Non-Contact Infrared Temperature Measurement

High Temperature IR-Imager with Wide Dynamic Range for Industrial Process Control 

Abstract

State of the art IR-Imager in the near infrared spectral range for monitoring high temperatures in industrial applications are characterized by a number of small measurement ranges. Scenes with a high temperature contrast require several measures switching between these range and result in pictures with under range and saturated parts.

A newly developed high temperature IR-Imager with a spectral range in the near infrared provides for a wide dynamic range by utilizing specialized signal processing. A continuous temperature measurement range from 600°C up to 1500°C is realized with a resolution of 640x480 points and a measuring frequency of 25Hz. Each resulting image contains the full dynamic range and is transmitted via a Fast Ethernet interface in real time.

Keywords: IR-Imager, NIR, Thermal imager, wide temperature range, high dynamic range, industrial process control

1. INTRODUCTION

Some IR-imagers on the market today utilize video detectors in their basic design. A special window in front of the detector is typically used to select the specific spectral region of near infrared (NIR) from 0.8 … 1.1µm. A radiometric calibration of such a modified imager leads to a measurement device, much more sophisticated than a simple imager. The standard measurement range starts normally at 600°C (1112°F) object temperature and has a span of approximately 300K (e.g.: range from 600°C (1112°F) to 850°C (1562°F)). The limitation of small measurement ranges is mostly solved by using two or more ranges starting at different temperatures (e.g. 2^nd range: from 800°C (1472°F) to 1150°C (2102°F ). The range is selected manually or in an automatic mode by selecting the “best” range dependent on the observed scene. The disadvantage of this behavior is the appearance of under ranged and saturated images for scenes

With a higher temperature dynamic. There are some imagers with a “sequential operation mode”; in which a single measurement is taken from each measurement range and combined together with a composite image which then needs to be calculated to produce a single measurement. The prolongation of the over all exposure time due to this switching method between the different measurement modes, is the disadvantage of this image measurement technique. Even the resulted multi-range composite image has often some artifacts especially in the parts where the measurement ranges are overlapped.

There are two different ways to overcome the situation:

The very high slew rate of the natural given characteristic between object temperature and emitted radiation makes the 1^st method nearly impossible because it is very difficult to handle it in an adequate manner. The 2^nd approach with the development of the customized detector and design of a camera was based on the various considerations and will be introduced here:

2. HIGH DYNAMIC RANGE DETECTOR

After various considerations (see above) a detector matching the main requirements for an IR-imager had to be designed. In cooperation with a local company the required detector was developed and manufactured. A patented pixel architecture was used to realize a non-linear characteristic to compensate the natural characteristic between object temperature and emitted radiation to an almost linear one. This design allows for a wide dynamic scene of over 120dB. To be covered. Even a linear operation mode is integrated for measurement jobs in a small temperature range. The detector dimensions including the pixel quantity are derived from the available optics: The detector array with 840 by 640 active elements uses only 640 by 480 elements.

The detector is made in a standard silicon CMOS technology and is designed for working in an extreme ambient temperature range from -40 to 125°C (-40 to 257°F). An internal fixed pattern noise cancelation is built-in as numerous operational modes are used for almost any measurement task.

The development and manufacturing of the detector is a precondition to enable IR-imager to measure temperatures. Therefore many different jobs had to be accomplished:

• The adjustment of the operating point: There are many parameters influencing the properties of the detector. It was to find the right balance of all parameters to get the best compromise between measurement range, accuracy, noise equivalent temperature difference (NETD), stability, ambient temperature dependencies and other target values.
• The wide ambient temperature range of the detector is mostly welcomed but there was a dependency of the signal from the detector temperature! So a compensation algorithm had to be found and realized.
• The wide temperature range led to difficulties for handling the radiation-temperature characteristic in the same way as for standard devices. A new description of the characteristic was found and implemented in the measurement software. In connection with this fact the (re-)calculation of object temperatures dependent on the emissivity had to be adapted.
• A non-uniformity correction of the image was necessary to fulfill the requirements regarding the picture quality. Even here a new procedure was introduced to avoid the detector generated pattern especially for low signals.
• Development of a calibration method fulfilled both the requirements for accuracy and for a time saving procedure for the serial production.

The final detector was successfully evaluated and tested under numerous conditions. The imager based on it is described in the following section.

3. HIGH DYNAMIC IR-IMAGER

The design goal of the IR-imager to be created was to serve industrial applications. It results in two versions:

• A standard model in a classic camera design consisting of optics and camera body.
• A model with cooling jacket and air purging for the use in furnaces, utility boilers or kilns; any industrial applications where a wall separates the “hot” process from the environment.

A separate section will describe these versions (refer below).

1.1   Camera Body

The heart of the camera is the detector head covering the following functions:

• Providing the detector with all necessary voltages and clocks
• Data gathering inclusive averaging (user selectable)
• Signal processing: non-uniformity correction and ambient temperature compensation

Furthermore an independent signal evaluation is implemented. There is the possibility to define up to 8 different regions of interest (ROI), with selected shapes i.e. rectangles, circles, square etc., superimposed over the thermal image to provide the minimum, maximum or average temperature and up to two threshold temperatures for each region. The evaluation will be performed in real-time, the results of it can be linked to the camera outputs (digital, electrically isolated) directly. This kind of signal evaluation needs only one time programming (using a PC) and provides “stand alone” operation without the need for a connected computer.

1.2   Camera Data Interface

The data interface of an IR-camera must fulfill various requirements:

1.     data transfer, able to transfer the native detector data rate

2.     transport medium must be applicable in different environments (from lab to heavy industries)

3.     transfer distance should be at least 50m with option to bridge much longer distances

4.     use of a wide range of computers and operating systems

5.     highly standardized protocol for avoiding installation trouble

6.     high degree of reliability

7.     possibility for camera maintenance (checking the camera, firmware update)

8.     should be well introduced with an high acceptance world wide

©2008. Process Sensors Corp.