COMMISSION INTERNATIONALE
DE L'ECLAIRAGE
INTERNATIONAL COMMISSION ON ILLUMINATION
INTERNATIONALE BELEUCHTUNGSKOMMISSION
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COMMISSION INTERNATIONALE
DE L'ECLAIRAGE |
INTRODUCTION
This workshop, organized jointly by CIE Div.2 and Div.1, is motivated by the report from R2-17 Aviation Photometry. The reporter noted recent activities in the United States to develop the measurement procedures for aircraft anticollision lights (SAE ARP5029) and the flashing light photometric standards, which attracted international attention. These procedures and standards were needed for enforcement of the federal regulation on the maintenance of the anticollision lights of all commercial aircraft, which affected airlines and aircraft manufactures worldwide.
During the development of ARP5029, many questions were raised on physical measurement methods of flashing lights and also on the visual perception of flashing lights. The effective intensity (defined by the Blondel-Rey equation) is widely used for flashing light measurements, but is based on experiments done for central vision almost ninety years ago. It addresses the visual perception only under limited conditions. Flashing lights are used in many other safety devices such as traffic barricade lights, which also require accurate intensity measurements. The issue of flashing lights is one of the long lasting problems in visual signaling and still has no perfect solution.
This workshop will investigate the problems in photometry of flashing lights from the viewpoint of new developments and research in both physical measurements and visual science, and will address future work for CIE.
The workshop is not just a series of presentations, but is designed to provide a forum for discussions. We will first have several introductory presentations by experts in the aspects of history of flashing light photometry, visual perception of flashing lights, recent developments in measurement technologies and standards, and industrial applications and needs. The presentations are followed by an open discussion session with participation from the audience. The following presentations are planned.
PRESENTATIONS
1. Historical Overview of Flashing Light Photometry
Starting from the experiments of Blondel-Rey, Allard, and others, the description of the effective intensity of flashing lights is derived. Including the experiments of Douglas, Projector, Kishto, and others, the formula for calculation of the effective intensity for any pulse shape will be described. As a sample the inclusion of this formula into the ECE-Regulations will be discussed.
2. Relevant Work in CIE Division 1 (Supplementary System of Photometry)
Division 1 is establishing a photometric system based on apparent brightness at any luminance levels, which is called "supplementary system of photometry." Temporal aspect of light is regarded as one of the critical factors affecting apparent brightness. A need to introduce efficiency of flashing lights into the supplementary system of photometry will be considered.
3. Development of SAE ARP 5029 on the Measurement of Aircraft Anticollision Lights
Anticollision lights have evolved from rotating incandescent beacons to the Xenon discharge lights that are used currently. The initial U. S. Federal Aviation Administration (FAA) requirements for effective intensity were 100 candelas of aviation red light in the horizontal plane based the Blondel-Rey relationship. Later revisions of FAA requirements increased the effective intensity requirements to 400 candelas of either red or white light. Most aircraft anticollision light measurements of effective intensity up to 1990 were done in laboratories of light manufactures or in other well-equipped and staffed laboratories. Test constraints necessary for relatively good agreement were communicated through technical society activities and the aircraft certification process. Maintenance of anticollision lights in service by the airlines until the early 1990s consisted of daily visual inspections, with service being performed when this inspection revealed a deficiency.
In the early 1990 period, the FAA developed new maintenance requirements that anticollision lights be maintained by the airlines at or above the required certification intensity of 100 or 400 candelas in the horizontal plane. This led to industry concern about the accuracy of measurements. The concern being that it is difficult to effectively maintain lights that may be only 25% above the required level initially when the uncertainty of the measurement plus other biases is near or above 25%. Establishment of a task group under the Society of Automotive Engineers (SAE) A20 Aircraft Lighting Committee to develop an SAE Aerospace Recommended Practice (ARP) was one result of the industry efforts. The purpose of SAE ARP 5029 is to define test constraints, develop detailed test procedures and equipment recommendations, specify a photometer calibration source and traceability, and estimate the uncertainty for laboratory, shop and field measurements. In conjunction with development of SAE ARP 5029, NIST has developed a flash photometer calibration process which is discussed by Y. Ohno of NIST.
4. Establishment of the NIST Flashing-Light Photometric Unit
Upon request from FAA, NIST undertook the task to establish flashing-light photometric standards to provide calibration services for anticollision light photometers. The work was completed in 1997. A flashing-light photometric unit (lux second, [lx·s]) has been realized based on the NIST detector-based candela, using four standard photometers equipped with current integrators. Two different approaches were taken to calibrate these standard photometers: one based on electrical calibration of the current integrator, and the other based on electronic pulsing of a steady-state photometric standard. The units realized using these two independent methods agreed to within 0.2 %. The relative expanded uncertainty (k=2) of the standard photometers, in the measurement of white xenon flash, is estimated to be 0.6 %. The standard photometers are characterized for temporal response, linearity, and spectral responsivity, to be used for measurement of xenon flash sources of various waveforms and colors. Calibration services have been established at NIST for flashing-light photometers with white and red anticollision lights.
