Measurement of ultraviolet radiation flux by distributed radiance method

Wang Zhen, Pei Tongsheng (State Key Laboratory of Modern Optical Instruments, Zhejiang University, Zhejiang University Tricolor Instrument Co., Ltd., Hangzhou, 310027)

Abstract: In recent years, with the biological effects of ultraviolet light sources, people have paid more and more attention, and its evaluation has become more important. The total radiant flux is an important technical indicator of the ultraviolet light source. In this paper, the basic principle and method of measuring ultraviolet radiation flux by distributed photometry are briefly described, and the cosine correction problem of ultraviolet detector is analyzed and discussed.
Key words: ultraviolet radiation; radiant flux; ultraviolet light power

1 Introduction <br> UV radiation is optical radiation wavelength range of about 10 ~ 400nm. In this wavelength range, different wavelengths of ultraviolet radiation have different effects. In research and application, ultraviolet radiation is often divided into: A-band (400-320 nm); B-band (320-280 nm); C-band (280-200 nm). ); vacuum ultraviolet band (200 ~ 10nm). Ultraviolet radiation having a wavelength of less than 200 nm cannot propagate in the air due to absorption by the atmosphere.
The development and application of ultraviolet light source is in a period of rapid development. The continuous development of ultraviolet light source has gradually filled the gap of light source in various ultraviolet bands. A series of different types of ultraviolet light sources are available: ultraviolet mercury lamps, ultraviolet metal halide lamps, ultraviolet fluorescent lamps, xenon lamps and other ultraviolet light sources.
A remarkable feature of ultraviolet light is that it has a biological effect [1], which refers to the physiological change of the human body or organism when the ultraviolet light illuminates the human body or the living body. For example, when ultraviolet rays are irradiated to the human body, the skin is pigmented and the skin is blackened. Another example is that the bacteria die soon after being irradiated by short-wave ultraviolet rays. Another example is that when the human body is irradiated with ultraviolet light of a certain wavelength, the disease resistance is enhanced, the skin regenerative power is strengthened, and the hair growth speed is accelerated. In particular, the short-wavelength (peak wavelength λ=253.7nm) ultraviolet light source has a strong biological effect, and has the characteristics of no pollution, convenient use, and energy saving. Therefore, the disinfection and sterilization of radiation sources of this wavelength are widely used in the fields of biology, medicine, health, and epidemic prevention in the international and domestic industries. [2] At present, the ultraviolet light source usually uses an ultraviolet radiation meter to measure the irradiance of a certain surface (unit: W/m2) to indicate the ultraviolet intensity of the ultraviolet light source. In the application of air sterilization, water treatment, photochemical reaction, etc., it is more important to know the range of ultraviolet light source to a certain range of space (solid angle), or even the intensity of ultraviolet radiation radiated from the entire space. This is to be expressed in terms of ultraviolet radiation flux. There are usually two methods for measuring the radiant flux of an ultraviolet light source [3]: one is to compare the measured light with a measured lamp with a known radiant flux in the integrating sphere to determine the radiant flux of the lamp under test. Due to the absorption of ultraviolet light by the inner wall coating of the integrating sphere, the UV reflection of the coating is relatively low. In addition, due to the excitation of ultraviolet light, the organic solvent and glue in the coating will produce a fluorescent effect, so the inner wall coating of the integrating sphere must use a special ultraviolet reflective high-reflection diffuse material. Another method is to use a distributed radiometer to measure the irradiance of the measured light source in all directions of the space, and calculate the radiation flux. The former method is relatively simple, but the measurement accuracy is not as good as the latter. So this paper describes a measurement method based on distributed radiance method - distributed irradiance method.

2 Distribution radiance measurement method
2.1 Measurement principle The measurement principle of the radiance method is: based on the spherical coordinate system, as shown in Figure 1.

Figure 1 spherical coordinate system

As shown, by measuring the irradiance on the spherical surface, if the light source is used as the coordinate origin, the spherical coordinate system is taken, For the polar angle, for Azimuth, the light source is at any position on the imaginary sphere ( , The illuminance produced on the basis is E ( , ). The polar angle of the detector scanning is from 0 to 2Ï€, and the scanning azimuth is from 0 to the radiant flux emitted by the light source:

The Beijing Institute of Metrology has established a value transfer based on ultraviolet (254 nm) irradiance, so the UV illuminance value on a certain surface can be measured by a standard irradiance meter.

2.2 Variable-angle radiance method In order to achieve irradiance measurement at various points on the spherical surface, it is necessary to have a motion system that can rotate around the angle of Figure 1 and the angle. There are usually four systems:
a) the lamp is fixed and the detector is moved around the measuring lamp;
b) The detector does not move, let the light source be with The direction moves around the center of the lamp itself;
c) combined motion of the detector and the source, such as a detector Angular movement while the light source is Angular movement;
d) The movement of the mirror (fixed by the detector) is equivalent to the motion of the detector.
Each of the above four systems has its corresponding characteristics. For example, for a light source with a long measuring size, if the detector moves, the moving arm length of the detector becomes very long, and the space requirement for the laboratory is It must be great. For the light source sensitive to the direction of the ignition point, if the light source is used, the radiant flux will cause fluctuations during the measurement, so the fluctuation of the flux must be corrected. If the mirror movement mode is adopted, the mirror must be required to have a high reflection ratio against the ultraviolet rays, and the manufacturing requirements of the mirror are high.

