The 1991 CIE : tandard observer Page 8 Spectral tristimului spectrum alues of equal-eDeres spectral tristimulus values of equal energ! spectzum: AC 20-74 leD/ Dm -
0.8700 .8103 .. . . ... ... ... ... ...
.3570
49.
300 $10 120 460 Par 3 Chap 1 1000 29 July 1971 0000 0000 0000 :0000 Totale ____ 21.9714 21.8715
SPECTRAL TRISTIMULUS VALUES Table 1.2
29 July 1971 AC 20-74 Capital letters X, Y, and 2 have been assigned as notation for the amounts of the three primaries required to visually match a color containing multiple wavelengths. These values can be plotted graphically if the following transformations are made:
× - (Z) y • (3) z • (4) X+Y+Z X+Y+Z X+Y+Z where X, I, and 2 are the amounts of the three primaries and x, y, and z are the 'chromaticity coordinates" in the CIE system. The horizontal of the graph becomes x and the vertical becomes y; ¿ may be obtained from the relationship x + y + z - 1. Such a graph, shown in Figure 1.5, 18 called a chromaticity diagran. Any color may be located on the diagram by specifying its chromaticity coordinates. The colors used for lighting of aircraft are shown with the limits as established by the airworthiness regulations.
On this diagram, there 1s a central white point marked I, where X = Y - Z corresponding to the color of a source having an equalenergy spectrum. As x + y + 2 = 1, this white point appears graphically at x • .333; y = .333 and represents a position of zero purity. The outer periphery of this diagram is a locus of points of 100% purity for visible wavelengths. A line drawn fron any point on this periphery to the point of equal energy (⅖) represents all purities between 100% and O for that dominant wavelength. For example, a line is shown for a light of dominant wavelength 0.9 micrometers with the 0%, 50%, and 100% purity points indicated. An aviation green light of this dominant wavelength would have to have a purity of only 32% to meet the regulatory requirements. Aviation red, however, would require a purity of nearly 100%.
The temperature scale along the bottom of the diagram, showing a range from 1000 k to infinity, contains calibrations for source color temperatures. Note that each temperature has a calibration mark on the curved line above. The curved line is called the "planckian locus," and represents the chromaticities of blackbodies at different temperatures.
In practical applications, it 1s accepted as representing the chromaticities of incandescent bodies much as lamp filaments. The numbers along this curve on the CIE diagram indicate the color temperature in Kelvins. CIE 1lluminant "A" can be represented on this curve at the 2854 K point, 11luminant "g" at about 5000 K and "C" at about 6800 k. Illuminant A represents the spectral distribution of typical tungsten-filament incandescent lamps, 1Iluminant B represents the spectral distribution of average noon sunlight, and illuminant C represents the spectral distribution of average daylight.
Chap 1 Par 3 445-499 0 - 71-3 Page 9
AC 20-74 29 July 1971 C.LE. CHROMATICITY DIAGRAM U.S. AVIATION COLOR LIMITS 55 .6 .57 'ELLOW .60 = 0.370 : 0.540 .61y=0.335 RED .3 raum
LIGHTING RED I URPLE
VIATION BLUE .8 AVIA,WHITE: C.I.E. CAROMATICITY DIAGRAM *EFFECTIVE AUGUST 197I
•A PRIOR TO HAUGUST IOTI Figure -.S chap i Page 10 Par !
29 July 1971 AC 20-74 The color of an object depends on the spectral characteristics of the 1lluminant as well as the nature of the object. For example, fabrics change color when moved from the light of ordinary tungstenfilament bulbs to fluorescent light. Any accurate system of color specification must account for this fact by relating the color to a specified light source. In specifying a color that is going to be used under known conditions of illumination, the tristimulus values for the standard illuminant conforming most closely to those conditions should be used. For example, in coloring an aviation red light cover, the color temperature of the light-source must be considered.
Some red glass filters change color as their temperatures change.
As the temperature increases, the light passing through some red filters becomes more red (longer wavelength) and less intense. For this reason, red filters are generally designed to be near the yellow limit (shortest wavelength) at room temperature. Then, if the operating glass temperature is consistently higher than room temperature, aviation red light will be produced under all operating conditions. By designing o this yellow limit, the maximum intensity will also be obtained. Glass of other colors changes slightly with temperature, but not enough to be a problem.
