Abstract
The spectral radiance is an important parameter in the thermal radiative property of a material. It plays an important role in the emissivity and temperature measurements made by a pyrometer, in the analysis of the radiative heat transfer between several objects, in an infrared missile, and so on[
The first spectrometers with complete structure have been developed by Kirchhoff and Bunsen in 1859. Spectral radiance measurement apparatus is very important to measure the spectral radiance of materials and analyze material composition in the fields of aerospace, remote sensing, materials, and the military[
In this work, a portable spectral measurement apparatus is designed to measure the spectral radiance of object in the wavelength range 2.1–4.1 μm.
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Figure
Figure 1.Diagram of spectral radiance measurement apparatus.
Spectral measurement apparatus proposed is improved as described next. MCT detectors with the wavelength range 2–20 μm must work in the condition of cooling by liquid nitrogen which cannot be provided in most occasions. PbSe detectors have a high response in the wavelength 1–5 μm and work at room temperature. So in the wavelength range from 2.1 to 4.1 μm, PbSe detectors but not MCT detectors are chosen. In order to minimize the size and weight of the apparatus and improve the application condition, Cores Luxell 256 module made by New Infrared Technologies Company in Spain is introduced to detect light signal and process the signal, which integrates 256 pixel line array PbSe detectors, amplifiers, analog-to-digital (AD) convertors, and Universal Serial Bus (USB) output interface.
PbSe detectors working under room temperature have a temperature drift, which affect its output. Electric aperture is applied to eliminate the effect of temperature drift and is normally closed. In every measurement period, the zero point is corrected first. Then electric aperture is opened. The light from the object is detected and converted by Luxell 256. The output of Luxell 256 is acquired with USB interface, displayed in the screen, and stored in the personal computer (PC) disk. Then electric aperture is closed. This measurement period ends.
Measurement principle is described next. According to Planck’s law, spectral radiance of a blackbody was calculated by the blackbody temperature[
The detector output proportional to the radiation of the sample or blackbody is given by
The output of the apparatus
In our work, the wavelength and the response function of the apparatus are calibrated.
The monochromator and the halogen lamp are used to calibrate the wavelength of the apparatus. The monochromator separates the light from halogen lamp into a desired monochromatic light, which enters the apparatus. By changing the monochromator wavelength from 2.1 to 4.1 μm, the peak output of the apparatus is detected. Meanwhile, the peak wavelength and corresponding pixels are recorded in Table
Peak wavelength (μm) | Pixel |
---|---|
2.101 | 31 |
2.625 | 81 |
3.149 | 131 |
3.644 | 181 |
4.102 | 230 |
Table 1. Peak Wavelengths and Corresponding Pixels
The wavelength resolution is 10 nm
Quadratic polynomial is used to calibration the relationship between the wavelength
After fitting, the relationship is obtained as Fig.
Figure 2.Relationship between the wavelength and the pixel.
We use the standard blackbody to calibrate the response function of the apparatus. The blackbody furnace with the diameter 80 mm is separately heated to the temperatures of 473 and 1073 K and set about 100 mm in front of the apparatus. The output of the apparatus
Figure 3.Output of the apparatus at 473 and 1073 K.
Figure 4.Spectral radiance of the blackbody at 473 and 1073 K.
Calibrated response function of the apparatus is shown in Fig.
Figure 5.Response function of the apparatus.
In order to verify the apparatus, we detect the blackbody at the temperature 973 K. Measured spectral radiance and theoretical spectral radiance are shown in Fig.
Figure 6.Measured spectral radiance and theoretical spectral radiance for the blackbody at 773 K.
In conclusion, a portable spectral radiance measurement apparatus based on Cores Luxell 256 is designed to measure the spectral radiance of the object in the wavelength range from 2.1 to 4.1 μm. This apparatus operates without cooling. Electric aperture is applied to eliminate the effect of temperature drift. The wavelength resolution is 10 nm. Response function of the apparatus is calibrated with the blackbody at 473 and 1073 K. The relative error between the measured and theoretical spectral radiance for the blackbody at 773 K is 2.3%. For its integration, the total weight of the apparatus is less than 6 kg, and the size is
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