IR Temperature Sensor FAQ

IR Temperature Sensor FAQ

Why Infrared (IR) Temperature Sensing?

Infrared Temperature Sensing Technology enables non-contact temperature measurements. Other methods of temperature measurement require the temperature transducer to make physical contact with the object of interest.

How does it work?

All objects above zero kelvin (-273.15C, -459F) radiate energy proportional to their temperature. This relationship is defined by the Stefan-Boltzmann Law. An IR Temperature Sensor, or pyrometer, absorbs the radiated energy in order to measure it. The TMP00x devices contain a thermopile which absorbs this energy and reports a corresponding voltage in the Sensor Voltage Register 0x00. Additional information is needed to convert this voltage to a temperature. See “What is Cold Junction Compensation?” for more information.

What is Cold Junction Compensation?

Thermocouple operation is described by the Seebeck effect. Two metals are chosen such that a voltage proportional to temperature is developed at their junction. One such junction, known as the hot junction, is used as the temperature sensor. There will be another, unavoidable, junction at the point where the thermocouple wiring connects to the measuring circuit. This junction is known as the cold junction. The voltage developed at the hot junction is dependent on the temperature of the cold junction, so it is necessary to measure the temperature of this cold junction by another means. In TMP00x devices, the hot junction and cold junction are integrated on a single MEMS device. The temperature of the cold junction is measured using an on-board semiconductor temperature sensor, and reported as Tdie Local Temperature Register 0x01.

What is a Thermopile?

A thermopile is several thermocouples connected in series.

What is Emissivity?

Emissivity describes the thermal radiation efficiently of a material. A material with an ideal emissivity equal to 1 will radiate 100% of the expected infrared radiation for its temperature. A material with an emissivity of 0.5 will radiate only 50%, and will appear to be much colder than it actually is. Polished metals have the lowest emissivity (typically below 0.1) while dark colored materials tend to have an emissivity above 0.9.

Is TMP00x a Multi-Chip-Module (MCM)?

No, TMP00x is a single chip MEMS device. The thermopile is constructed in the center, and the supporting electronics (ADC, local temp sensor, digital logic) are implemented in the area surrounding it.

How far away (distance) can TMP00x sense temperature?

TMP00x is primarily deployed in applications with distances less than six inches. The maximum distance is dependent on system design. In an experiment using a crude Fresnel lens with the Thru-Hole Layout, we observed a thermopile response to a human body six feet away.

What is the accuracy?

The Object Temperature accuracy of TMP00x devices is specified as +/-1°C typical and +/-3°C max. This is given for a Die Temperature between 20 and 40°C, and an Object Temperature between 20 and 60°C. This specification is based on data collected in a reference system, with the TMP00x used in the Top View/No Shield Layout, and may not reflect performance in a given application.

Is calibration necessary?

The relationship between radiated energy received and actual object temperature varies based on several system parameters.

  • Emissivity describes how efficiently an object radiates thermal energy.
  • Field-Of-View (FOV) is used to describe the area that the sensor can see.
  • Distance is important, because there is decay in signal through atmosphere.
  • Material and Thickness affects the transmittance of any covers or lenses.

A one-time calibration is necessary to establish the coefficients needed for a given system design. The default coefficients will give a reasonable Object Temperature result when evaluated near room temperature in most system designs. Once the system is stimulated, the default coefficients often return an obviously wrong Object Temperature.

How to calibrate?

A reference object temperature is needed for calibration. The TMP007EasyCalEVM includes a TMP112 Local Temperature Sensor for this purpose. The reference temperature sensor must make thermal contact with the object being measured by the TMP00x. Then, data collection can begin at a rate of one sample per second. The following information is required for each sample:

  • Sensor Voltage in microvolts (uV) from TMP00x Register 0x00
  • Local Temperature in Celsius (°C) from TMP00x Register 0x01
  • Reference Temperature in Celsius, such as TMP112 Register 0x00

During logging, the system must be exercised to capture a temperature change. This can mean different things for different applications. Data should be collected and evaluated for the full temperature range expected in the application.

The data will be analyzed to select coefficients. These coefficients must then be programmed into TMP007, or used in the calculation performed by the host in the case of TMP006.

Collected data can be analyzed by the TMP007EasyCalEVM software even if a TMP006 is used or if the EVM hardware is not available. Data must be formatted as Comma Separated Values (CSV) text file with one sample per line break. Using the Analysis tab in the EasyCal software, choose Load Data File, and then Run Calibration to obtain coefficients on the right side of the screen. See the TMP007EasyCal User’s Guide (SBOU149) for more information.

Alternatively, see the TMP007 Calibration Guide (SBOU142) for information on manual methods of obtaining coefficients.

Can I improve performance with calibration?