Page 11(and 12)
29 July 1971 A2 20-74.
• CHAPTER 2
MEASUREMENT CONSIDERATIONS GENERAL. The measurement of visible radiated energy is called photometry. Instruments for measurement of the amount of a light, regardless of their calibration, measure only flux, and care must be taken when converting to various photometric units. An additional precaution is necessary because photosensitive devices, unless properly filtered, do not have the same characteristics as the human eye.
There are four general types of light measurements:
Luminous Intensity - The luminous flux emitted per unit solid angle in a given direction (candelas).
b. Illuminance - The luminous flux incident per unit area on a surface at some distance from the source (footcandles).
Luminance - Luminous intensity per nit projected area of surface (candelas per square foot).
d. Chromaticity - The color quality of light determined by its chromaticity coordinates.
In addition to the above light measurements, the efficiency of light covers 1s sometimes measured. There are two terms associated with such efficiency measurements; spectral transmittance (T) which refers to the ratio of transmitted to incident power at one wavelength or a very narrow band of wavelengths, and luminous transmittance (I) which is the ratio of transmitted to incident total light power. The transmittance of a light cover indicates the decrease in light due to material and color, and can be used to adjust luminous intenaity values measured without the cover. The filtered light may be measured directly provided the photometer is equipped with filters which accurately match the luminous-efficiency function.
INTENSITY. The instrument used to measure the luminous intensity of lights is called a photometer. Photometers can be placed in two general classes; "visual" and "direct-reading photoelectric".
Visual Photometers. Before the invention or photoelectric cells, most instruments for measuring intensity employed the principle of balancing two adjacent fields visually. With these photometers, a test light and a standard light of know intensity were viewed Chap 2 Page 13 Par 4
AC 20-74 29 July 1971 simultaneously. By adjusting the relative distances from the viewing point, a balance was obtained, and by applying the inverse square law, the intensity of the test light could be determined.
Although visual photometers have generally been superseded by direct-reading photoelectric type instruments, many are still being Direct-Reading Photoelectric Photometers. Photoelectric devices are now being used to detect and convert light energy to electrical energy for measurement. A photometric system for precisely measuring the magnitude and coverage of aircraft lights usually consists of the following:
(1) Goniometer. This device includes an attachment position for and or one post on m the light unit which can be moved around two axes of freedon.
The vertical and horizontal positions are calibrated, and in some cases the information is remoted to recorders so that a plot of intensity vs. direction can be made directly. A goniometer of this type is installed in the photometric laboratory of the National Bureau of Standards in Washington, Tunnel and Track. A dark tunnel, including baffles to eliminate stray light, is located in line with the goniometer.
A photodetector is placed on a carrier which can be moved on a track to vary the distance between the goniometer and the photodetector. Position information is usually remoted to a recorder.
(3) Photodetector. Photodetectors are of several types, and read-out in units such as milliwatts, microamperes, etc.
By conversion factors, footcandles (1llumínance) can be computed from the read-out. Then, by means of the distance information, a calculation of intensity can be made. For steady lights, measurements are concerned with rate of flow of light entering the eye, analogous to gallons per minute into a container. For flashing lights, however, the measurements are concerned with the quantity of light per flash entering the eye, analogous to gallons. The quantities are proportional to footcandles and footcandle-seconds respectively. Since the instantaneous illuminance varies during exposure to flashing lights, integration is necessary to determine the footcandle-seconds. As discussed in paragraph 2 of Chapter 1, the computation of effective intensity requires a candela-second measurement. Candela-seconds Chap 2 Page 14 Par 5
29 July 1971 AC 20-74 can be computed from footcandle seconds by multiplying by the distance (in feet) squared. Care must be taken not to overload a photodetector by too much 11ght as it can saturate and its response become non linear.
Integrating photometers are available which electronically perform the integration and read out the total exposure in microcouloubs.
These photometers are used only on short duration strobes where the entire flash is integrated. A microcoulomb is a microampere-second, and a calibration factor is used to convert a microcoulomb reading to footcandle-seconds. Then, multiplying by the distance squared gives candela-seconds corresponding to the numerator of the equation for effective intensity.
