AC NO: 20- 14 DATE: 29 July 1911
• ADVISORY -~ ~ CIRCULAR• AJ;BCB,AfI f9§JIJQN 4liP ANTTCQL1jISTQN LJQHT MEA§YBEMENI§
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION Initiated by: FS-130
29 .July 1971 AC 20-74
FOREWORD
1. PURPOSE. this advisory circular contains useful information concerning mea1urementa for intensity, covering and color of aircraft 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. BAQCGROUND.
a. this advisory circular has been developed as a reference for those concerned with data on measurements of aircraft position and anti• collision lights. Light measurement is quite complex, and users of this advisory circular will have various degrees of experience and training. Por 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. Ch.apter two includes information qn types of measurements·and descriptions of the equipment used to make them.
c. Chapter three ia devoted to discussions on measurement data. First, the Federal Aviation Regulations are shown by pictorial representa· tions. Following this, measurement data considerations are given including the precautions which should be observed when making measurements.
d. The appendices include a 1101sag of terms, a conversion table, a biblioaraphy and a discussion of tristimulus colorimetry as applied to aircraft position and anticollision ligt.t measurements.
Service
...· 29 J'uly 1971 TABLE OF CON'lENTS
LIST OF ILLUSTRATIONS
LIST OF TABLES CHAPT!lt 1. PROPER.TIES OF LIGHT
1. General
2. Intensity
3. Color CIIAPTEB. 2. MEASUl!NEN'l CONSIDBIATIONS
4. General
5. Inte-sity
6. P0lor ~
7. Teuaperature Effect•
8. General
9. Position Light Data
10. Anticolllslon Liaht Data APPENDIX 1. GLOSSARY OF TERMS APPENDIX 2 • PBOTCJmTlUC TERMS APPENDIX 3. TllISTIMtJLUS COLOR.IMB'l'R.Y AND AVIATION LIGHTS APPENDIX 4. BIBLIOGIAPBY AC 20· 74 Page iii V l 2 2 6 13 13 13 16 19 21 21 21 24 (2 pages) l (1 page) l (2 pages) l (2 pages) 1
29 .July 1971 AC 20- 74 LIST OP ILLUfflATIONS Ill!& The Visible Spectrum Standard •Observer Lum1iaou1•Efficiency Curve (C.I.E.)
Intensity and Illumination Spectral Tri1timulua Value• According to the 1931 C.I.E. Standard Obaerver C.I.B •. ChrotM.ticity Diagram Strip Recording of Pla1h Poaition.Ligbts, Minim&pa tnten11ty in the Horisontal Plane Position Lights, Minimum lntensities in Vertical Plane• Anticolliaion Light•, Minimum tntenaity in Vertical Plane Typical Borisontal Calldlepower Distribution•Poaition Light Typical Vertical c:anctlepowr Distribution•Poaition Light Vertical Distribution of Rotating Beacon Time vs. Intenaity curvea o0 ancl s0 for Rotating Beacon Time va. Intenaity • 2Q 0 for Rotating Beacon Typical Plashtube Measurement Data Sheet rICURE
1.1
1.2
1.3
1.4
1.5
2.1
3.1
3.2
3.3
3.4
3.5
3.6
3. 7
3.8
3. 9 PAGE - 1 3 5 7 10 15 27 28 29 30 31 32 33 34 35 Page iii(and iv)
29 July 1971 AC 20-74
LIST OF TABLES PACE!!!!::! ~ 4 Spectral Luminous Efficiency, V(A) 1.1 8 Computation Form 2.1 .17 Spectral Tristimulus Values 1.2 Page v (and vi)
•••• AC 20-7429 July 1971 CHAPTEI. 1 PllOPEB.'IIES,OF LIGHT
1. GEND.AL. Light is visually evaluated radiant energy •. Like other forms of radiant energy, light travels through space at a constant velocity of 300,000 1 000 me..ters 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 vuual sensation of brightneS8 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 sti111Ulation. The wavelength of light may be expressed in micrometers (~m), equal to 10-6 meters, in nanometers (am), equal to 10-9 meters, or in angstroms on, equal to 10-10 •tera. ·
WAVELENGTH IN METERS 10-1 104 10• lt&OIO WAVII •ttR&•ltl'.D •-c••o•• CIRCUITS ......I ......I ....I ... ....I ....
I .... ....
I ' .... I ....
I VISIBLE SPECTRUMI ' ',....
700 I .... < ..HN Yl&.LOW Olt&N& I II..,." ....
400 500 100
• WAVELENGTH IN NANOMETERS
1.11!,VISIBLE SPECTRUM !'igure 1.1 t--1--~-..,..,.--,,'--- uu••v10Ln .t.-,---4..--f..-__,J--...___,,..J-_--4 ...,.
Chap l Page 1Par 1
29 July 1971AC .20• 74 ,,.,,,.. ....The sensitivity of the eye, bovever, varies withia. this visible spectrum.
Radiant energy at.differeut 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 shaw that the eye is twice as sensitive to a yellow-green of 555 nanometers, as it is to a green of 510 nm. This cur.-e is referred to u "spectral sensitivity of the human eye" or "luminous efficienc.y". 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 IILISt be auch higher in power to appear equally as bright as a green light. The detecting device, therefore, 1111st be corrected for the response of the staa.dard observer if the reading is to indicate luminous (visual) output. · The numbers associatedwith Figure 1.2 and Table 1.1 are referred to as "spectral luminous efficiencies". 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 pover. This is evidenced in the conversion from watts to lwaens:
luminous flux in lumens • 680 V().) times the radiant flux in.watts.
