rus
Issue #8/2017
S.G.Kireev, V.P.Arkhipov, S.G.Shashkovsky, N.P.Kozlov
Measurement of Spectral and Energy Characteristics of Pulsed Radiation Sources of Continuous Spectrum
Measurement of Spectral and Energy Characteristics of Pulsed Radiation Sources of Continuous Spectrum
The method of measuring spectral distribution of the radiation energy of pulsed light sources of continuous spectrum is described in the article. The method is tested on a radiation pulse with a duration of about 120 μs.
Теги: measurement of spectral and energy characteristics of pulsed rad spectrometer uf-radiation source источники излучения уф-диапазона спектрально-энергетические характеристики импульсных источников спектрометр
INTRODUCTION
The constant expansion of the spheres of application of artificial radiation sources in everyday life once again shows the importance of light sources in our life. Radiation sources (RS) are used in such spheres as lighting, film projection technology, control of various photochemical and photobiological processes, photo etching and photolithography, creation of electronic components. In recent decades, pulsed RSs of the continuous spectrum, used in film projection technology, photography, medical technology [1,2], laser pumping, etc., have been widely used.
A fairly wide range of metrological equipment and techniques has been developed for recording the radiation from continuous combustion RSs. For example, successfully applied photophysical methods based on the internal or external photoelectric effect, and photochemical recording of radiation based on changes in the optical or physical properties of a substance under the influence of light are well-known [3]. The above methods make it possible to obtain reliable results when recording sources of continuous combustion, in particular, linear or monochrome emission spectra. However, recording the RSs of the pulsed continuous spectrum causes difficulties and leads to significant errors.
The need to determine the quantum yield of the reaction as a function of the spectral composition of the radiation, the requirements for the thermal stability of the material used, the impossibility of using the substance repeatedly, and the practical inapplicability of existing actinometers to detect short-wave ultraviolet (UV) radiation in the range 200–240 nm [4] make the photochemical registration method time-consuming intensive and inapplicable.
The most widespread among devices based on the photophysical method of recording radiation received photodiodes, the principle of which is based on the appearance of EMF in the illumination of the semiconductor surface. Convenience in application, repeatability of results, speed, as well as a wide range of the offered photodiodes for different spectral ranges by such well-known companies as Sglux and Hamamatsu make us pay close attention to this area. Main application: measurement of irradiance generated on the receiving surface from such common monochromatic radiation sources as low-pressure mercury lamps, excimer lamps, lasers. In this case, ignoring the fact of broadening of the spectral lines, considering the known absolute sensitivity of the photodetector at the radiation wavelength Kλ in W/m2 · V and the detected value of the signal from the photosensor U in V, the irradiance value of E = Kλ · U is calculated. By taking into account the spectral broadening of the lines and the relative spectral characteristics of the photodetector, one can increase the accuracy of the measurement.
However, the measurement of irradiance from the polychromatic RS introduces significant errors due to the impossibility of considering the spectral distribution of radiation over the spectrum.
Pyroelectric sensors are often used to measure the radiation power of lasers operating based on the appearance of electric field in a crystal with a change in its temperature. The good sensitivity and stability of the readings provided pyroelectric sensors a wide spread in the field of measuring the laser power. To date, many companies offer pyroelectric heads with a wide range of energy sensitivity, time resolution, spectral range (Fig. 1). Furthermore, the algorithms incorporated into the processing computer allow choosing the wavelength of the laser, thereby considering the spectral sensitivity, which greatly improves the user-friendliness of the sensor. All the above facts allow you to select a receiver depending on the RS parameters.
Depending on the using a pyroelectric sensor to measure the radiation of pulsed RS, there is a number of limitations. First, in addition to the sensor itself, it is necessary to purchase a special computerized device that processes the signal from the sensor and considers its calibration parameters. The cost of such a set often exceeds 4000 US dollars. Second, the sensor is affected by any radiation sources falling within the spectral range of its sensitivity. When working with sufficiently powerful RS, any heating elements (e. g., electrodes, bulb) will introduce a significant error in the measurements by means of radiation in the infrared range. Third, unlike photodiode receivers of radiation, exceeding the threshold energy density leads to damage to the sensitive surface and to a change in its characteristics. Fourth, when recording the radiation of continuous spectrum, a constant spectral sensitivity of the sensor is necessary. For the receiver with the BB Pyro head (Fig. 1), which has the most constant sensitivity, the maximum difference is ≈ 8%, which introduces an additional error in the measurement result. Fifth, there is no possibility of obtaining energy characteristics in certain spectral ranges, even considering the possibility of using optical glasses.
