About the Proper Wavelength for Pyrometry on Shock Physics Experiments

Abstract

Usually one wants to measure the thermal radiance emitted by a hotsurface at a wavelength as short as possible, since the uncertainty in the true temperature due to unknown emissivity decreases with decreasing wavelength. Unfortunately the radiance also decreases with decreasing wavelength, and hence the signal-to-noise ratio becomes worse with shorter wavelengths. Depending on what temperature range is to be covered, a reasonable compromise can be found for most applications. When pyrometry is applied to shock physics experiments, there is an additional factor that has to be taken into consideration. Due to the nature of shock physics experiments, one has to deal with background light caused by flashes from air lighting up, highexplosive light, and muzzle flash from a powder gun, etc. In addition, even if the experiment is designed appropriately, there is often a temperature nonuniformity as well as thermal radiation from transparent anvils that are used to increase the interface pressure. In most cases, there is no engineering approach to minimize these temperature non-uniformities. The sensitivity to these non-uniformities increases with decreasing wavelength for the very same reason that the sensitivity to uncertainties in emissivity is increasing. This paper describes the above problems, deals with the problem of temperature non-uniformity in detail, and presents arguments why single-wavelength pyrometry in shock physics experiments can be very deceiving even in well designed experiments.

Use of IR pyrometry to measure free-surface temperatures of partially melted tin as a function of shock pressure

“…These experiments, which can also quantify the percent age of surface area at higher radiance as well as the radi ance (temperature) of the hot=spots, were conducted and they confirmed the occurrence of hot-spots. The temper atures of the hot-spots and their fractional areas of the surfaces appear to be in good agreement with estimates obtained from the free-surface temperature at different wavelengths, assuming a simple two-temperature model [35].…”

Shock and release temperatures in molybdenum: Experiment and theory

“…Thermal radiation is described by the Stefan-Boltzmann relation, where the total power is proportional to the fourth power of temperature. Pyrometry measurements are often more accurate at shorter wavelengths where the power varies with higher powers of temperature [11]. Thus unquantified temperature variations (spatial or temporal, within the respective resolutions of the detectors) lead to an overestimate of the mean temperature.…”

Pyrometric measurement of the temperature of shocked molybdenum

“…(This can be inferred from the response of different wavelengths shortly after the time of shock breakout. For more details on spatial temperature non-uniformities see reference 20.) The background light at the free surface experiments was most likely caused by thermal radiation from APIEZON Q c , a soft, black, putty-like substance that was intended to absorb thermal radiation from hot jets formed at the edge of the sample/window interface.…”

“…FIG.6: Apparent window temperature as function of wavelength (squares) from experiment #06. The solid line is the result from fitting the measured data with a simple two temperature model20 , the results of the fitting process are presented in the text below.…”