| Abstract | | | | one horizontal line and at the point where the |
| When looking to select an infrared camera, it is | | | | temperature delta between the reference and |
| extremely important to better understand the | | | | the ambient targets no longer creates a |
| attributes of these cameras that most impact the | | | | measureable signal the NETD is determine by the |
| quality of the infrared images that are produced. | | | | measured temperature difference between the |
| This paper covers the three primary areas that | | | | reference and the ambient reference targets. |
| influence thermal image quality: pixel resolution, | | | | MRTD - Minimum Resolvable Temperature |
| thermal sensitivity and fixed pattern noise. Each | | | | Difference |
| area has a significant impact on thermal image | | | | This is a system test. An observer is asked to |
| quality. | | | | assess the minimum temperature difference at |
| If you’ve purchased a digital camera in the | | | | which a 4 bar target can be resolved by watching |
| past, your purchase was likely influenced by your | | | | the video output displayed as the temperature |
| belief that the number of pixels was the most | | | | set points of the reference and the ambient |
| important specification when trying to judge | | | | targets are brought close together. This minimum |
| image quality between all the camera choices | | | | difference will change with the spatial frequency |
| offered. For anyone that reads Consumer | | | | of the bar target used. A curve of MRTD against |
| Reports™ and their detailed evaluation of | | | | spatial frequency is obtained which characterizes |
| digital cameras you'll appreciate that camera | | | | the performance of the imaging system. Modern |
| performance includes careful analysis of much | | | | infrared imaging systems can have low spatial |
| more than the pixel count. Because a thermal | | | | frequency MRTDs of tens of milli-kelvins. |
| camera is basically an image converter (radiant | | | | The benefits of large format cameras is |
| thermal energy to visible image), you need to | | | | significant we you combine the need for high |
| understand what are the primary attributes that | | | | sensitivity while viewing high spatial frequencies. |
| determine thermal image quality and how they | | | | To simplify explaining the fundamentals of thermal |
| each contribute to the image quality that you | | | | sensitivity let's focus on a single pixel of the |
| may be experiencing in your application. | | | | infrared sensor in an uncooled infrared camera. |
| Pixel Resolution | | | | Each pixel in an uncooled focal plane array image |
| The first consideration is the number of pixels. | | | | sensor is essentially a resistor fabricated using |
| Today there are three resolution standards (some | | | | MEMS (micro electro mechanical systems). |
| manufacturers' cameras deviate slightly) | | | | The basis structure of a thermal uncooled camera |
| - Low Resolution - 160x120 (19,600 pixels) | | | | pixel is a microscopic bridge structure on which a |
| - Medium Resolution - 320x240 (76,800 pixels) | | | | thin resistor material and an absorbing layer have |
| - High Resolution - 640x480 (307,200 pixels) | | | | been deposited. Legs suspend the deck of bridge |
| How much resolution you need (verses want) is | | | | above an integrated circuit and provide electrical |
| primarily determined by your application and by | | | | connection between the resistive bridge and the |
| the value you give to image quality. When | | | | silicon readout circuit. The readout IC controls the |
| evaluating a digital camera with 5 verses 10 mega | | | | voltage that biases the thin film resistor and |
| pixels most users will never benefit by purchasing | | | | multiplexes all the pixel signals out to the cameras |
| a camera with 10 million pixels because they will | | | | imaging electronics. |
| never print the images on large enough paper | | | | As infrared radiation is absorbed by each pixel its |
| where the resolution would provide better print | | | | temperature changes as the photon energy (8-14 |
| quality. Whereas you will always print and display | | | | micron wavelength) is converted to heat which in |
| the full resolution of an infrared camera since the | | | | turn changes the resistance of the pixel's thin film |
| highest resolution available is relatively modest by | | | | resistor. The readout IC sends a voltage across |
| today's digital camera standards. Even at | | | | each "micro bolometer" element and a |
| 640x480 pixel resolution a high definition thermal | | | | signal proportional to heat absorbed by each |