None of the methods listed above makes it possible to obtain reliable results of measuring the radiation parameters of continuous spectrum pulsed radiation sources in view of, at least, spectral energy distribution.
The energy distribution over the radiation spectrum can be taken into account in the RSs subject to the approximation of absolutely black body (black body). In this case, the energy brightness measured [6] makes it possible to construct the emission spectrum and, considering the known relative spectral sensitivity of the photodetector, to calculate the characteristics of the radiation incident on the photodetector pad. The departure from the black body approximation and, often, the complexity of measuring the energy brightness of sources with a freely expanding radiation body [7], introduce significant errors in the output parameters.
A technique is known for measuring absolute values of energy illumination using a combination of a monochromator that emits a narrow spectral range and a pyroelectric sensor. The authors of article [8], consistently isolating 10-nm range sections from the emission spectrum using a monochromator, irradiated the receiving area of the pyroelectric radiation detector, thereby obtaining energy illumination at the sensor level with a step of 10 nm (Fig. 2a).
However, this technique creates several problems. A slight discrepancy in the form of the spectra of energy illumination (Fig. 2) is due to the unstable sensitivity of the receiving head of the pyroelectric sensor. A more significant disadvantage is the time spent on obtaining the spectrum of energy illumination: to obtain a spectral distribution in the 80-nm range, the authors were forced to repeat the radiation detection operation 8 times. An increase in the accuracy of registration, and therefore a decrease in the spectral step, will lead to a proportional increase in the number of operations and time to obtain the result.
THEORETICAL SUBSTANTIATION
OF THE METHOD
As an alternative method of obtaining absolute characteristics of radiation from the continuous spectrum, it is suggested to use a set of obtained data from a spectrometric device recording the spectral distribution of the optical signal by its decomposition on the dispersion element and the focusing on the photosensitive matrix, and a photodiode-based photophysical receiver (PBPR) with a RC-circuit with the output voltage equal to the integral of the charging current of the capacitor.
The main advantage of the spectrometer is that the manufacturer considers the spectral sensitivity of the photodetector pad, which makes it possible to speak of the independence of its sensitivity of the wavelength. Thus, the spectrometer makes it possible to obtain the real spectral distribution of the incoming radiation in relative units.
The choice of the spectrometer is based on three basic requirements: the spectral sensitivity of the device must fall within the spectral range of the lamp radiation, the integration time must significantly exceed the pulse length, the sensitivity of the receiving matrix must not be saturated with the incident radiation.
The photodiode receiver is selected based on the requirements for exceeding the RC-chain time constant over the duration of the radiation pulse, but less than the flare period and on the occurrence of the spectral sensitivity of the PBPR in the spectral range of the radiation detection by the spectrometer.
The PBPR, depending on the spectral sensitivity range, is first verified by the approved method [9], [10], which provides the spectral distribution of the relative sensitivity Sλ, normalized to its maximum value, and the value of the volt sensitivity, which characterizes it in relation to the circuit of switching on the receiver, at the wavelength of the maximum sensitivity Smax [V · m2/W].
The photodetectors are located at a distance sufficient to hit the entire RS in the effective field of view of the receiver in such a way that the cosine of the angle of incidence of the radiation on the photosensitive surfaces can be neglected.
Let us write down the RC-circuit equation by expressing the signal obtained at the output through the photodiode pad reaction [11]:
, (1)
where URC is the signal at the RC-circuit output, V; Uin is the signal at the RC-circuit input, V; τ is the time constant of the chain, s; t1 are the instants of time characterizing the beginning of the radiation pulse and the termination, s.
Let us express the PBPR reaction at the RC-circuit input through the incident spectral distribution of the radiation power, taking into account the normalized relative sensitivity:
, (2)
where Φ (λ, t) is the spectral illuminance, W/(m2 · nm); Sλ (λ) is the distribution of the normalized relative sensitivity of the sensor; λ1 and λ2 are the wavelength ranges within the spectral sensitivity of the sensor.
Considering that the time integral of the spectral distribution of irradiance is the energy illumination, we substitute (2) in (1) and, taking into account the known value of the volt sensitivity of the PBPR, we express the integral of the spectral distribution of the energy radiation:
, (3)
where E (λ) is the spectral distribution of the energy illumination, J/(m2 · nm).
