Peter Y. Y. Ngai

Spatial distribution of fluence rate from upper-room ultraviolet germicidal irradiation: Experimental validation of a computer-aided design tool

A commercial computer-aided design tool used by the lighting industry was modified to predict fluence rates for upper-room ultraviolet germicidal irradiation. Experimental validation based on more than 1600 measurements and 3 types of commercial ultraviolet fixtures, which was done in an experimental chamber and in a homeless shelter having fixtures in continuous use for over 7 years, showed differences in measured and predicted average upper-room fluence rates of less than 10%. The computer-aided design tool, however, was not very successful at predicting fluence rates at specific room locations, a capability that is needed for mating computational fluid dynamics with ultraviolet germicidal irradiation. Although not an objective of this study, it was also found that the three types of fixtures used in this study have surprisingly significant differences in efficiency based on fixture ultraviolet power output and electrical input. One fixture type had an efficiency that was more than five times that of another. For comparison purposes, a standard method for measuring and reporting fixture efficiency is needed.

A Radiometry Protocol for UVGI Fixtures Using a Moving-Mirror Type Gonioradiometer

Ultraviolet germicidal irradiation (UVGI), 254 nm UV-C, is increasingly used as an infection control strategy to reduce the spread of airborne pathogens such as tuberculosis (TB), influenza viruses, and measles. With the appearance of multidrug-resistant TB and emerging infectious disease such as severe acute respiratory syndrome (SARS) and H1N1 influenza viruses, engineering controls using 254 nm UV-C lamps within specialized luminaires, herein designated UVGI fixtures, are being installed in high-risk settings such as homeless shelters, hospitals, jails and prisons, and schools. Studies have established that a relatively uniform spatial distribution of UV-C in the upper room can effectively cleanse the air of aerosolized pathogens. However, for planning purposes, the placement of multiple UVGI fixtures in a space, to achieve uniformity of UV-C energy distribution using currently available lighting software, is not yet practical because no industry-wide standard method exists for radiometric measurement of commercial UVGI fixtures. In this article, standard methods for photometry and reporting of general fluorescent lighting luminaire photometric data are adopted to provide UVGI fixture spatial emission distribution data in an electronic file format. The ultimate expectation of the authors is that the results will lead to a software program for fixture placement, comparable to and as easy to use as the corresponding software used for general interior lighting applications. To accomplish this goal, a radiometry measurement system is developed to obtain the radiant intensity distributions of UVGI fixtures in a three-dimensional space. This system includes a moving-mirror Type C goniometer, a mirror, a radiometer, a desktop computer, the mechanical control hardware, and the data acquisition/presentation software. Repeated measurements were made on each of three exemplary UVGI fixtures, and measurement variation did not exceed ± 2.0%.

On Equivalent Sphere Illumination

Under the present equivalent sphere illumination (ESI) system, the actual visibility of a visual task is not known. The author recommends that for every different task and viewing angle, the maximum contrast sensitivity of the task be published; thus, one can determine the ESI required for any given task in order to achieve a certain visibility, or one can obtain task visibility by knowing its ESI value. This broadens the application of the ESI system by enabling conversion between ESI and visibility for a specific task and also conversions among different tasks and viewing angles. Moreover, differences in individual contrast sensitivity response may result in different ESI and visibility from the same task, with the same viewing angle, at the same location under the same luminous environment.

Some Issues on the Foundation of Visibility and Task Contrast

The paper demonstrates that adaptation luminance should be used instead of background luminance in determining supra-threshold visibility. Issues are raised on subjects of task luminance, background, adaptation luminance, luminance contrast, equivalent contrast and visibility level when adaptation luminance is considered.