2. INTENSITY. The luminous flux being emitted from a point source, if the light is ·emitted equally in all directions, may be represented 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 l/4w 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 1a.
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 illum:inance to decrease to one fourth. As measurements of a light source are usually done with instruments which measure illuminance, the distance ai.st 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 Par 1 Page 2
29 July 1971 AC 20-74 ,( -> • >u 2 LIJ - u I&.- 0.6 I&.
LIJ u, :, 0 z 2 - 0.4 :,_, _, ct a: t-u LLI G..
Cl) Chap 1 Par 2 ,.o
0.9
0.8
0.1
0.5 VIOLET BLUE GREEN YELLOWi I I I I ,r \ I 1 \ I \ I \ \ REO ..
0.3
0.2 O.t J I \ I \ V \ L/ '0 380 420 460 500 540 580 620 660 700 740
WAVELENGTH OF RADIANT ENERGY IN NANOMETERS STANDARD-OBSERVER LUMINOUS EFFICIENCY CURVE ( C.1.E.)
Figure. 1.2.
AC 20- 74 29 July 1971 1931 C'.I.!. STANl>AlU) OBSP.B.VEB.
WAVELENGTH
NANOMETEIS 380 390 400 410 420 430 440 450 460 470 480 490 500 510 S20 S30 540 sso 560 570 Page 4 SPECTllAL LUMINOUS WAVEI.Blln'B EFFICIENCY, V(l) IWICltBTDS
0.0000 S80
0.0001 590
0.0004 600
0.0012 610
0.0040 620
0.1116 630
0.0230 640
0.0380 650 ..
0.0600 660
0.0910 670
0.1390 680
0.2080 690
0.3230 700 O.S030 710
0.7100 720
0.8620 730
0.9540 740
0.99SO 7SO
0.99S0 760
0.9520 770 SPECTRAL LUMINOUS EFFICIENCY• Table 1.1 SPBCTllAL LUMINOUS El'PICIENCY, V (),)
0.8700
0.7570
0.6310 O.S030
0.3810
0.2650
0.1750
0.1070
0.0610
0.0320
0.0170
0.0082
0.0041
0.0021
0.0010
o.ooos
0.0003
0.0001
0.0001
0.0000 V00 Chap 1 Pu 2
29 .July 1971 AC 20- 74 .----INTENSITY •I CANDELA,= I L:.UMEN/STERADl.ff ~t----------lMETER ________ ..,.
........,_____ IFT. ___....., . I CM AREA=ICM 1 ILLUMINANCE• I LUMEN/CM 1 AREA:t Fll IL LUMINANCE: I LUMEN/FT 1 AREA= I METERt ILLUMINANCE a f LUMEN/M 1
INTBNSITY AND ILLUMDIATION Figure 1.3 Fluhing light• are u1ed exten1ively a1 1ignals and warnings because of their superiority ira. attractuag attention. Becauae of thia characteristic, flashing beacoa.e were eatabliahed •• the required lighting for anticollision light8. These beacona are of Hveral types: rotating, flashing incandes cent. oscillating and gu discharge (strobe).
When a light aignal cOQ.lists of separate flashes, cile maximum intensity during the flash must be greater than the inteueity of a steady light to have the sam apparent inteneity. It is convenient to evaluate flashing light1 in terms of their EFFECTIVE INTENSITY, I 8 or EFI, the intensity of a eteady light which will appear equally bright when viewed at threshold, and is expressed in candelas.
Chap 1 Par 2 Page 5
29 .July 1971 A.C 20-74 Current airworthiness requirements for aircraft anticollision lights specify the following equation (known as the Blondel Rey equation) for the computation of effective inteu.eity:.,.,..
where: le• effective intensity I(t) • instantaneous intensity as a function of time t2 - ti• 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 tz and t1. For short time flashes, t2 t 1becomes insignificant compared to 0.2 seconds, and the total flash is intearated.
Short-duration flashtube lights have been used primarily as supple mentary 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 comDOn in the small supplementary lights.
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 is said to have three psychological c0111ponents: !!!:!!., (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 spectrum required to be mixed with the equal-energy(white) source to produce the color; this wavelength is called the dominant wavelength of the color. -Saturation ia satisfactorily specified by the ratio of the distance on the CIE chromaticity diagram (See Figure l.S) from the equal-energy point to the color point, to the distance from equal-energy point, in the same direction, to the spectrum locus. This ratio is called purity.
Chap l Par 3Page 6
29 July 1971 AC 20•74 It can be shown that any color can be matched by combining three properly choeea. colors called pr111111ry colors. :If a colored sample is placed in OGe half of a photo11etric field. a mixture ~f three primary colors such as red. green. and bbe, 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 pdury colors. A problem with this method is that the matching judgment of one obaerver cannot be taken as representative of the average person: Only when a large number of observers are used in each experi•nt can consistent values be obtained for the relative brightne88 of primary colors required to match any given color. In order to avoid this difficulty, the tristimulus method was revised and standardiHd. 'ftlree primary colors were agreed upon; thea., by expert•nta 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 COllliaaion Internationale de L'Eclairage (CIE), a more objective and ecoaom.cal teclmici\\e is available to specify color. Figure 1.4 is a graphical presentation of the relative amounts of each primary color required to •tch any wavelength. Standard notation for these functions is: i for the red primary. y for the green primary. and z for the blue primary. It should be noted th.at 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.
eoo eoo ~nm
SPECTRAL TRISTIMULUS VALUES ACCORDING TO THE 1931 CIE STANDARD OBSERVER Figure 1.4 Chap l Page 7 Par 3