It is necessary to perform a one-time calibration of the system design in order to meet datasheet specifications. The accuracy can be improved beyond datasheet specifications by calibrating each system in production to remove device-to-device variations.

Why is PCB layout important?

Since IR Temperature Sensors rely on Cold Junction Compensation, it is critical that the device does not experience radical temperature changes. The PCB Layout recommended by TI literature is intended to perform the following.

  • Provide Additional Thermal Mass to slow down thermal changes
  • Thermally Isolate TMP00x and the added thermal mass from other heat sources on the PCB

It is possible to reduce, redesign, or otherwise change the parameters of the recommended layout while maintaining good thermal performance. Other techniques, such as the use of a shield, offer greater advantages than layout alone. A system design which fully encapsulates the sensor with an external housing may not benefit from PCB layout techniques at all.

What’s the deal with the copper fill underneath the device?

The copper area shown in TI Literature helps to thermally stabilize the device. It is a critical feature of the recommended layout. This copper does not serve an electrical purpose, and should not be covered with solder paste. In fact, solder paste applied in this location has the potential to damage the thermopile and/or prevent proper operation of the device. We have elected to not cover this area with solder masking in our EVM PCB, but we have built other PCBs with masking that have acceptable performance.  Since our EVM PCB is gold plated, this copper fill has a very low emissivity and uniform finished thickness. The low emissivity will provide the least amount of undesirable (offset) radiation to the sensor. If a high quality finish plating is not available, then solder masking is recommended.

What is the Through-Hole Layout?

The construction of the TMP00x devices results in a thermopile that is closer to the PCB than it is to the top of the device. For this reason, radiated energy must travel through silicon substrate to reach the thermopile. The Through-Hole Layout is a new approach to PCB layout. A via is placed beneath the TMP00x device that directs energy through the PCB and directly onto the thermopile without passing through silicon substrate first. This layout offers a 2x improvement in signal, and allows the system designer a convenient way to control FOV. This layout requires that a cover or shield be placed over the device to block unwanted energy. For more information, see TMP006 and TMP007: Through-Hole Mounting Method (SBOA164.)

Where can I buy metal shields?

Our partner, Wurth Elektronik, has released their version of the metal shield to DigiKey. There are four variations available. The version without a hole, which is listed as solid on DigiKey, will block infrared energy from the top of the device, and is intended for use with the Through-Hole Layout. The other three versions, which are listed as vented on Digikey, are intended to reduce the Field-of-View in top view applications. The variations are as follows.

3690103020 – no hole

3690103021 – 0.99mm

3690103022 – 0.76mm

3690103023 – 0.51mm

http://www.digikey.com/product-search/en?keywords=369010302

I want to cover or protect the sensor. What window materials can I use?

The TMP00x devices are sensitive to medium wavelength infrared energy in the 4-16um range. Many materials that appear clear do not allow transmission of energy at this wavelength. A material that passes near wavelength infrared, such as the 940nm produced by an IR LED, may not be suitable for sensing temperature. Materials intended for infrared imaging are suitable. A low-cost material that we have evaluated is a kind of polypropylene sold by Edmund Optics.

Low Cost Window Material from Edmund Optics

Infrared Optics at Edmund Optics

 

Can I improve performance or distance with a lens?

Yes, the range and performance of TMP00x can be improved by a lens. Unfortunately, the cost of such lenses does not make sense for most customers. Please see offerings from Edmund Optics, ThorLabs, and others in the optics business.

Infrared Optics at Edmund Optics

 

How to solder TMP007?

Please see the following application report for information on WCSP devices like TMP007 and TMP006.

AN-1112 DSBGA Wafer Level Chip Scale Package

 

What EVM (Evaluation Modules) are available?

TMP007EasyCalEVM

TMP007 Evaluation Module for Infrared Thermopile Sensor with Integrated Math Engine

TMP006 Evaluation Module

Sensor Hub BoosterPack

Educational BoosterPack MKII

Sensors BoosterPack Plug-In Module

SimpleLink™ Bluetooth low energy/Multi-standard SensorTag

TMP007 Evaluation Module for Infrared Thermopile Sensor with Integrated Math Engine

Where can I learn more (literature)?

http://www.ti.com/product/TMP007/technicaldocuments

http://www.ti.com/product/TMP006/technicaldocuments

TMP006 and TMP007 Layout and Assembly Guide

TMP007EasyCal User's Guide

TMP006 and TMP007: Through-Hole Mounting Method

TMP007 Calibration Guide

https://en.wikipedia.org/wiki/Thermocouple

https://en.wikipedia.org/wiki/Thermoelectric_effect

https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

https://en.wikipedia.org/wiki/Pyrometer

https://en.wikipedia.org/wiki/Emissivity

http://www.thermoworks.com/learning/emissivity_table

Additional Reference (Images)