| image will only take up a fraction of today's | | | | detector is the basis of a real time video image. |
| computer displays and the resulting thermal image | | | | The electrical circuit of an infrared sensor is very |
| print quality will always be fully realized. Therefore | | | | simple, a voltage is turned on to each pixel and a |
| when evaluating a thermal camera the number of | | | | change in resistance of the thin film resistor based |
| pixel is relevant and increased resolution is the | | | | on the pixels temperature is sampled and |
| most significant consideration in improving image | | | | converted into a digital value. All analog signal |
| quality. | | | | carry some level of noise along with the signal |
| Another benefit to high resolution is the ability to | | | | generated by the sensor. The ratio of signal to |
| zoom into a scene and maintain good image | | | | noise strongly impacts the image quality of a |
| quality. The majority of thermal cameras feature | | | | camera because the noise level is usually a fixed |
| a standard optic with a horizontal field of view of | | | | amount and as the detector gain is increased the |
| approximately 25°. Regardless of pixel | | | | system will begin to display the signal noise and |
| resolution the performance of a 640x480 camera | | | | you'll begin to see "snow" in the image. |
| set to 2X digital zoom is going to equal the | | | | The signal level of this noise is commonly specified |
| performance of a 320x240 resolution camera | | | | as Noise Equivalent Temperature Difference. |
| with an optional (and often costly) 12° (2X) | | | | Like any electrical circuit there are a lots of |
| lens. If you anticipate the need for imaging | | | | opportunities for electrical noise to get into |
| objects at distances further than 20 feet you | | | | systems, but the quality (signal to noise) of the |
| should consider the increased costs of a 2X lens | | | | signal coming directly off the infrared pixel has the |
| for a 320x240 thermal camera when comparing | | | | most impact on thermal sensitivity, since nearly all |
| the total costs between 320x240 and 640x480 | | | | camera developers have access to the same |
| systems. | | | | electronic components with which to create a |
| The second major issue that impacts image | | | | camera. Therefore the thermal sensitivity in large |
| quality is thermal sensitivity. While there are a | | | | part is based on the quality of the infrared imager |
| number of tests used to quantify this | | | | array. |
| specification, thermal sensitivity basically defines | | | | Other issues like the f number of the lens also |
| how well the camera will image as you increase | | | | impact thermal sensitivity. Your camera's lens is |
| image contrast. Thermal sensitivity varies with | | | | likely ƒ1.0 (the focal length is equal to the |
| object temperature, as object temperature | | | | lens diameter) which is considered a |
| increases the slope of the signal output of the | | | | "fast" lens. By comparison the f |
| detector increases with increased temperature. | | | | number in your digital camera is likely between |
| This means that the signal (increasing) to noise | | | | ƒ3 and ƒ5 while the cameras used in |
| (fixed) ratio improves as you view hotter objects. | | | | cell phones and other low cost systems can be as |
| However this is not usually a benefit because the | | | | high as ƒ20! As application demands lead to |
| applications where better thermal sensitivity can | | | | longer focal length lenses it is practical to go to |
| be exploited are low temperature (room | | | | "slower" optics in order to reduce the |
| temperature) applications where the thermal | | | | size, weight and cost of telephoto lenses and |
| contrast (temperature delta within an image) is | | | | trade off some thermal sensitivity. For example, |
| very low. Typical low thermal contrast applications | | | | an F1.4 optic will result in 2X reduction in thermal |
| include building diagnosis where the camera is | | | | sensitivity and an F2.0 optic a 4X reduction in |
| imaging interior walls with very little temperature | | | | thermal sensitivity. Therefore a system with |
| variations or emissivity differences and issues like | | | | 50mK sensitivity using a standard lens will still |
| moisture or insulation quality can only be visualized | | | | maintain good sensitivity (100mK) when a |
| by increasing the contrast to the point where the | | | | ƒ1.4 telephoto lens is attached to the |
| cameras thermal sensitivity limits the useful | | | | camera verses another camera whose thermal |
