Handheld thermal imaging cameras are an important tool for the first responder community. As their use becomes more prevalent, it will become important for a set of standard test metrics to be available to characterize the performance of these cameras. A major factor in the performance of the imagers is the quality of the image on a display screen. An imager may employ any type of display screen, but the results of this paper will focus on those using liquid crystal displays. First responders, especially firefighters, in the field rely on the performance of this screen to relay vital information during critical situations.
Current research on thermal imaging camera performance metrics for first responder applications uses trained observer tests or camera composite output signal measurements. Trained observer tests are subjective and composite output tests do not evaluate the performance of the complete imaging system. It is the goal of this work to develop a non-nondestructive, objective method that tests the performance of the entire thermal imaging camera system, from the infrared destructive, sensor to the display screen. Application of existing display screen performance metrics to thermal imaging cameras requires additional consideration. Most display screen test metrics require a well defined electronic input, with either full black or white pixel input, often encompassing detailed spatial patterns and resolution. Well characterized thermal inputs must be used to obtain accurate, repeatable, and non-destructive display screen measurements for infrared cameras. For this work, a thermal target is used to correlate the measured camera output with the actual display luminance.
A test method was developed to determine display screen luminance. A well characterized CCD camera and digital recording device were used to determine an electro-optical transfer function for thermal imaging cameras. This value directly relates the composite output signal to the luminance of the display screen, providing a realistic characterization of system performance.
The accurate measurement of spatial resolution can be critical in the characterizations of projection display systems. Techniques can be used to determine resolution of projection systems by measuring the contrast of alternating grille patterns or fully modulated sine waves of various spatial frequencies. Unfortunately, the measurement of the contrast of these patterns may be influenced by stray light, either from ambient and reflected light in the environment, or from veiling glare (light scattering) in the lens of the light-measuring device. Such stray-light corruption can lead to large errors in contrast determination, providing an inaccurate and misleading characterization of the projector. For large-area measurements, various tools and methodologies have been employed, including the use of frustums and masks, to minimize such unwanted effects and provide a more accurate measurement. With some modifications, these same tools may be used for small-area measurements with similar results. Results will be shown comparing resolution determinations using different test methodologies with and without stray light compensation.
When characterizing or calibrating electronic displays, the technical staff needs to understand the capabilities and limitations of the measuring instrumentation used. Once these parameters are established, the metrologist can make meaningful decisions about the appropriateness of the instrument, identify potential errors, and remedy solutions if applicable. Several tools are introduced to facilitate the assessment of display measurement systems: the display measurement assessment standard (DMATS), the gamut assessment standard (GAS) and several stray-light elimination tools. Employing these tools, one can better understand the measurement capabilities of the facility and possibly make desired improvements.
With the advent of digital cinema, medical imaging, and other applications, the need to properly characterize projection display systems has become increasingly more crucial. Several standards organizations have developed or are presently developing measurement procedures (including ANSI, IEC, ISO, VESA, and SMPTE). The National Institute of Standards and Technology (NIST) has played an important role by evaluating standards and procedures, developing diagnostics, and providing technical and editorial input, especially where unbiased technical expertise is needed to establish credibility and to investigate measurement problems.
Light-measurement instrumentation based upon high-quality charge-coupled-devices (CCD) is currently in use for measuring the characteristics of electronic displays. When such array detectors are used to measure scenes or images having high contrasts or wide color variations, they can suffer from the effects of veiling glare or lens flare and thereby inaccurately measure the darker luminances because of a mixing of the scene luminances or colors. A liquid-filled simulated-eye design (SED) camera was constructed to improve the ability to measure such images by reducing the internal scattering that contributes to the veiling glare. This paper discusses the investigation of the use of various liquids, in particular the effects of scattering within the liquids.
In earlier papers, NIST proposed a standard illumination source and optical filter targets with which to assess the state-of-the-art of display measurement. The Display Measurement Assessment Transfer Standard (DMATS) was designed to present the display metrologist with a rectangular array of targets such as color filters, polarizers, and grilles, back-lighted by uniform illumination, to be measured using methods and instruments typically used in display performance measurement. A “round robin” interlaboratory measurement exercise using the “standard” artifact suite would enable a first order assessment of display measurement reproducibility, i.e., measurement variability within the electronic display community. The rectangular array design of the DMATS was anticipated to present stray light and color contamination challenges to facilitate identification of error sources deriving from measurement protocols, laboratory environment, and equipment. However, complications in dealing with heating problems threatened to delay the planned laboratory intercomparison. The Gamut Assessment Standard (GAS) was thus designed as an interim solution to enable the NIST scientists and participating measurement laboratories to begin collecting data. The GAS consists of a 150 mm diameter integrating sphere standard illumination source with a stray light elimination tube (SLET) mounted at the exit port. A dual six-position filter wheel is mounted at the SLET exit port. One wheel holds a series of neutral density filters and a second interchangeable wheel holds various color filters. This paper describes the design and construction of the GAS, its initial performance characterization by NIST, and comparison measurements made at NPL. Possible design changes suggested by the results of the preliminary intercomparison are discussed, as are plans for future interlaboratory comparisons and potential use of the GAS as a transfer standard for laboratory self-certification.
