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Method for the Measurement of the Output of Monochromatic (254 nm) Low-Pressure UV Lamps#

Contributing authors on the IUVA Manufacturers’ Council

Oliver Lawal (Aquisense), Bertrand Dussert (Xylem), Craig Howarth (Hanovia), Karl Platzer (Light Sources), Mike Sasges (Trojan), Jennifer Muller (Trojan), Elliott Whitby (Calgon Carbon), Richard Stowe (Fusion UV), Volker Adam (Heraeus), Dave Witham (UVDI), Stuart Engel (Sanuvox), Phyllis Posy (Atlantium) and Arjan van der Pol (Phillips)

Revised by James Bolton (Bolton Photosciences) and Michael Santelli (Light Sources)

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Figure 1


Figure 2

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  • # A draft of this protocol was originally published in 2008.
  • * A glossary of terms is provided in Appendix A.
  • 1 For the condition where D = 2L, the field of view is ±14° (28° full view). The recommended cosine response should be an f2 factor (according to CIE publication 69 or DIN 5032) of 1.5 percent or less for class A rating, within this field of view. The cosine response will be verified in the Validation of Cosine Response section of this protocol.


This protocol has been developed to present a consistent methodology for the determination and benchmarking of UV lamp output from monochromatic (254 nm) lamps operated by a corresponding power supply (ballast). The protocol can be used for testing and comparing different lamp and ballast combinations, to compare test results from different laboratories and to compare operation under different ambient conditions. The protocol is not intended for general manufacturing quality control or quality assurance testing, or as a replacement for biodosimetry for determining the dose in UV water treatment systems. The protocol should be conducted by someone who is a UV expert and preferably by a third-party independent contractor.

These tests only yield information on the UV efficiency of the lamp being tested in the orientation in which it is tested (often but not exclusively horizontally) under open-air conditions where the lamp is allowed to achieve a steady-state with the surrounding still air at room temperature. The results cannot be extrapolated to the UV efficiency of UV lamps under other conditions, such as inside a quartz sleeve in a UV reactor, unless thermal operating conditions of the lamp are taken into account under each circumstance.

Monochromatic lamps include tubular low pressure and low pressure high output (e.g., amalgam) lamps that are typically used in water and air treatment (e.g., disinfection, AOP etc.) applications. The protocol described herein is not recommended for medium pressure, pulsed, folded, non symmetrical or other special lamps (e.g., excilamps – also called excimer lamps).


Based on the work of Keitz (1971), the following formula is recommended for calculating the total UV output from a UV lamp with a monochromatic (e.g., 254 nm) output. The lamp output power P can be calculated from Equation 1 (the Keitz formula)*:

where (see Figure 1)

  • E is measured irradiance (W m-2)
  • D is distance (m) from lamp center to the UV sensor.
  • L is the lamp arc length (m) from electrode tip to electrode tip.
  • α is the half angle (radians) subtended by the lamp at the sensor position. That is, tan α = L/(2D)

This expression has been tested by comparing with goniometric measurements of lamp output, and by comparing results from laboratories in different countries (Sasges et al. 2007). The results are considered accurate within 5 percent and have shown good agreement between laboratories.

Necessary conditions

  1. Measurements shall be conducted in still room air, not in a moving air stream.
  2. The lamp orientation shall be horizontal.
  3. Reflected light must be avoided (e.g. through use of baffles, differential measurement with beam stops). Appendix A examines two possible methods that can account for or minimize reflection.
  4. The UV sensor must have an adequate cosine response for the lamp length and distance used. Cosine response means that the UV meter should have an output that is proportional to the cosine of the angle of the input beam to the normal to the UV meter surface.
  5. In order to assure that the UV meter can “see” the entire arc length of the UV lamp, D should be at least twice L.1

Temperature conditions
Low-pressure and amalgam lamps are affected by their operating temperature, which is in turn affected by their surroundings, air temperature, etc. These lamps generally will exhibit increasing UV output with increasing temperature after ignition until an optimum temperature is reached, and then a decreasing output with further increases in lamp temperature. This behavior is shown in Figure 2, denoted as a “slightly overheated lamp.” It is desirable to measure a lamp under these slightly overheated conditions, so that the maximum output can be measured. Lamps shall be measured at a stable and constant air temperature. The entire warm-up curve of irradiance vs. time shall be reported including the maximum peak. Room temperature shall be documented and included in the report.


The lamp output reported shall be measured after a new-lamp 100 h burn-in period. The lamp output reported shall be based on lamps operating under air conditions, in which the lamp has reached a maximum output and then decreases to steady state, indicating that the lamp has passed through an optimum into an overheated condition. This will generate a UV irradiance curve as a function of time, which will illustrate the maximum and steady state output values.

Lamp and ballast efficiency
Lamp output power is generally compared with the electrical (line) power consumed in order to calculate the efficiency of the lamp/ballast system, or compared to the electrical power delivered to the lamp to calculate the efficiency of the lamp. It is necessary that either electrical power be measured accurately, so the efficiency may be accurately determined. This electrical power measurement must be done using calibrated instruments for power measurement, using equipment such as power analyzers with adequate frequency response. In particular, it is not sufficient to measure the ballast voltage and current to obtain the lamp power by multiplication.

