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Understanding Infrared Camera Thermal Image Quality
Abstract
When you select an infrared camera, it is extremely important to improve the properties of these cameras understand that most affect the quality of the infrared images produced. This document relates on the three primary areas that affect thermal image: pixel resolution, thermal sensitivity and fixed pattern noise. Each area has a significant influence on the thermal image quality.
If you have a digital camera bought in the past, your purchase was probably influenced by your belief that the number pixels is the most important specification when trying to decide between all camera image quality choices offered. For anyone Consumer Reports ™ and their detailed evaluation Digital cameras can read will understand that camera performance includes careful analysis of much more than the number of pixels. Because a thermal camera is actually an image converter (radiant thermal energy to visible image), you should understand what are the primary attributes that determine thermal image quality and how they each contribute to the quality that you experience in your application.
Pixel Resolution
The first consideration is the number of pixels. Today there are three standard resolution (some cameras manufacturers 'something different)
- Low Resolution - 160x120 (19,600 pixels)
- Medium Resolution - 320x240 (76,800 pixels)
- High resolution - 640x480 (307,200 pixels)
How much resolution you need (like verses) is primarily determined by your application and the value you add to the picture quality. When evaluating a digital camera with 5 megapixels 10 verses most users will never benefit by purchasing a camera with 10 million pixels, because they will never print the photos on large enough paper in which the resolution was a better print quality. Whereas you always print and display the full resolution of an infrared camera, as the highest resolution available is relatively modest by today's digital camera standards. Even at 640x480 pixel resolution, a high definition thermal image will take only a fraction of today's computer displays and the resulting thermal image printing will always be fully realized. Therefore, when assessing a thermal camera the number is relevant and higher pixel resolution is the key consideration to improve the image quality.
Another advantage is the possibility of high resolution to zoom into a scene and maintaining a good image quality. The majority of the thermal camera with an optical standard with a horizontal field of view approximately 25 °. Regardless pixel resolution of 640x480 the performance of a 2X digital zoom camera is set to equal the performance of a 320x240 resolution camera with an optional (and often expensive) 12 ° (2X) lens. If you anticipate the need for imaging objects at a distance more than 20 meter you should consider the higher cost of a 2X lens for a 320x240 thermal camera when comparing the total cost between 320x240 and 640x480 systems.
The second important issue affecting quality is thermal sensitivity. Although a number of tests used to quantify this specification, thermal sensitivity principle determines how well the camera image and more image contrast. Thermal sensitivity varies with temperature object, object as the temperature of the slope of the signal from the detector increases with increased temperature. This means that the signal (more) noise (solid) relationship improves when you look hotter objects. This is usually not an advantage, because the applications where improved thermal sensitivity can be exploited low temperature (room temperature) applications where the thermal contrast (temperature delta within an image) is very low. Typical applications include low thermal contrast when the camera is building diagnostic imaging walls with low temperature or emissivity differences and issues such as moisture or insulation quality can only be visualized by increasing the contrast to the point where The camera's thermal sensitivity limits the useful temperature span settings.
As you review published sensitivity thermal camera specifications see specifications lie between 0.25 ° C (250mK) and 0.05 ° C (50mK). While one quarter of the degree would be sufficient to consider thermal sensitivity when you look at a low contrast scene you'll find the image quality as the adverse effects of noise starts to dominate the picture.
Thermal Imaging usually display images in pallets consists of 256 discreet color or grayscale. Suppose your target is a difference in temperature between 0 ° C and 256 ° C each gray or color level would represent 1 degree temperature difference. Now applying this same color mapping in one scene with temperatures between 25 ° C and 35 ° C or 10 degrees. Each color now represents 0.03 ° C (10 ÷ 256 ° C), a value lower than the most sensitive uncooled cameras. The result is a representation of sound. There are many applications where it is very important to Set the span as narrow as possible to see the smallest temperature variations. If you have a camera with 0.25 ° C sensitivity and would maintain the same level of noise you should get to a set temperature range of 65 ° C (150 ° F), which will probably result in a very low contrast image. You must recognize the difference between a camera with a camera with 50mK sensitivity verses 100mK sensitivity is 100% better and not as 0.05 ° C better.
Thermal sensitivity
NETD is the scene temperature difference is equal to either the internal noise of the detector (detector NETD) of the total electronic noise of a measurement system (system NETD). If you have a camera Buyer should evaluate system NETD. The test setup consists of a temperature reference black and kind of ambient (passive) object that a simple crack target for the camera allows to visualize. The temperature of the black body is adjusted until it almost equals the ambient temperature target. An oscilloscope measures the analog video output of a horizontal line and at the point where the temperature delta between the reference and the environmental objectives no longer provides a measurable signal of the NETD is determined by the measured temperature difference between the reference and the ambient benchmarks.
