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OCR for page 12
12 CHAPTER 2 Photometric Characterization Experiment The purpose of this experiment was to photometrically lated. The effective intensity was calculated for each of the characterize the light sources that are typically used in warning- flashing lights using four of the previously mentioned methods light conditions. The photometric measures include the color (Allard, modified Allard, Blondel-Rey-Douglas, and Form characteristics and the effective-intensity characteristics. A Factor). The spectral waveform and the time-based waveform more detailed description of the experiment and the results is were also included in the analysis. provided in Appendix C. Summary of Results Experimental Method To summarize the data, the results of the measurements The photometric characterization of the light sources were characterized by the lamp type and the color. Figure 1 was performed on 35 different devices. However, with al- shows effective intensity calculated by each of the four methods. ternative flash patterns, over 135 different flash patterns With the exception of the strobe lights, the results showed fairly have been investigated. The purpose of this work was to similar patterns, with the values from the Form Factor method identify suitable light sources to be used in the screening being just slightly lower than values from the Allard method, and the dynamic experiments. The technologies used in followed by the modified Allard and the Blondel-Rey-Douglas these devices included rotating beacon, strobe, halogen, methods. For the strobe systems, the Form Factor method and LED. The data measured for each device included time- provided much higher values than the other methods. It was averaged chromaticity, time-averaged luminous intensity, rep- also notable that the blue and red lights had lower excitation etition rate, and waveform. These measurements were made levels than the amber and white for all of the light sources tested, at the Center for High Accuracy Retroreflection Measure- and the LED had significantly lower values than the halogen ments (CHARRM) facility of the National Institute of Stan- and the strobe. dards and Technology (NIST). The photometer was capable Color characterization of the sources was also considered. of positioning the devices 0.5 to 30.5 m away from the light The colors for the red and the amber light sources all match detector. fairly closely, but the blue and the white colors were very widely dispersed. Based on these measurements, the relationship of the meas- Data Analysis ured data to the human response was considered. The most For each light source, the time-averaged chromaticity, the appropriate metric was determined through the correlation time-averaged intensity, and the effective intensity were calcu- of the response and the photometric data.

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13 2500 2000 Effective Intensity (cd) 1500 Form Factor Allard Modified Allard 1000 Blondel Rey Douglas 500 0 Red Red Red Amber Blue Amber Blue Amber Blue White White White Halogen LED Strobe Lamp Type Figure 1. Effective-intensity calculation by all four models.