AC NO: 20- 74 DATE: 29 July 1971
ADVISORY
CIRCULAR
- AIRCRAFT POSITION AND ANTICOLLISION LIGHT MEASUREMENTS
DEPARTMENT OF TRANSPORTATION
FEDERAL AYIATION ADMINISTRATION Initiated by: FS-130
29 July 1971 AC 20-74
FOREWORD PURPOSE. This advisory circular contains useful information concerning measurements for intensity, covering and color of sircraft position and anticollision lights.
2. REFERENCES. Federal Aviation Regulations FAR 23.1385 through 23.1397 and 23.1401. FAR 25.1385 through 25.1397 and 25.1401. FAR 27.1385 through 27,1397 and 27.1401. FAR 29.1385 through 29.1397 and 29.1401.
Advisory Circular AC 20-30A.
3. BACKGROUND.
a. This advisory circular has been developed as a reference for those concerned with data on measurements of aircraft position and anticollision lights. Light measurement is quite complex, and users of this advisory circular will have various degrees of experience and training. For these reasons, chapter one contains educational and reference material on the properties of light. It includes a description of light and discusses the general parameters.
b. Chapter two includes information on types of measurements and descriptions of the equipment used to make them.
Chapter three is devoted to discuasions on measurement data. First, the Federal Aviation Regulations are shown by pictorial representations. Following this, measurement data considerations are given including the precautions which should be observed when making measuremencs.
d. The appendices include a glossary of terms, a conversion table, a bibliography and a discussion of tristimulus colorimetry as applied to aircraft position and anticollision light measurements.
Acting Director, Flight Standards Service
29 July 1971 AC 20-74
TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS ¡ii
LIST OF TABLES CHAPTER 1. PROPERTIES OF LIGHT 1 General
2. Intensity
3. Color CHAPTER 2. MEASUREMENT CONSIDERATIONS 13
4. General S. Inte-sity
6. Color
7. Temperature Effects 19 CHAPTER 3. MEASUREMENT DATA 21
8. General 21
9. Position Light Data 21
10. Anticoll1sion Light Data 24 APPENDIX 1. GLOSSARY OF TERMS (2 pages) 1 APPENDIX 2. PHOTOMETRIC TERMS (1 page) APPENDIX 3. TRISTIMULUS COLORIMETRY AND AVIATION LIGHTS (2 pages) 1 APPENDIX 4. BIBLIOGRAPHY (2 pages) 1 Page 1 (and 11)
29 July 1971 AC 20-74
LIST OF ILLUSTRATIONS
TITLE FIGURE PAGE The Visible Spectrum 1.1 Standard-Observer Luminous-Efficiency Curve (C.I.E.) 1.2 3 Intensity and Illumination 1.3 Spectral Triatimulus Values According to the 1931 C.I.E. Standard Observer 1.4 7 C.l.E. Chromaticity Diagran 1.5 10 Strip Recording of Flash 2.1 15 Position Lighta, Minim Intensity in the Horizontal Plane 3.1 27 Position Lights, Minimm Intensities in Vertical Planes 3.2 28 Anticollision Lights, Minimum Intensity in Vertical Plane 3.3 29 Typical Horizontal Candlepower Distribution-Position Light 3.4 30 Typical Vertical Candlepovar Distribution-Position Light 3.5 31 Vertical Distribution of Rotating Beacon 3,6 32 Time vs. Intensity curves 0° and 5° for Rotating Beacon 3.7 33 Iime va. Intenaity - 20° for Rotating Beacon 3.8 344 Typical Flashtube Measurement Deta Sheet 3.9 35 Page 111 (and 1v)
29 July 1971 AC 20-74
LIST OF TABLES
TITLE TABLE PAGE Spectral Luminous Efficiency, V(A) 1.1 Spectral Tristimulus Values 1.2 8 Computation Form 2.1 .17 Page v (and vi)
29 July 1971 AC 20-74 CHAPTER 1
PROPERTIES OF LIGHT GENERAL. Light 18 visually evaluated radiant energy. Like other forms of radiant energy, light travels through space at a constant velocity of 300,000,000 meters per second. Light energy may be considered as having a sinusoidal wave form, stimulating vision only over a narrow band of wavelengths (Fig. 1.1). Within this band, the amplitude affects the visual sensation of brightness and the wavelength, the visual sensation of hue. The longest waves produce a sensation of red, and the shortest a sensation of violet. By definition, white occurs when all visible wavelengths are combined in equal amounts. The appearance of white, however, may be produced when certain critical ones are combined. If the light covers a narrow band of wavelengths, a certain hue is seen.