MRTD - minimum resolvable temperature difference
This is a system testing. An observer is asked to assess the difference in minimum temperature with a 4 bar target can be resolved by looking at the video output displayed when the set temperature points of reference and the surrounding air targets close together. This minimum difference will change with the spatial frequency of the bar target used. Plot MRTD against spatial frequency is obtained, which characterizes the performance of the imaging system. Modern infrared imaging systems can have dozens of low spatial frequency MRTDs milli-kelvin.
The advantages of large format cameras is significant we need to combine high sensitivity during viewing of high spatial frequencies.
To simplify the explanation of the fundamentals of thermal sensitivity to focus on a single pixel of the infrared sensor in an uncooled infrared camera. Each pixel in an uncooled focal plane array image sensor is essentially a resistor fabricated using MEMS (micro-electro mechanical systems).
The basic structure of an uncooled thermal camera pixel is a microscopically thin bridge structure in which a resistance and an absorbent material layer are deposited. Legs suspend the deck of the bridge over an integrated circuit and provide electrical connection between the resistive bridge and the silicon readout circuit. The readout IC controls the voltage that biases the thin film resistance and multiplexes the pixel signals to the camera's imaging electronics.
As infrared radiation is absorbed by each pixel the temperature changes as the photon energy (8-14 microns wavelength) is converted into heat which in turn changes the resistance of thin film resistance of the pixel. The readout IC sends a voltage across each micro bolometer "element and a signal proportional to heat absorbed by each detector is based on a real-time video image.
The electrical circuit of an infrared sensor is very simple, a voltage is switched at each pixel and a change in the resistance of the thin film resistance temperature based on the pixel is sampled and converted into a digital value. All analog signal carry a certain level of noise, along with the signal generated by the sensor. The ratio of signal / noise significantly affects the quality of a camera, because the noise is usually a fixed amount as the detector gain is increased the system will begin to display the signal noise and you'll begin to "snow" in the picture.
The signal level of this noise is usually specified as Noise Equivalent Temperature Difference.
Like any electrical circuit, there are many opportunities for electrical noise to get into the system, but the quality (signal / noise) of the signal comes directly from the IR pixel has the most influence on the thermal sensitivity, because almost all Camera developers have access to electronic components that allow a camera. Therefore, the thermal sensitivity to a large extent based on the quality of the infrared imager array.
Other issues like the f-number of the lens also affects thermal sensitivity. Lens of your camera is probably ƒ1.0 (the focal length equal to the lens diameter), a fast considered "lens. In comparison, the f-number in your digital camera is probably between ƒ3 and ƒ5, while the cameras are used in mobile phones and other systems, such as low cost can amount to ƒ20! As application requirements lead to longer focal length lenses it is convenient to go to "slower" Optics in the size, weight and cost of telephoto lenses to reduce the trade off some thermal sensitivity. For example, an optical F1.4 will result 2X reduction in the thermal sensitivity of F2.0 and a 4X optical reduction of thermal sensitivity. Therefore, a system with 50mK sensitivity with a standard lens still maintaining good sensitivity (100mK) ƒ1.4 when a telephoto lens attached to the camera verses another camera whose thermal sensitivity began to 100mK and 200mK when viewing through a "slower" (ƒ number greater than 1).
As you can see the various issues raised in this document the nature of the thermal sensitivity is very complex, but in the real world, the human eye is very good at distinguishing small differences in image quality, you know what you (good sensitivity) when you see.
Non Uniformity Correction
As the number of pixels increases their sensitivity and improves the quality of the image is increasingly dependent on a Not Uniformity process called calibration or NUC. As we previously described a microbolometer imaging array is essentially an array of small resistors, and because the micro level of these devices, there are differences in how each pixel reacts to infrared energy of an object. During manufacture of the infrared sensor cameras must be standardized, meaning that differences in response and DC output for each detector are set at zero. Thermal cameras feature of an internal flag or iris periodically placed before the detector as a constant reference to zero temperature differences between the pixels. This is a fine tuning of the plant NUC process is sometimes referred to as a "touch-up."
Since the retouching source in the lens, higher image quality improvements in the performance of a touch-up calibration by the lens, either by a lens cover or expose the camera on a large uniform surface. As camera improves the performance of the non-uniformities created by the lens will begin to be seen for the ultimate picture quality and a simple through the lens calibration step will ensure the highest picture quality of the camera is able to generate.
Benefits increased high quality
- Much more flexibility to inspect several goals distances
- Ability visualize low thermal contrast targets
- More intuitive diagnosis of heat-related problems
- Enhanced infrared image rendered visible through better alignment of infrared and visible Camera resolution ..
- Flexibility to include lower costs and lighter weight lenses optional
- More intuitive diagnosis of the temperature anomalies
For complete article with pictures and reference materials, visit www.electrophysics.com / Tiqab
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