<br> based measurement system 3 characteristics of the above four measurement system, we use the method of moving the light source, as shown in FIG. The measuring light source is rotatable about the same horizontal axis as the axis of the light source and about a vertical axis perpendicular to the axis of the light source. When the lamp is rotated about the vertical axis, the detector at the same level as the center of the turntable measures the irradiance value in each direction on the horizontal plane. When the lamp is rotated about the horizontal axis, the detector measures the irradiance values ​​in all directions on the vertical plane. And both the vertical and horizontal axes can rotate continuously within ±180°. In addition, a reference radiance meter fixed in position relative to the light source is used to monitor changes in the measured light source during the measurement process.

The structure of the entire measurement system is shown in Figure 3. The radiation signal in a certain direction of the space emitted by the ultraviolet light source placed on the rotating table is received by the ultraviolet radiation detector. After the electrical signal output by the detector is sampled by the radiometer system, the sampled data is transmitted to the upper microcomputer system. The upper computer processes the measurement data and calculates the radiant flux according to formula (1). At the same time, it can also give the radiation intensity distribution curve on any section of the space and the irradiance distribution map on a certain working surface in practical application. At the same time, the upper microcomputer system controls the turntable to rotate in two directions through the turntable control system, so that the system can measure the irradiance of the light source in any direction of the space.

4 Cosine correction of the detector <br> A radiation intensity is Point light source at distance to it Irradiance produced on the plane Radiation intensity with the radiation source In proportion to, and distance The inverse of the square is expressed as:
If the normal to the plane is at an angle to the ray , the irradiance produced by the point source on the plane is:

The above formula is called the cosine law of irradiance [4]. In order for the measurement results to comply with this rule, a cosine modifier must be added before the UV radiation probe. For short-wave ultraviolet irradiance probes, materials such as quartz quartz glass or UV-transparent PTFE are commonly used, as shown in Figure 4. Since the transparency of the quartz glass itself is very good, even if the double-side sanding is poor in scattering, when measuring an ultraviolet light source having a large size, the angle of view is large, which causes a large error. In order to get a good cosine correction effect, people tend to work hard on the structure, but the results are not satisfactory. The integrating sphere is a good diffuser with a sharp-edged entrance hole whose directional response approximates the ideal "cosine" response. [5] So we use the method of adding an integrating sphere in front of the detector to correct the cosine, as shown in Figure 5. For the cosine response of the UV detector, we tested it at 5° intervals using an ultraviolet variable angle radiation test bench. The test results are shown in Figures 6 and 7.

The dotted line in Fig. 6 is the normalized ultraviolet cosine response curve measured after adding the integrating sphere, and the solid line is the ideal curve. Fig. 7 is the cosine response curve of the ultraviolet irradiance meter probe which is conventionally produced by the planar diffuser. Comparing Fig. 6 with Fig. 7, it can be seen that the integrating sphere is an ideal cosine modifier, and the cosine error is less than 3% in the normal 50° range. However, the ultraviolet diffusing material sprayed on the inner wall of the integrating sphere is obviously affected by environmental dust and temperature, and is generally suitable for laboratory precision measurement applications.

4 Measurement results <br> The detector leaves the center of the variable angle turntable at ?0 meters. After the calibration of the instrument, we measured 4 straight tubular 254 UV lamps (2 30W, 2 5W), and Each lamp was measured repeatedly three times. Radiant flux average of 5W254 UV lamp 0.740W, repeatability of 2.10%, average radiation efficiency 15.9%; average flux of 30W UV lamp 6.707W, repeatability of 2.96%, average radiation efficiency 22.9%. The measurement data of the three lamps is shown in Table 1.

Table 1 Measurement data

Voltage V

Current I

Power P

Radiant flux

Radiation efficiency

Maximum radiation intensity

5W

31.7v

160mA

4.6W

0.704W

15.3%

0.088w/sr

5W

32.1v

160mA

4.7W

0.776W

16.5%

0.092W/sr

30W

102.0v

297mA

29.2W

6.599W

22.6%

0.95W/sr

30W

102.5v

300mA

29.5W

6.815W

23.1%

0.96W/sr

Finally, we also obtained the radiation intensity distribution curve of a 30W straight tubular UV lamp in polar coordinates (Fig. 9).

The measured data is stored in a format similar to IES luminosity data, so the existing illuminating design software can be used to simulate the illuminance distribution of the UV germicidal lamp in the actual workplace. As shown in Fig. 8, the illuminance illuminance distribution map of the ultraviolet mercury lamp horizontally placed on the working surface at a distance of 6 meters from the lamp tube. There are related requirements for the UV intensity required for effective killing of different microorganisms and bacteria. Therefore, based on the ultraviolet radiation intensity distribution data measured by the actual ultraviolet lamp, the suspension height of the lamp, the spatial distribution of the lamp, and the required irradiation time can be determined. Therefore, the distributed radiation intensity method can not only accurately measure the ultraviolet radiation flux, but also provide detailed angular distribution data for practical application design.

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