Black is usually treated as the absence of stimulation. The wavelength or 11ght may be expressed in micrometers (um), equal to 10-6 neters, in nanoneters (nm), equal to 10-9 meters, or in angstroms (¿°), equal to 10-10 meters.
WAVELENGTH IN METERS Ю-н 10-12 10-* Ю~ 10-1 10-2 10* 10% 10°
NAYS X-RAYS ULTRA- VIOLET RATS - RED RADAR RADIO WAVES CIRCUITS A-C
VISIBLE SPECTRUM
VIOLET OLUE OREEN YELLOW ORANSE RED 400 500 600 700
- WAVELENGTH IN NANOMETERS
THE VISIBLE SPECTRUM Figure 1.1 Chap 1 Par 1 Page 1
AC 20- 74 29 July 1971 The sensitivity of the eye, however, varies within this visible spectrum.
Radiant energy at different wavelengths produces varying sensations of brightness even though the amount of energy received is the same at each wavelength. Figure 1.2 and Table 1.1 show that the eye is twice as sensitive to a yellow-green of 555 nanometers, as it 1s to a green of S10 nu. This curve 1s referred to as ispectral sensitivity of the human eye" or "luminous efficiency". When the intensity of colored lights is measured, this variable sensitivity of the eye must be taken into consideration. In other words, a red light must be much higher in power to appear equally as bright as a green light. The detecting device, therefore, must be corrected for the response of the standard observer if the reading 1s to indicate lumínous (visual) output. The numbers associated with Figure 1.2 and Table 1.1 are referred to as ¿spectral luminous efficienciesi. It should be remembered that lumens and candelas are associated with visible light, and as the sensitivity of the eye decreases, so does the amount of lumens for the same radiant power. This is evidenced in the conversion from watts to lumens:
luminous flux in lumens • 680 V()) times the radiant flux in watts.
2. INTENSITY. the light a ented quafly bail entered ta, ma per sourent if by a sphere. Light flux is rate of flow of visible energy. The basic unit of flux is the lumen which by definition is equal to 1/4n times the total flux emitted by a uniform point source of one candela. The flux emitted by a point source per unit solid angle (steradian) is called intensity. A steradian is defined as that solid angle originating at the center of a sphere and subtending an area on the sphere surface equal to the square of the sphere radius. Intensity is measured in lumens per steradian, and a uniform point source equal to one candela has an intensity in every direction of one lumen per steradian. Intensity in a given direction is usually expressed in candelas and is often called candlepower.
The luminous flux density received on a surface (illuminance) varies with the intensity of the source, and inversely as the square of the distance from the source to the surface. Illuminance is expressed in lumens per unit area or footcandles. Figure 1.3 shows that as the distance from a source increases, one lumen is spread over increasing areas, and the illuminance decreases. The relation of illuminance to distance from the source is referred to as the "Inverse square law". The illuminance from a given source varies inversely as the square of the distance from that source. Doubling the distance causes the illuminance to decrease to one fourth. As measurements of a light source are usually done with instruments which measure illuminance, the distance must be known before the intensity of the source can be determined. In other words, footcandles times the distance squared gives candelas. For example, if a footcandle reading of 10 is measured at 10 feet from the source, the intensity is 1000 candelas.
Chap 1 Page 2 Par 1
29 July 1971 AC 20-74 VIOLET BLUE 1 GREEN YELLOW RED 1,0
SPECTRAL LUMINOUS EFFICIENCY, V(A) 380 420 460 500 540 580 620 660 700 740
WAVELENGTH OF RADIANT ENERGY IN NANOMETERS STANDARD-OBSERVER LUMINOUS EFFICIENCY CURVE (C.LE.)
Figure 1.2 Chap 1 Par 2 Page 3
AC 20-74 29 July 1971 1931 C.I.E. STANDARD OBSERVER
WAVELENGIH SPECTRAL LUMINOUS WAVELENGIE SPECTRAL LUMINOUS NANOMETERS EFFICIENCY, V (N NANOLETERS EFFICIENCY, V.(2) 380 0.0000 580 0.8700 390 0.0001 590 0 - 370 400 0.0004 600 0.6310 410 0.0012 610 0.5030 420 0.0040 620 0.3810 430 0.1116 630 0.2650 440 0.0230 640 0.1750 450 0.0380 650 0.1070 460 0.0600 660 0.0610 470 0.0910 670 0.0320 480 0.1390 680 0.0170 490 0.2080 690 0.0082 500 0.3230 700 0.0041 510 0.5030 710 0.0021 520 0.7100 720 0.0010 530 0.8620 730 0.0005 540 0.9540 740 0.0003 550 0.9950 750 0.0001 560 0.9950 760 0.0001 570 0.9520 770 0.0000
SPECTRAL LUMINOUS EFFICIENCY, VA Table 1.1 Chap 1 Page 4 Par 2
29 July 1971 AC 20- 74
- INTENSITY = I CANDELA, = 1 LUMEN/STERADIAN
• 1 METER 1 CM- W = 1 STERADIAN FLUX= ILL AREA: | CM?