The NIST Flat Panel Display Laboratory (FPDL) is operated through the Display Metrology Project (DMP) of the Electronic Information Technology Group in the Electricity Division of the Electronics and Electrical Engineering Laboratory of NIST. The DMP works to develop and refine measurement procedures in support of ongoing electronic display metrology, and applies the results in the development of national and international standards for flat panel display characterization.
The accurate measurement of small area-black levels is important in projection display characterization. For example, techniques can be used to determine resolution of projection systems by measuring the contrast of alternating grille patterns or fully-modulated sine waves of various spatial frequencies. Unfortunately, the measurement of the contrast of these patterns may be influenced by stray light, either from ambient and reflected light in the environment, or from veiling glare scatter in the lens of the light-measuring device. Such stray-light corruption can lead to large errors in contrast determination, providing an inaccurate characterization of the projector. For large-area measurements, various techniques have been employed, including the use of frustums and masks, to minimize such unwanted effects and provide a more accurate measurement. With some modifications, these same tools may be used for small-area measurements with similar results. The design, construction, and implementation of these tools will be discussed. Results will be shown comparing small-area contrast measurements of projection systems, including resolution determination, with and without stray light compensation, for different measurement instrumentation.
A prototype display measurement assessment transfer standard (DMATS) is being developed by the NIST to assist the display industry in standardizing measurement methods used to quantify and specify the performance of electronic display. Designed as an idealized electronic display, the DMATS illumination source emulates photometric and colorimetric measurement problems commonly encountered in measuring electronic displays. NIST will calibrate DMATS units and distribute them to participating laboratories for measurement. Analysis of initial interlaboratory comparison results will provide a baseline assessment of display measurement uncertainties. Also, diagnostic indicators expected to emerge from the data will be used to assist laboratories in correcting deficiencies or in identifying metrology problem areas for further research, such as measurement techniques tailored to new display technologies. This paper describes the design and construction of a prototype DMATS source and preliminary photometric and colorimetric characterization. Also, this paper compares measurements obtained by several instruments under constant environmental conditions and examines the effects of veiling glare on chromaticity measurements.
Flying-spot displays use narrow-spectrum high-power sources that scan the image across the display screen. They can provide a bright display with a large color gamut. When such displays are measure with conventional light-measuring devices (LMDs) such as luminance or illuminance meters, there is concern that the LMD may not accurately measure the display's photometric and colorimetric output. The unique characteristics of the source may exceed the limitations of the instrumentation. A series of diagnostics has been developed that allows for an evaluation of LMDs for use in measuring flying-spot displays. Limitations resulting from LMD saturation, timing, and tristimulus or photopic filters can be revealed, and in some cases, specific causes can be identified. Each diagnostic will be demonstrated using several instruments, including luminance meters, illuminance meters, colorimeters and spectro radiometers. Using a simple comparison box, flying-spot displays can be viewed side-by- side with steady-state sources in a bipartite image. After the sources have been visually matched in color and luminance, the two images can be measured with a particular LMD, and results compared. Any significant difference between results would indicate a limitation of the LMD. Further diagnostics, using integrating spheres, neutral density filters and interference filters, are used to aid in identifying the nature of the limitation, and in some cases, point to solutions.
Flying-spot displays scan an image across the display screen using a high-energy beam. Each pixel can be a narrow, submicrosecond pulse. When such displays are measured with conventional light-measuring devices (LMDs), such as luminance or illuminance meters, there is concern that the LMD may not accurately measure the display light output because of the unique characteristics of the source. The LMD may be unable to properly integrate the narrow pulses, or the high-energy signal may saturate the detector. As in all areas of metrology, it is essential to verify that the instruments used are providing the desired information. A diagnostic has been developed that allows for an evaluation of LMDs for use in measuring flying-spot and similar displays. This method tests for both integration and saturation errors using a bipartite comparator and a neutral density filter. Errors resulting from the saturation of the LMD by the flying-spot display are demonstrated. The construction and procedure of the diagnostic is described. Limitations of the technique as well as sources of error are presented.
Electronic front-projection display specifications are often based on measurements made in ideal darkroom conditions. However, not everyone has access to such a facility. In many environments, ambient light from other sources in the room illuminates the screen. This includes room lights directly illuminating the screen and the reflection of these light sources off of walls, floors, furniture, and other objects. Additionally, back-reflections from the projection screen must be considered. These stray light components contribute to the measured value, giving an inaccurate measurement of the projector light output. Thus, these conditions may make the task of adequately comparing and evaluating different projection systems difficult. We can better verify whether the projector is operating according to its specifications or compare its performance with other projectors by compensating for stray light. A simple projection mask constructed from black plastic and a stray-light elimination tube are presented as solutions that can provide an accurate measurement of projector light output in many ambient light conditions.
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