By Lianfeng Zhang, Laboratory of Ecology and Environmental Protection, Research Institute of Tsinghua University

Measurement of the output of UV lamps is critical to the design of standards and guidelines for evaluating and regulating the function of UV devices. Herein, measurement terms are defined, methodology is discussed and standards are reviewed.

UV Output/Emittance and Irradiance/Fluence Rate

UV lamps emit UV photons and are characterized by their UV output. It is impossible to design or calculate a UV reactor without knowing the UV output of the lamps. In literature, there are many alternative terms for the UV output, such as source radiant power ¹, effective lamp power ², total UV output ³, UV light power ⁴, photon rate ⁵, radiant power ⁶, total UV power output ⁷, total power UV-C ⁸, the rate of energy emission ⁹, power output ¹⁰ and energy output. ¹¹ There also is the term “emittance,” which is the UV output divided by the surface area of the lamp. ¹² To characterize the radiation field around a UV lamp or UV device, the physical concepts of irradiance and fluence rate have been defined.

Irradiance is defined as “the total radiant power from all directions incident on an infinitesimal element of surface area dS containing the point under consideration, divided by dS.” ¹ Fluence rate is defined as “the radiant power passing from all directions through an infinitesimally small sphere of cross-sectional area dA, divided by dA. ¹ The concepts of irradiance and fluence rate describe the UV field itself, independent of the UV sources.

Measurement of UV Output of UV Lamps

There is no direct way to measure the UV output of UV lamps. A viable approach to obtain UV output is to establish a theoretical or mathematical relationship between the measured irradiance or flux rate and the UV output or emittance. So far, there is only one method to derive the UV output from measured irradiance: the Keitz equation, as seen in Equation 1 (see the IUVA protocols ^13-15 and the ISO standard ¹⁶).

Evaluation of UV Devices for Water Disinfection

There are various methods and probes to detect or measure irradiance and fluence rate in or around UV devices, such as photoelectric probes, particulate actinometers, chemical actinometers, biodosimetry and others.

Fluence rate measurement

Both biodosimetry and particulate actinometers can measure fluence rate, which is crucial for evaluating the UV reactors used for water disinfection. “Biodosimetry involves measuring the inactivation of a challenge microorganism after exposure to UV light in a UV reactor and comparing the results to the known UV dose-response curve of the challenge microorganism [determined via bench-scale collimated beam testing].” ¹⁷ Biodosimetry is included in the national standards of Austria, Canada, China and Germany. ^18-21 The particle actinometer uses fluorescent microspheres as surrogate microorganisms for measurements. ²²

Irradiance measurement

Photoelectric radiometers are the most commonly used UV measurement tool. Its probe has a small, flat window that serves as the receiver of UV photons, so the measured result is irradiance. Irradiance measurements can be an effective method for evaluating UV disinfection devices if the probes are positioned strategically. The measured readout is irradiance, but in water disinfection, fluence rate is the physical quantity that represents the disinfection effect of UV reactors. Therefore, a physical and mathematical relationship needs to be established between the fluence rate in water and the measured irradiance.

A photoelectric radiometer can measure the irradiance along the axis of the cylinder in a cylindrical UV reactor. Such measurements can be used to perform a comprehensive evaluation of UV reactors, which are influenced by four factors: reactor fabrication, fluid dynamics, UV transmission of the target water and the emission of UV lamps. Such measurements also can be used to evaluate lamps, provided the three factors, other than the lamp, are fixed. However, such an assessment has a limitation: It can make only relative comparisons among UV lamps. So far, there is no “bridge” that can convert the irradiance measured along the lamp axis into the UV output or emittance of the lamp. “Along the lamp axis” implicitly means that the probes are placed near the lamps. The Keitz equation is invalid for short measurement distances. ^13-16 In fact, the multiple irradiance values measured along the lamp axis provide limited information about how the lamp’s emission varies along the axis.

Three factors may cause variation in emissions along the axis:

1. A Faraday dark zone exists along a UV lamp ²³, meaning that, strictly speaking, the longitudinal UV emission is never uniform.
2. UV lamps are not perfectly straight. Quality control (QC) and quality assurance (QA) processes at lamp manufacturers set the maximum allowable limit for curvature.
3. It is reasonable to assume that the thickness of the lamp wall is not perfectly uniform.

Observing such academic or well-controlled variations/errors reveals that they possibly are too small to be detected in industrial measurements, such as simply placing a radiometer along the axis of a lamp. In literature, the longitudinal UV emittance always is considered to be uniformly distributed. ^3,6,24-25

Standards, Protocols and Guidelines of Nations, Organizations and Associations

In the IUVA protocols ^13-15 and the ISO standard ¹⁶, the method for measuring the UV output of UV lamps based on the Keitz equation has been proposed. This is the only globally recognized method. In fact, no other methods have been reported in literature or in government/organizational documents. Some researchers have attempted to use integrating spheres for such measurements. This approach is less feasible in the UV industry because it requires a standard lamp with known UV output. If the method based on the Keitz equation is used for metrological traceability in the measurement with integrating spheres, the measurement still is the IUVA protocols ^13-15 and the ISO standard. ¹⁶

Several countries have issued national standards/guidelines for evaluating and regulating “the characteristics and function of UV devices” (ÖNORM M 5873-1:2020 and DIN 19294-1:2020) ^{18, 21}, “UV equipment for drinking water applications” (Guidelines for Ultraviolet Disinfection of Drinking Water) ¹⁹ and “ultraviolet disinfection equipment” (HJ2522-2012). ²⁰ Although their specifications focus on the evaluation and regulation of UV devices for water disinfection, the annexes of ÖNORM, DIN and HJ2522-2012 include the method for measuring the UV output of UV lamps based on the Keitz equation (see Table 1). In other words, the measurement of the UV output of UV lamps is the key factor that links them. The IUVA protocols and ISO standard complement the national standards and guidelines for evaluating and regulating UV devices.

Summary

• IUVA and ISO each have released protocols and standards for measuring the UV output of UV lamps, the method in which provides the only method to convert the irradiance measured around the lamp into its UV output. ^13-16
• Austria, Canada, China and Germany each have issued national standards or guidelines for evaluating and regulating UV devices. ^18-21
• The national standards and guidelines, excluding the Canadian standard, incorporated the measurement methods for lamps from the IUVA protocol (in air) and the ISO standard as annexes. ^18-21

References

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The Correlation of UV Output and UV Delivered

UV Output: A 100-watt UV lamp consumes 100 W of electrical power (energy per unit time). This energy changes into three forms: heat, UV radiation and visible light. The portion of energy converted into UV radiation is called the UV output, typically measured in Watts (W).

UV Delivered: UV Delivered is a determination of how much UV power (W) reaches the surface at which it is directed. Since the surface has an area, the UV amount commonly is expressed as UV power per unit area (W / m²).

Relationship between UV Output and UV Delivered: UV output is an inherent property of a UV lamp, independent of its surroundings. UV delivered refers to the amount of UV radiation received by an illuminated surface, whether from a single UV lamp or multiple lamps. While UV delivered depends on the presence of UV lamps, it is not a direct property of any specific lamp. (Of course, without UV lamps, no UV can be delivered … but, that’s another story.)