The reaction spectrometer is proportional to the incident radiation:
, (4)
where Kspec is the coefficient of proportionality of the reaction of the spectrometer, J/ref.unit; Espec (λ) is the reaction of the spectrometer to the incoming spectral distribution of the energy illumination, ref.unit/(m2 · nm). By substituting (4) into (3) and expressing Kspec, we obtain:
. (5)
By substituting the calculated value of the proportionality coefficient in (4), we obtain the spectral distribution of the energy illumination coming to the receiving area of the spectrometer.
EXAMPLE OF OBTAINING SPECTRAL
AND ENERGY RADIATION DISTRIBUTION
As a continuous spectrum RS, a pulsed xenon flash lamp with an interelectrode distance of 120 mm and an inner diameter of 5 mm was studied. The lamp shell was made of quartz glass with absorption in the UV–C region of not more than 15%. The discharge circuit provided a radiation pulse with a duration of about 120 microseconds.
The radiative characteristics were recorded using a light-emitting optical fiber spectrometer with a high sensitivity in the ultraviolet range AvaSpec-ULS2048-USB and a spectral error of no more than 0.05 nm and an PBPR UV Sensor "TOCON-C6" recording radiation in the range of 220–275 nm with a maximum sensitivity of 255 nm and a time constant of 31.4 ms [12]. The PBPR was carried out in the All-Russian Scientific Research Institute of Optical and Physical Measurements (ARSRIOPM) in advance, and a distribution of the relative sensitivity Sλ normalized to the maximum value (Fig. 3) and the value of the volt sensitivity at a wavelength of 253.7 nm were obtained.
URC taken from the radiation pulse oscillograph pattern using PBPR (see Fig. 4) was 0.3 V. The relative spectral distribution recorded by the spectrometer taking into account the normalized relative sensitivity of the PBPR is shown in Fig. 5.
Substitution of the integral of the spectral distribution of radiation, taking into account the PBPR relative sensitivity, the volt sensitivity, the radiation maximum signal with the PBPR, and the time constant into formula (6), makes it possible to calculate the value of the coefficient of the calibration coefficient for the spectrometer Kspec. Calculating the value of the energy illumination by formula (5) and taking into account the distance to the RS, we obtain the spectral distribution of the energy luminosity per unit of solid angle of the pulsed xenon lamp (Fig. 6).
CONCLUSION
Even though the photodiode receiver registered the radiation in a relatively narrow spectral range, it was possible to obtain absolute values of the spectral energy distribution in a wider range of spectrometer registration.
A method of measuring the spectral distribution radiation energy of the continuous spectrum pulsed light sources is obtained and tested. The method is characterized by its simplicity, reliability of preliminary calibration and efficiency of obtaining the result. The method is tested on a radiation pulse with a duration of about 120 µs, but it can be used to record both shorter and longer pulses, since it is limited only by a temporary resolution of the equipment used.
The constant expansion of the spheres of application of artificial radiation sources in everyday life once again shows the importance of light sources in our life. Radiation sources (RS) are used in such spheres as lighting, film projection technology, control of various photochemical and photobiological processes, photo etching and photolithography, creation of electronic components. In recent decades, pulsed RSs of the continuous spectrum, used in film projection technology, photography, medical technology [1,2], laser pumping, etc., have been widely used.
A fairly wide range of metrological equipment and techniques has been developed for recording the radiation from continuous combustion RSs. For example, successfully applied photophysical methods based on the internal or external photoelectric effect, and photochemical recording of radiation based on changes in the optical or physical properties of a substance under the influence of light are well-known [3]. The above methods make it possible to obtain reliable results when recording sources of continuous combustion, in particular, linear or monochrome emission spectra. However, recording the RSs of the pulsed continuous spectrum causes difficulties and leads to significant errors.
The need to determine the quantum yield of the reaction as a function of the spectral composition of the radiation, the requirements for the thermal stability of the material used, the impossibility of using the substance repeatedly, and the practical inapplicability of existing actinometers to detect short-wave ultraviolet (UV) radiation in the range 200–240 nm [4] make the photochemical registration method time-consuming intensive and inapplicable.
The most widespread among devices based on the photophysical method of recording radiation received photodiodes, the principle of which is based on the appearance of EMF in the illumination of the semiconductor surface. Convenience in application, repeatability of results, speed, as well as a wide range of the offered photodiodes for different spectral ranges by such well-known companies as Sglux and Hamamatsu make us pay close attention to this area. Main application: measurement of irradiance generated on the receiving surface from such common monochromatic radiation sources as low-pressure mercury lamps, excimer lamps, lasers. In this case, ignoring the fact of broadening of the spectral lines, considering the known absolute sensitivity of the photodetector at the radiation wavelength Kλ in W/m2 · V and the detected value of the signal from the photosensor U in V, the irradiance value of E = Kλ · U is calculated. By taking into account the spectral broadening of the lines and the relative spectral characteristics of the photodetector, one can increase the accuracy of the measurement.
