescent Treated Surface.
6. Liquid Crystal Thermography
LC thermography works on the basis of visible light reflected from a surface treated with the LC material. The system captures the reflected wavelengths, and based on internal calibration, converts them into temperatures. Liquid crystals (LCs) are cholesteric materials. When applied to a heated surface they realign and reflect light at a different wavelength. The reflected light shows the standard colors seen in a rainbow. Figure 7 shows the application of LCs on an IC.
Figure 7. Color display of liquid crystal applied on an IC. Blue reveals the circuit’s hottest point, and black shows that the temperature is outside the range of the crystal material .
The following are salient features of LC material:
- Liquid crystals are organic compounds that can be poured like a liquid, yet reflect light like a crystal.
- Changes in LC optical properties can be produced by externally applied fields (e.g., electrical, magnetic, and thermal).
- Cholesteric liquid crystals progressively exhibit all colors of the visible spectrum when heated over their event temperature range.
- Width and placement of the event temperature range can be controlled by selecting and mixing the appropriate liquid crystals.
- Liquid crystals are commercially available with event temperatures ranging from below 0oC to 160oC, with spans ranging from 1 to 50oC.
An LC thermography system such as the one shown in Figure 8 can provide a very effective temperature-mapping system.
Figure 8: The thermVIEW System for Macro and Microscopic (down to 1um) Surface Temperature Measurement
Figure 9 shows a typical result of LC thermography at die level:
Figure 9. Temperature distribution across a memory chip (5 x 5 mm) at Tambient = 25oC, as shown using Liquid Crystal Thermography .
Like any other system for temperature measurement, LC thermography offers distinct advantages and disadvantages. One salient advantage of LC thermography is that its not dependent on surface emissivity. A second is that at micron and submicron levels, though not a trivial task, LC thermography allows easier and less costly temperature measurements while enabling 1Âµm or smaller spatial resolution. One disadvantage of LC thermography is that it is not a pick-up-and-measure system like IR. One must apply the calibrated liquid crystal material to a surface in order to perform the measurement. But, this is similar to the IR system, in that you need to make the surface emissivity uniform if the measured surface has multi-emissivity (e.g., a die or PCB).
In this article, we reviewed six different probes/techniques for temperature measurement. It would be a missed opportunity not to include a word on calibration. Irrespective of the types of measurement and sensors one uses, calibration is of utmost importance. Pay special attention to the calibration process and ensure that the sensors are properly calibrated. Further, one needs to ensure that the chosen sensor is suitable for the type of measurement. As Prof. Frank White states in his Viscous Flow book, Bad data is worse than no data at all .
To catch up on the series or read from the start, you can click to Part 1 of our series or click over to Part 2 of our series.
- Klinger, D., Nakada, Y., Menendez, M., AT&T Reliability Manual, Van Nostrand Reinhold, 1990.
- Azar, K., Thermal Measurement in Electronics Cooling, CRC Press, 1997.
- Advanced Thermal Solutions, Inc., Tutorial Series, Principles of Temperature Measurement.
- thermVIEW System, product of Advanced Thermal Solutions, Inc.
- White, F., Viscous Fluid Flow, McGraw-Hill, 3rd Ed., 2005.
If you are in need of sensors for thermal measurement, click now to ATS’ sensor family. To learn more about ATS ‘ thermVIEW liquid crystal thermal analysis system, click to thermVIEW System. You can also email or call us with your questions on temperature measurement and one of our engineers will be happy to help you. Email us at firstname.lastname@example.org or call us at 781-769-2800.