5. Frequency-Dependent Photocurrent Measurements of Flashing Lights
Two circuits for flashing light measurements will be discussed and compared. The first circuit has two stages. The first stage is a current-to-voltage converter that includes the photodiode sensor. This stage can be used to measure the frequency components of flashing light if the sensor has fast enough responsivity. The second stage is a voltage integrator with two controlled switches. One switch shorts the integrating capacitor before start and the other one controls the timing of the integration (measurement) cycle. In the second circuit, the sensor and the integrating capacitor are combined in one stage.
The fundamental gain equations (current-to-voltage gain, loop gain, and closed-loop voltage gain) of the two different measuring circuits will be determined for the frequency interval of the frequency components of the flashed pulse to be measured. Accuracy requirements for the photocurrent-to-voltage conversion will be discussed for the interval of all frequency harmonics of the flashing light pulse.
The effect of pulse length and amplitude of the flashing signal and the capacitance of the photodiode sensor for the measurement nonlinearity will be discussed briefly. Results of related publications will be evaluated.
6. Conspicuity of Point-Sequential Light Signals Used to Mark Emergency Vehicles
A high intensity discharge (HID) short-arc lamp coupled with a fiber-optic distribution system interrupted by rotating dichroic filter assemblies can provide rapidly alternating sequences of intense light pulses of different colors at the focal point of a single lens. Such signals have unique properties that strongly influence the way they function as emergency vehicle markers. In a point-sequential light signal (PSL), pulses of different colors alternate without apparent temporal gaps at a single point. There are interactions of psychophysical properties across the color boundaries. These include sequential color and brightness effects that are quite striking, and not predicted by any current theory. These sequential signal interactions in turn affect the function of the light as an emergency vehicle marker. Flashing signal lights embody trade-offs of conspicuity and trackability. A highly conspicuous flash pattern may be hard to track in traffic where there is considerable visual background noise. Using point-sequential lights (PSLs), it is possible to design signals which are optimum for particular background and traffic environments. PSL cycle times and pulse duration ratios can be modified over a wide range of values.
This initial series of investigations studied flash patterns incorporating two colors presented within a basic cycle time of 800 ms. The shorter duration color was present for 3%, 17%, 36%, 62% or 100% of the time occupied by the longer duration color. Perceptions of image size, color saturation and apparent point of origin varied with these ratios. A signal consisting of long red pulses interrupted by short white pulses seemed to provide a good combination of conspicuity and trackability.
Photometric documentation of these signals is complex, since the changes in intensity and color at the pulse boundaries include a brief dark phase, and the light pulse onsets and offsets are asymmetrical and nonlinear.
7. Photography and Flash Exposures
From the perspective of photographic film, essentially all exposures are "flash exposures", at least in the sense of being short duration. While xenon flash lamps are important "flash exposure" sources and will be discussed, steady state light (like daylight) sources plus some shuttering mechanism are more common. High precision in measurement and control of these short duration exposures is extremely important in the "flash exposing devices", the "sensitometers", used by those who design and build new films, where subtle differences in photographic properties between different coating formulations, etc. are easily masked by unintended variability in exposure sources. From a measurement perspective, there is much similarity in the metrology/photometry of flashes from xenon lamps and from flashes from a sensitometer. Some spectrally weighted integrated quantity of light, its spectral characteristics, and reproducibility of "flashes", from time to time and from one source to another, are similar concerns. For historical reasons, film sensitometry involves red, green and blue spectral sensitivity of films (itself derived from "spectral sensitometry") and measurements in photometric units. "Photographic flash lamp photometry" should include either spectral data or spectral weightings related to R, G, B film spectral sensitivities. Even flash duration may be similar; concern with "reciprocity effects" (different photographic sensitivities from the same quantity of exposing light, delivered over different exposure times) leads to sensitometer exposure times of 10-5 sec at Kodak, derived from cw light-source sensitometers. Shorter times still, representative of laser printing, are used in "laser sensitometers." There are obvious differences as well: sensitometers generally have a low flash repetition rate, usually have (shuttered or scanned) continuous light sources (greater stability), and invoke no psychophysics.
Granted the need for high precision in sensitometry in creating new films, and the need to define exposures accurately to determine ISO speeds accurately, the photographic industry's greatest need in flash light photometry will be improved sensitometer photometry and spectral radiometry.
Archive: First Announcement, Dec. 2, 1998