However, the measurement of irradiance from the polychromatic RS introduces significant errors due to the impossibility of considering the spectral distribution of radiation over the spectrum.
Pyroelectric sensors are often used to measure the radiation power of lasers operating based on the appearance of electric field in a crystal with a change in its temperature. The good sensitivity and stability of the readings provided pyroelectric sensors a wide spread in the field of measuring the laser power. To date, many companies offer pyroelectric heads with a wide range of energy sensitivity, time resolution, spectral range (Fig. 1). Furthermore, the algorithms incorporated into the processing computer allow choosing the wavelength of the laser, thereby considering the spectral sensitivity, which greatly improves the user-friendliness of the sensor. All the above facts allow you to select a receiver depending on the RS parameters.
Depending on the using a pyroelectric sensor to measure the radiation of pulsed RS, there is a number of limitations. First, in addition to the sensor itself, it is necessary to purchase a special computerized device that processes the signal from the sensor and considers its calibration parameters. The cost of such a set often exceeds 4000 US dollars. Second, the sensor is affected by any radiation sources falling within the spectral range of its sensitivity. When working with sufficiently powerful RS, any heating elements (e. g., electrodes, bulb) will introduce a significant error in the measurements by means of radiation in the infrared range. Third, unlike photodiode receivers of radiation, exceeding the threshold energy density leads to damage to the sensitive surface and to a change in its characteristics. Fourth, when recording the radiation of continuous spectrum, a constant spectral sensitivity of the sensor is necessary. For the receiver with the BB Pyro head (Fig. 1), which has the most constant sensitivity, the maximum difference is ≈ 8%, which introduces an additional error in the measurement result. Fifth, there is no possibility of obtaining energy characteristics in certain spectral ranges, even considering the possibility of using optical glasses.
None of the methods listed above makes it possible to obtain reliable results of measuring the radiation parameters of continuous spectrum pulsed radiation sources in view of, at least, spectral energy distribution.
The energy distribution over the radiation spectrum can be taken into account in the RSs subject to the approximation of absolutely black body (black body). In this case, the energy brightness measured [6] makes it possible to construct the emission spectrum and, considering the known relative spectral sensitivity of the photodetector, to calculate the characteristics of the radiation incident on the photodetector pad. The departure from the black body approximation and, often, the complexity of measuring the energy brightness of sources with a freely expanding radiation body [7], introduce significant errors in the output parameters.
A technique is known for measuring absolute values of energy illumination using a combination of a monochromator that emits a narrow spectral range and a pyroelectric sensor. The authors of article [8], consistently isolating 10-nm range sections from the emission spectrum using a monochromator, irradiated the receiving area of the pyroelectric radiation detector, thereby obtaining energy illumination at the sensor level with a step of 10 nm (Fig. 2a).
However, this technique creates several problems. A slight discrepancy in the form of the spectra of energy illumination (Fig. 2) is due to the unstable sensitivity of the receiving head of the pyroelectric sensor. A more significant disadvantage is the time spent on obtaining the spectrum of energy illumination: to obtain a spectral distribution in the 80-nm range, the authors were forced to repeat the radiation detection operation 8 times. An increase in the accuracy of registration, and therefore a decrease in the spectral step, will lead to a proportional increase in the number of operations and time to obtain the result.
THEORETICAL SUBSTANTIATION
OF THE METHOD
As an alternative method of obtaining absolute characteristics of radiation from the continuous spectrum, it is suggested to use a set of obtained data from a spectrometric device recording the spectral distribution of the optical signal by its decomposition on the dispersion element and the focusing on the photosensitive matrix, and a photodiode-based photophysical receiver (PBPR) with a RC-circuit with the output voltage equal to the integral of the charging current of the capacitor.
The main advantage of the spectrometer is that the manufacturer considers the spectral sensitivity of the photodetector pad, which makes it possible to speak of the independence of its sensitivity of the wavelength. Thus, the spectrometer makes it possible to obtain the real spectral distribution of the incoming radiation in relative units.
The choice of the spectrometer is based on three basic requirements: the spectral sensitivity of the device must fall within the spectral range of the lamp radiation, the integration time must significantly exceed the pulse length, the sensitivity of the receiving matrix must not be saturated with the incident radiation.
The photodiode receiver is selected based on the requirements for exceeding the RC-chain time constant over the duration of the radiation pulse, but less than the flare period and on the occurrence of the spectral sensitivity of the PBPR in the spectral range of the radiation detection by the spectrometer.