The following traceability of calibration, standard method must be confirmed, showing calibration within one year:

  1. Radiometer with a detector traceable to a national laboratory (e.g. NIST, PTB, NPL, NRC etc.). The calibration for the UV radiometer used must be valid and traceable for calibration in the UVC range, and it must include a wavelength of 254 nm. If a spectroradiometer is used, then only the output between 250 and 260 nm shall be included in the calculated output.
  2. The radiometer or spectroradiometer calibration to be validated by a qualified third party and/or accredited facility.
  3. Confirmation for calibration of the power analyzer.

Detailed method

General remarks

  1. Avoid reflected light during the measurement of the UV light.
    1. Use non-reflecting materials, such as flat black paint, for walls, floor, baffles.
    2. Be aware that the UV reflectance may be different from reflectance in the visible range.
    3. Test: to check the amount of reflected light, compare the sensor signal to that measured when direct irradiation is blocked out, where a suggested procedure is described in Appendix B, part 1. Report the corrected result.
    4. If reflection reduction is desired, a suggested method is described in Appendix B, part 2.


  1. Do not expose uncovered skin or eyes to UV radiation.
  2. Use adequate protective equipment, such as a UV safety shield, gloves and UV goggles. Not all plastic or glass safety glasses block UV below 300 nm. The transmission characteristics of safety glasses should be checked prior to use.


  1. Adjust the lamp and detector at a suitable height (e.g., 0.5 to 1.0 m) over the ground.
  2. Check validity of calibration for all devices that influence results:
    1. radiometer
    2. spectroradiometer
    3. electrical equipment (power analyzer, multimeter)
    4. thermometer
    5. Make sure the electrical power measurement equipment is appropriate for the desired measurement.
  3. Warm up all devices.

Validation of cosine response

  1. The cosine correction for radiometers and spectroradiometers is critical to the proper measurement of the UV irradiance. The cosine correction must be confirmed by the following method for each lamp and detector combination, so that the lamp measurements are consistent within and between laboratories.
  2. Validation of cosine response and the resulting minimum distance Dmin where measurements for a given combination of lamp and detector can be performed as follows:
    1. Take readings of the UVC Detector for different distances (detector position perpendicular to lamp axis), recommended range from D = L/2 to 4 L.
    2. Take several readings of the UVC irradiance. For example, moving the detector from the closest point to the most remote point and then back again.
    3. Average the irradiance readings for each distance.
    4. Calculate the UVC power from the measured irradiances using Equation 1 (the Keitz formula) for each distance.
    5. Plot calculated UVC power versus distance.
    6. At a certain distance (Dmin) the UV output should become independent of distance. Generally one finds that Dmin > 2L.
    7. Measure at least at one distance greater than Dmin.
  3. The distance derived by this method is valid for the combination of this lamp length and this individual detector.

Measurement procedure
(instructions to the person supervising the tests)

  1. Record or monitor ambient temperature (±1 °C tolerance). The measurement thermometer should be near the test apparatus but not in a region where the temperature would be affected by heat from the lamp.
  2. Determine that the distances for radiometer readings are valid.
  3. Start recording the readings (UVC irradiance, electrical measurements, etc.) after the lamp is turned on.
  4. Sampling rate: should be matched to the rate of changing of the UV intensity readings.
  5. Rate of ~1 reading every 10 s is often sufficient to mark the maximum.
  6. Record the ambient temperature again after the lamp has been turned off.
  7. Calculate peak and steady state UV power using the Keitz formula. The peak UV power value is the value where the influence of temperature is reduced to a minimum and which can be compared to results of other laboratories.
  8. Calculate the lamp efficiency either based on lamp power (Equation 2a; top) or power from the wall (Equation 2b; bottom) (optional) as:

Report content

Measurement report to include:

  1. Description and certification of the organization and persons supervising the tests.
  2. Full and detailed information about the lamp (e.g., manufacturer, identification etc.).
  3. Full and detailed information about the ballast (e.g., manufacturer, identification etc.).
  4. Lamp orientation during testing (horizontal required).
  5. Active arc length L (between the ends of the filaments for “linear” lamps).
  6. Measurement of the distance D from lamp center (with tolerance) to the “calibration plane” of the radiometer detector.
  7. Room temperature (°C).
  8. Sensor and radiometer brand, model number and serial numbers for the radiometer, detector and any filters or other optical elements (e.g., diffuser) on the detector.
  9. Valid, traceable radiometer or spectroradiometer calibration documentation.
  10. Plot of irradiance vs. time after ignition, with an indication of the peak irradiance values and the point on the curve where the efficiency calculations were made.
  11. Calculated peak UV power with uncertainty, and fraction of reflected light subtracted to arrive at power reading. The uncertainty should include equipment uncertainty. The method to determine the reflected light should be stated.
  12. Electrical power meter (e.g., brand, model number and serial numbers for the power meter). Confirmation of calibration or calibration certificate for the electrical power meter.
  13. Measured voltage and current into the ballast.
  14. Measured electrical power across the lamp and “from the wall” with uncertainty.
  15. Calculated lamp efficiency (%) both with respect to the electrical power consumed by the lamp and the “from-the-wall” (optional) electrical power.


Keitz. H.A.E. 1971. Light Calculations and Measurements, Macmillan and Co. Ltd., London, UK.

Sasges, M.R., van der Pol, A., Voronov, A. and Robinson, J.A. 2007. Standard Method for Quantifying the Output of UV lamps, Proc. International Congress on Ozone and Ultraviolet Technologies, Los Angeles, CA, August, International Ultraviolet Association, PO Box 28154, Scottsdale, AZ, 85255.