Description
Measurements of radiometric characteristics of emitters in the range of 180 - 1100 nm
Precision photometry with a distribution radiometer
In the conditions of the emergence of modern objects of research - LEDs and devices based on them, whose parameters stand apart from all the light sources existing before them, there is an urgent need to develop methods for measuring their parameters, because the classical options for using traditional measuring instruments are applicable only in general cases and, as experience has shown in their use in parallel with radiometric methods, they cannot be highly reliable in comparison with the latter.
The conversion factor of photometers (the value in the dimensions of the electric current (voltage) proportional to the illumination of the sensitive part of the photometer) is determined with some error relative to a standard source of type “A”, corresponding to the emission spectrum of an absolutely black body (blackbody) with a color temperature of 2856K Consequently, any deviation of the spectral distribution of the radiation power (luminous flux) of the measured source from the specified one will cause an increase in the measurement error. And if we talk about such quasi-monochromatic sources as LEDs, with a half-width of the spectrum in 10-25 nm, then the error can increase by an order of magnitude, especially at the edges of sensitivity: in the zone of short-wave blue and long-wave red. An increase in the error will also inevitably occur when measuring the white LEDs of the luminescence color on the basis of the phosphor due to a significant fraction of blue in the emission spectrum. To reduce the measurement error of illumination (luminous intensity) by such photometers, an adjustment is made to the conversion coefficient of photodetectors with known values relative to standard sources with reference to a specific emission spectrum of the measured sources. However, it requires an accurate measurement of the spectrum of their radiation, which also introduces its error. And, nevertheless, if all the calibrations and calculations are performed correctly, then the described refinement to the value of the conversion coefficient will significantly reduce the measurement error.
However, the cases of measurements using a photometer given here are private in more wide-range radiometry, which covers the entire range of electromagnetic radiation. And, if we limit this range to visible light, then radiometric methods can be successfully applied for photometric purposes. First of all, this is due to the absence of any optical filter at the input of the radiometric head, which uniquely excludes a whole class of errors associated with the accuracy of the correction of the OFS receiver as close as possible to the V (l) function. Of course, the measurement of the spectrum of the measured source is mandatory here, because the calculation of the conversion coefficient of the radiometer will be based on its results. However, to increase the accuracy of measurements with a photometer, it will also be necessary, therefore, according to the labor intensity, these 2 processes can be approximately equated with the only difference that it will be necessary to calculate photometric values from the energy using the spectral luminous efficiency found during the measurements. Thus, we can conclude that direct measurements of energy units using a radiometer are much more accurate and correct from the point of view of the physics of the measurement process. It should be noted that all the above reasoning is valid provided that the gonioradiometric method is used to measure the integrated radiation power in the far field, as the most accurate of the existing ones. It is this method that is used in the ARCHILAYT laboratory using Speckord installations (State Register of SI No.39537-08) and Flax goniophotometric installations (State Register of SI No.39536-08 and No. 39535-08) (Patent No. 130394), and Together with the above-mentioned version of the use of radiometers of a special design (Patents No. 130404, No. 130394), it allows to measure photometric and radiometric units with the highest accuracy.
Measurement of radiometric parameters of irradiators according to GOST R 8.760—2011 in comparison with the proposed gonioradiometric method
This standard defines the method for measuring the energy illuminance and radiation power of irradiators using radiometers - dosimeters or irradiation meters. The document recommends a number of formulas for calculating the designated units, however, key dependencies have extremely uncertain, and even dubious variables, the values of which are not intended to be measured during the study - they must be taken from some passports of measuring instruments, or taken as a certain fact, the definition of which refers to goniometric method. Examples of marked formulas are shown below, where conditionally accepted variables are shown in red.
Dependence for calculating the energy illumination
Ec = (Ei-Ej) K (ψ, φ) / Kτ
K (ψ, φ) is the relative angular correction factor (from
SI passports).
Dependence for calculating the radiation power
P = γ Ec R2
γ is the geometrical factor determined when measuring the angular distribution of the radiation flux using a goniometer.
An illustration demonstrating how the method recommended in the standard is private: a correct measurement can be made only if the source size (for example, a bactericidal lamp) and radiometric distance are appropriate. However, the normalization of exposure from bactericidal lamps is carried out, as a rule, at the same distance - 1m. Under this condition, the reliability of measurements of lamps of various sizes will be extremely low.
In contrast to the described method, the use of a distribution radiometer allows not only to exclude the vast majority of methodological measurement problems, but also significantly expands the possibilities of using the information obtained in the study. For example, the design of the parameters of the energy illumination of premises by irradiation facilities in the DIALux environment directly in units of energy illumination. An example of such use of the data is shown in the figure.
Thus, emphasizing the versatility of the proposed method of measuring radiometric parameters of irradiators using a distribution radiometer, unlike the standard GOST R 8.760—2011 standard that regulates this type of measurement, it can be summed up that in our case, the accuracy of determining radiometric characteristics does not depend on:
- Forms of the spatial radiation pattern of the source
- source sizes
- Distances from the source
- Spectral distribution of radiation (180 - 1100 nm)
- The range of measured values (0.01 - 10 000 W / sr)
- Allows simultaneous receipt of the entire set of parameters
The elements of the Radiometry and Precision Photometry Program have been successfully used by ARCHILAYT for measurements of radiation energy exposure for the purposes of photobiological safety studies within the framework of the mandatory standard GOST R IEC 62471 - 2013 "Photobiological safety of lamps and lamp systems", as well as when performing measurements and calculations of parameters of lighting devices and systems for lighting and additional illumination of plants in greenhouses (Program No. 14).
Detailed examples of the various dependencies and characteristics contained in the measurement reports for this Program are given in the Samples of Appendices to the Measurement Protocol.