The PBPR, depending on the spectral sensitivity range, is first verified by the approved method [9], [10], which provides the spectral distribution of the relative sensitivity Sλ, normalized to its maximum value, and the value of the volt sensitivity, which characterizes it in relation to the circuit of switching on the receiver, at the wavelength of the maximum sensitivity Smax [V · m2/W].
The photodetectors are located at a distance sufficient to hit the entire RS in the effective field of view of the receiver in such a way that the cosine of the angle of incidence of the radiation on the photosensitive surfaces can be neglected.
Let us write down the RC-circuit equation by expressing the signal obtained at the output through the photodiode pad reaction [11]:
, (1)
where URC is the signal at the RC-circuit output, V; Uin is the signal at the RC-circuit input, V; τ is the time constant of the chain, s; t1 are the instants of time characterizing the beginning of the radiation pulse and the termination, s.
Let us express the PBPR reaction at the RC-circuit input through the incident spectral distribution of the radiation power, taking into account the normalized relative sensitivity:
, (2)
where Φ (λ, t) is the spectral illuminance, W/(m2 · nm); Sλ (λ) is the distribution of the normalized relative sensitivity of the sensor; λ1 and λ2 are the wavelength ranges within the spectral sensitivity of the sensor.
Considering that the time integral of the spectral distribution of irradiance is the energy illumination, we substitute (2) in (1) and, taking into account the known value of the volt sensitivity of the PBPR, we express the integral of the spectral distribution of the energy radiation:
, (3)
where E (λ) is the spectral distribution of the energy illumination, J/(m2 · nm).
The reaction spectrometer is proportional to the incident radiation:
, (4)
where Kspec is the coefficient of proportionality of the reaction of the spectrometer, J/ref.unit; Espec (λ) is the reaction of the spectrometer to the incoming spectral distribution of the energy illumination, ref.unit/(m2 · nm). By substituting (4) into (3) and expressing Kspec, we obtain:
. (5)
By substituting the calculated value of the proportionality coefficient in (4), we obtain the spectral distribution of the energy illumination coming to the receiving area of the spectrometer.
EXAMPLE OF OBTAINING SPECTRAL
AND ENERGY RADIATION DISTRIBUTION
As a continuous spectrum RS, a pulsed xenon flash lamp with an interelectrode distance of 120 mm and an inner diameter of 5 mm was studied. The lamp shell was made of quartz glass with absorption in the UV–C region of not more than 15%. The discharge circuit provided a radiation pulse with a duration of about 120 microseconds.
The radiative characteristics were recorded using a light-emitting optical fiber spectrometer with a high sensitivity in the ultraviolet range AvaSpec-ULS2048-USB and a spectral error of no more than 0.05 nm and an PBPR UV Sensor "TOCON-C6" recording radiation in the range of 220–275 nm with a maximum sensitivity of 255 nm and a time constant of 31.4 ms [12]. The PBPR was carried out in the All-Russian Scientific Research Institute of Optical and Physical Measurements (ARSRIOPM) in advance, and a distribution of the relative sensitivity Sλ normalized to the maximum value (Fig. 3) and the value of the volt sensitivity at a wavelength of 253.7 nm were obtained.
URC taken from the radiation pulse oscillograph pattern using PBPR (see Fig. 4) was 0.3 V. The relative spectral distribution recorded by the spectrometer taking into account the normalized relative sensitivity of the PBPR is shown in Fig. 5.
Substitution of the integral of the spectral distribution of radiation, taking into account the PBPR relative sensitivity, the volt sensitivity, the radiation maximum signal with the PBPR, and the time constant into formula (6), makes it possible to calculate the value of the coefficient of the calibration coefficient for the spectrometer Kspec. Calculating the value of the energy illumination by formula (5) and taking into account the distance to the RS, we obtain the spectral distribution of the energy luminosity per unit of solid angle of the pulsed xenon lamp (Fig. 6).
CONCLUSION
Even though the photodiode receiver registered the radiation in a relatively narrow spectral range, it was possible to obtain absolute values of the spectral energy distribution in a wider range of spectrometer registration.
A method of measuring the spectral distribution radiation energy of the continuous spectrum pulsed light sources is obtained and tested. The method is characterized by its simplicity, reliability of preliminary calibration and efficiency of obtaining the result. The method is tested on a radiation pulse with a duration of about 120 µs, but it can be used to record both shorter and longer pulses, since it is limited only by a temporary resolution of the equipment used.
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