A thermal imaging device’s NETD (Noise Equivalent Temperature Difference) rating is one of the most important factors to take into consideration when it comes to choosing a device that is right for you. But what exactly is an NETD rating and how does it affect the performance of a thermal imager. In this blog, we’ll try to break down what this rating means and why it is important when it comes to deciding which product is best for your desired application. For ease, a definition of certain terms can be found at the end of this article.
An NETD rating is used as an indicator of a thermal imaging system’s sensitivity, denoting when the temperature value signal is equal to the noise signal. NETD also denotes the minimum detectable temperature difference between an observed object and the background said object is against.
A thermal imaging device registers signal and noise, the latter of which can be detrimental to its picture quality. When a device registers a noise signal equal to the smallest detectable temperature difference it is not able to discern a useful thermal signal to produce a high-quality image of the object. Thus, the higher the noise level, the higher the NETD level, and the less able your device becomes when discerning small temperature differences.
This means that when the temperature of an observed object drops low enough, its heat signature may merge with noise in such a way that the thermal imaging device is unable to differentiate between the object’s signal and noise. This results in a scene in which the background and observed objects merge and become practically indistinguishable from one another. The lower the NETD value of a thermal imager, the better the sensor’s ability to register small temperature differences.
Why is NETD Important?
NETD is an important factor when it comes to object detection, recognition, and identification as, at long ranges, extreme atmospheric attenuation will cause the device to become sensitivity limited and therefore it will struggle to achieve any of the above-mentioned tasks.
A thermal imaging device with a lower NETD value will achieve better image quality with greater levels of visual information, more contrast, and better visibility in conditions with minimal temperature differences, such as cold weather, heavy rain, or dense fog).
Take for example Pulsar devices. Pulsar devices are available with varying NETD ratings, this quick list and selection of images should allow you to gain an understanding of the image quality each rating is capable of achieving.
- <18 mK (Exceptional)
- <25 mK (Outstanding)
- <40 mK (Excellent)
- <50 mK (Good)
- <60 mK (Acceptable)
- <80 mK (Satisfactory)
How is NETD Measured?
When a device’s NETD rating is being measured, the temporal noise is divided by the signal transfer function (SiTF). The blackbody is set to a specific temperature and a series of frames are taken over a predefined duration. These frames are then used to generate a histogram of each of the pixel values. The subsequent NETD rating is determined through the standard deviation of this histogram converted into millikelvin.
Terms
Blackbody – Energy radiated by an object or system that absorbs all incident radiation. “Blackbody” usually refers to the spectrum of light emitted by any heated object. The amount of radiation a particular wavelength emits from a surface can be affected by the type of material absorbing the incident radiation and the temperature of the surface; meaning different materials can emit different amounts of energy, even if they are at the same temperature. An object that emits the maximum amount of energy for its absolute temperature is called a blackbody.
Histogram – A graph that uses vertical or horizontal bars to indicate quantity by length. The histogram is one of the most common formats used to represent statistical data. A histogram will both clearly display the largest and smallest categories when collecting data and provide an immediate impression of the distribution of said data.
Millikelvin (mK) – One thousandth of a Kelvin (the base unit of measurement for temperature in the International System of Units. An absolute temperature scale that starts from 0 K and rises by exactly 1 K per 1°C)
NETD – Noise Equivalent Temperature Difference
Noise – Random variations of brightness or colour information that result in a grainy texture to an image. Noise is most likely to occur when using a device with a small sensor in low-light conditions.
Signal – A value related to the amount of thermal radiation received by the thermal imaging device.
Signal Transfer Function (SiTF) – A measure of the signal output versus the signal input of a thermal imaging system.
Temporal Noise – Random noise that varies from image to image.
Thermal Radiation – The process by which energy in the form of electromagnetic radiation is emitted by a heated surface in every direction.