ILLUMINANCE = | LUMEN/CM?
AREA = 1FT 3 ILLUMINANCE = I LUMEN/ FT?
AREA = I METER?
ILLUMINANCE = 1 LUMEN/ M?
INTENSITY AND ILLUMINATION Figure 1.3 Flashing lights are used oxtensively as signals and warnings because of their superiority in attracting attention. Because of this characteristic, flashing beacons were established as the required lighting for anticollision lights. These beacons are of several types: rotating, flashing incandescent, oscillating and gas discharge (strobe).
when a light signal consists of separate flashes, the maximum intensicy during the flash must be greater than the intensity of a steady light to have the same apparent intensity. It 18 convenient to evaluate flashing lights in terms of their EFFECTIVE INTENSITY, le or ETI, the intensity of a steady light which will appear equally bright when viewed at threshold, and is expressed in candelas.
Chap 1 Par 2 Page 5
AC 20- 74 29 July 1971 Current airworthiness requirements for aircraft anticollision lights specify the following equation (known as the Blondel Rey equation) for the computation of effective intensity:
where: le • effective intensity I(t) = instantaneous intensity as a function of time t2 - t1 - flash time interval (seconds) The maximum computed value of effective intensity is obtained when t2 and t1 are chosen so that the effective intensity is equal to the instantaneous intensity at t2 and t1. For short time flashes, t2 - t1 becomes insignificant compared to 02 seconds, and the total flash is integrated.
Short-duration flashtube lights have been used primarily as supplementary lights. Since flashtubes in general produce relatively small proportions of red light, about 90 percent of the light is lost in passing it through a red filter. Therefore, in order to use this source for an anti-collision light, it is necessary to operate them at higher energy levels than has been common in the small supplementary 11ghts.
3 . COLOR. The sensation of color is closely related to the wavelength of light and varies with the individual and the conditions of viewing.
Usually a color 1s said to have three psychological components: hue (red, blue, orange, etc.), brightness, and saturation (the amount a color differs from a grey of the same brightness). A measure of hue, sufficiently reliable for signal-color specification, is the wavelength of the part of the spectrun required to be mixed with the equal-energy (white) source to produce the color; this wavelength is called the dominant wavelength of the color.
ratio of the distance on the CIE chromaticity diagram (See Figure 1.5) from the equal-energy point to the color point, to the distance fron equal-energy point, in the same direction, to the spectrum locus. This ratio is called purity.
Chap 1 Page 6 Par 3
29 July 1971 AC 20-74 It can be shown that any color can be matched by combining three properly chosen colors called primary colors. It a colored sample is placed in one half of a photometric field, a mixture of three primary colors such as red, green, and blue, in the other half of the field can be made to match the colored sample to the satisfaction of the eye. This is done by varying the relative brightness of the three primary colors. A problem with this method is that the matching judguent of one observer cannot be taken as representative of the average person. Only when a large number of observers are used in each experiment can consistent values be obtained for the relative brightness of primary colors required to match any given color. In order to avoid this difficulty, the tristimulus method was revised and standardized. Three primary colors were agreed upon; then, by.
experiments with a number of normal observers, standard values for the relative amounts of each primary color were established to match each wavelength in the visible spectrum. These numbers were associated with a "standard observer". With these values made available in 1931 by the Commission Internationale de L'Eclairage (CIE), a more objective and economical technique 18 available to specify color. Figure 1.4 is a graphical presentation of the relative amounts of each primary color required to match any wavelength. Standard notation for these functions 18: & for the red primary, y for the green primary, and 2 for the blue primary. It should be noted that the f function has been adjusted to correspond to the luminous-efficiency function (Figure 1.2). Table 1.2 shows the values of these functions in tabular form.
the of we time natured toù 1O a
SPECTRAL TRISTIMULUS VALUES ACCORDING TO THE 1931 CIE STANDARD OBSERVER Chap 1 Par 3 Page 7