| temperature span settings. | | | | sensitivity started at 100mK and becomes 200mK |
| As you review published camera specifications | | | | when viewing through a "slower" |
| you will see thermal sensitivity specifications range | | | | (ƒ number higher than 1). |
| between 0.25°C (250mK) and 0.05°C | | | | As you can see from the various issues raised |
| (50mK). While you might consider a quarter of | | | | within this paper the nature of thermal sensitivity |
| degree to be adequate thermal sensitivity as soon | | | | is very complex but in the real world the human |
| as you look at a low contrast scene you'll | | | | eye is extremely good at differentiating small |
| discover the image quality adversely effects the | | | | differences in image quality that you'll know it |
| image quality as noise begins to dominate the | | | | (good sensitivity) when you see it. |
| image. | | | | Non-Uniformity Correction |
| Thermal imagers usually display images in palettes | | | | As the number of pixels increases and their |
| comprised of 256 discreet color or gray levels. | | | | sensitivity improves the quality of image is |
| Imagine your target has a temperature difference | | | | increasingly dependent on a process called Non |
| between 0°C and 256°C each gray or | | | | Uniformity Calibration or NUC. As we described |
| color level would represent 1 degree of | | | | earlier a microbolometer imaging array is |
| temperature difference. Now apply this same | | | | essentially an array of tiny resistors, and because |
| color mapping into a scene with temperatures | | | | of the micro scale of these devices, there are |
| between 25°C and 35°C or 10 degrees. | | | | variations in how each pixel responds to the |
| Each color now represents 0.03°C (10°C | | | | infrared energy from an object. During |
| ÷256), a value lower than the most | | | | manufacturing the infrared camera's sensor must |
| sensitive uncooled cameras. The result is some | | | | be normalized, meaning that the differences in |
| display of noise. There are many applications in | | | | response and DC output for each detector must |
| which it is very important to set the span as | | | | be zeroed out. Thermal cameras typically feature |
| narrow as possible in order to see the smallest | | | | an internal flag or iris that periodically is positioned |
| temperature variations possible. If you are using a | | | | in front of the detector as a constant |
| camera with 0.25°C sensitivity and wanted to | | | | temperature reference to zero out differences |
| maintain the same level of noise you would have | | | | amongst the pixels. This is a fine tuning of the |
| to set a temperature range of 65°C | | | | factory NUC process and is sometimes referred |
| (150°F) which would likely result in a very low | | | | to as a "touch up." |
| contrast image. You should recognize that the | | | | Because the touch up source is inside the lens, |
| difference between a camera with 50mK | | | | additional image quality improvements are possible |
| sensitivity verses a camera with 100mK | | | | when performing a touch up calibration through |
| sensitivity is 100% better and not as 0.05°C | | | | the lens either using a lens cap or exposing the |
| better. | | | | camera to a large uniform surface. As camera |
| Thermal Sensitivity | | | | performance improves the non-uniformities |
| NETD is the scene temperature difference equal | | | | created by the lens will begin to be seen and for |
| to either the internal noise of the detector | | | | the ultimate image quality a simple through the |
| (detector NETD) or the total electronic noise of a | | | | lens calibration step will ensure the highest image |
| measurement system (system NETD). As a | | | | quality the camera is capable of generating. |
| camera buyer you need to evaluate system | | | | Benefits of high increased image quality |
| NETD. The test setup consists of temperature | | | | - Much greater flexibility to inspect targets are |
| control blackbody reference and some type of | | | | varying distances |
| ambient (passive) object that creates a simple slit | | | | - Ability to visualize low thermal contrast targets |
| target for the camera to visualize. The | | | | - More intuitive diagnosis of heat related problems |
| temperature of the black body is adjusted until it | | | | - Improved infrared visible fused image quality due |
| nearly equals the ambient target temperature. An | | | | to better matching of infrared and visible camera |
| oscilloscope measures the analog video output of | | | | resolution.. |