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ASTM E1791-96(R2021) pdf free download

ASTM E1791-96(R2021) pdf free download.Standard Practice for Transfer Standards for Reflectance Factor for Near-Infrared Instruments Using Hemispherical Geometry
1. Scope
1.1 This practice covers procedures for the preparation and use of acceptable transfer standards for NIR spectrophotom- eters. Procedures for calibrating the reflectance factor of materials on an absolute basis are contained in CIE Publication No. 44 (9). Both the pressed powder samples and the sintered PTFE materials are used as transfer standards for such calibra- tions because they have very stable reflectance factors that are nearly constant with wavelength and because the distribution of flux resembles closely that from the perfect reflecting diffuser. 1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. 1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appro- priate safety, health, and environmental practices and deter- mine the applicability ofregulatory limitations prior to use. 1.4 This international standard was developed in accor- dance with internationally recognized principles on standard- ization established in the Decision on Principles for the Development of International Standards, Guides and Recom- mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
3. Terminology
3.1 Definitions—Terms and definitions in Terminology E284 are applicable to this practice.3.2 Descriptions of Terms Specific to This Standard—The following definitions are particularly important to this practice. 3.2.1 linearity—the ability of a photometric system to yield a linear relationship between the radiant power incident on its detector and some measurable quantity provided by the system. (E131) 3.2.2 near-infrared, adj—the region of the electromagnetic spectrum for radiation of wavelengths between 780 and 2500 nm (0.78 and 2.50 µm). 3.2.3 perfect reflecting diffuser—ideal reflecting surface that neither absorbs nor transmits light, but reflects diffusely, with the radiance of the reflecting surface being the same for all reflecting angles, regardless of the angular distribution of the incident light. 3.2.4 reflectance, r, n—ratio of the reflected radiant or luminous flux to the incident flux in the given conditions (1). The term reflectance is often used in a general sense or as an abbreviation for reflectance factor. Such usage may be assumed unless the definition is specifically required by the context. 3.2.5 reflectance factor, R, n—ratio ofthe flux reflected from the specimen to the flux reflected from the perfect reflecting diffuser under the same geometric and spectral conditions of measurement (2).
4. Summary of Practice
4.1 Procedures for the preparation of packed powder samples of barium sulfate and PTFE can be found in Practice E259. Sintered PTFE samples are commercially available. Reflectance data for this material are given in Table 1. These materials provide close approximation to the optical properties of the perfect reflecting diffuser and may be used to transfer a scale of reflectance factor to another material or instrument. 4.2 Sintered carbon-black doped PTFE samples are also commercially available and are described in Table 2. These materials provide close approximation to the optical properties of a perfect reflecting diffuser with spectrally neutral absor- bance features and may be used to transfer a scale of linearity in reflectance factor to another material or instrument.
5. Significance and Use
5.1 Most commercial reflectometers and spectrophotom- eters with reflectance capability measure relative reflectance. The instrument reading is the ratio of the measured radiation reflected from the reference specimen to the measured radia- tion reflected by the test specimen. That ratio is dependent on specific instrument parameters. 5.2 National standardizing laboratories and some research laboratories measure reflectance on instruments calibrated from basic principles, thereby establishing a scale of absolute reflectance as described in CIE Publication No. 44 (5). These measurements are sufficiently difficult and of prohibitive cost that they are usually left to laboratories that specialize in them. 5.3 A standard that has been measured on an absolute scale could be used to transfer that scale to a reflectometer. While such procedures exist, the constraints placed on the mechanical properties restrict the suitability of some of the optical properties, especially those properties related to the geometric distribution of reflected radiation. Thus, reflectance factor standards that are sufficiently rugged or cleanable to use as permanent transfer standards, with the exception ofthe sintered PTFE standards, depart considerably from the perfect diffuser in the geometric distribution of reflected radiation.

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