Learn the basics of uncooled microbolometers, how they work, their key characteristics, and applications in infrared imaging and temperature measurement.
Microbolometer are thermal detectors used in infrared imaging and temperature measurement applications. A microbolometer consist of an array of heat-sensitive electrical restistance. The incoming infrared radiation on the detector surface changes the resistance of the individual bolometer pixels, which then leads to a voltage change. The electrical signals are then processed by the camera and converted into a digital image. This makes the intensity of the infrared radiation visible, allowing for the detection of heat variances and thermal patterns.
Rolling-Readout characteristics of a microbolometer
Almost all microbolometers used in the field use the Rolling- Readout method. This means that the readout of the pixels via the bolometer array is staggered. Instead of reading out all elements simultaneously, they are read out individually or in groups (line by line), similar to a camera with rolling shutter.
This staggered approach helps to reduce the impact of readout noise on the signal because the noise is distributed over time, and the bolometer elements have time to recover before their next readout.
The Rolling-Readout function requires precise synchronization between the scanning process and data acquisition to ensure accurate measurements.
The thermal time constant of the bolometer is between 7 and 12 ms depending on the camera type.
Typical parameters of a microbolometer
Here are the key characteristics of a microbolometer:
Responsivity
Responsivity refers to the sensor’s ability to convert changes in temperature into changes in electrical signal. It is typically measured in volts per watt (V/W) and is a measure of the sensitivity of the microbolometer. Higher responsivity indicates greater sensitivity to temperature variations.
Thermal Sensitivity
Thermal sensitivity is a measure of how small a temperature change the microbolometer can detect. It is usually specified in units like mK (millikelvin) per watt. A higher thermal sensitivity means the sensor can detect smaller temperature differences.
Noise Equivalent Temperature Difference (NETD)
NETD is a crucial parameter that quantifies the smallest temperature difference that the microbolometer can detect in the presence of noise. It is usually measured in mK and is inversely proportional to the sensor’s thermal sensitivity. Lower NETD values indicate better performance in low-temperature contrast conditions.
Spectral Range
Microbolometers are typically designed to operate in specific infrared wavelength ranges, such as long-wave infrared (LWIR), mid-wave infrared (MWIR), or short-wave infrared (SWIR). The spectral range determines which types of heat sources or objects the sensor can detect.
Frame Rate
The frame rate of a microbolometer represents how quickly it can capture and process thermal images. It is typically measured in frames per second (FPS) and varies from sensor to sensor. Higher frame rates are desirable for applications that require real-time imaging or video.
Thermal Time Constant
The thermal time constant of a microbolometer is the time it takes for the sensor to reach approximately 63.2% of its final temperature in response to a step change in incident radiation. It is an important parameter for understanding the sensor’s response time. The thermal time constant of the IRSX-I camera is up to 12 ms.
Pixel Size
Microbolometers are arrayed sensors with multiple individual pixels. The size of each pixel determines the spatial resolution of the sensor. Smaller pixel sizes can provide higher spatial resolution but may sacrifice sensitivity.
Non-Uniformity Correction (NUC)
Microbolometers can exhibit pixel-to-pixel variations in sensitivity, leading to image artifacts. Non-uniformity correction algorithms are used to compensate for these variations and provide a uniform image.