Strip recorders or recording oscilloscopes are generally used for longer duration flashes, such as rotating beacons, and mechanical integration 1s then perforned.
800 INTENSITY 500 400 200 I INCH 12-11 -.05 SEC.
STRIP RECORDING OF FLASH figure 2.l Figure 2.1 shows an example of a recording made by a strip recorder.
The paper is moved horizontally at a linear rate so that each diviaion represents approxdmately ,05 seconda. The recorder is calibrated so that each vertical division represents 100 candlas. After the flash 1s recorded, a mechanical integrator (planineter) 1s used to trace the recording and obtain the area, in candela-seconds, between the limits of t2 and t1. This ares divided by 0.2 + t2 - t1 gives the effective intensity. The values of t2 and t1 are determined by experinentation, and are selected to maximize the computed effective intensity. In the Chap 2 Par 2 Page 15
29 July 1971 AC 20- 74 example of figure 2.1 the flash has an area between t2 and t1 of approximately Si candela-seconds. When divided byQ3 seconds (0.2 +
0.1) the effective intensity is 170 candelas. The peak intensity of 850 candelas, therefore, has an effective intensity of only 170 candelas. A method for selecting t2 and t1 is given in Chapter 3.
when measuring colored flashing lights, the photodetector cannot always be relied upon to give accurate results, even with luminous-efficiency-correction filters. The alternative is to measure the effective intensity with a clear cover, and then with a spectrophotometer, determine the relative luminous transmittances of the two covers The effective intensity value measured through the clear cover is then reduced by this luminous transmittance factor.
Spectrophotometers are discussed in paragraph 3., Color.
3. COLOR. Color can be evaluated with a visual photometer by comparing • to le efor temperature a sample against a filter of known color. In such comparisons, the same, correct color temperature should be used for both sources. However, there has been increasing interest in the use of photoelectric instruments for the measurement, of chromaticity of colored lights.
The following is a discussion of some of these instruments:
Spectrophotometer. Spectrophotometers break up a self-contained light source into a spectrum by prisms or gratings, so that narrow bands wíthin the visible spectrum can be individually applied to a test ware (such as a light cover). Although these are not single wavelengths, they are very narrow bands approximately five nanometers wide. Determining either luminous transmittance (I), or chromaticity coordinates (x, y, and z) with a spectrophotometer involves the measurement of 40 transmittances (t), spaced 10 nanometers apart, throughout the visible spectrum. Mathematically, these transmittances are used as follows:
X - Е360 КЕТАЛ (5) Y • 1380 FETA) (6) Z = [388 ZЕТAЛ (7) The values for x, y, and z are found in Table 1.2, and represent the relative amounts of primaries required at each wavelength. The E in the equations represents the spectral distribution of the light source, and is published for certain Illuminants such as C.I.E. Source Tables are available which list the products II, yE, and zi for certain illuminants (NBS Monograph 104). These numbers are sometimes found on a computation form as shown in table 2.1. If the 11ght 18 not a standard source, a table must be developed from measurements of the energy distribution. When each of the 40 lines have measured values for t logged, the remainder of the sheet can be filled out with data computed by hand, calculating machine, or computer. For example, assume a measured value for 1 at 380 nanometers of 0.078.
Then IET would be 0.078, JET would be 0.000, and IET would be 0.468.
Chap 2 Page 16 Par 5
29 July 1971 AC 20-74 Observer: C.I.E. Standard' 1931 Guide for Computation of x, 1, and T Sample Basic Stimulus: Equal Energy Illuminant: Planck 2856 K (C.I.E. Standard A) Wave Length Nanome ters ŸE XET ŻET ZET 380 390 0. 23 400 19 93 410 0. 340 420 o. 262 1256 430 O. 649 3167 440 926 61 4647 450 1031 117 5435 460 0. 1019 210 5851 470 O. 776 362 5116 480 O. 428 622 3636 490 O. 160 1039 2324 500 O. 27 1792 1509 510 57 3080 969 520 o. 425 4771 525 530 o. 1214 6322 309 540 o. 2313 7600 162 550 3732 8568 75 560 O. 5510 9222 570 O. 7571 9457 580 o. 9719 92 590 11579 8540 600 o. 12704 1547 610 0. 12669 6356 620 O. 11373 507 630 O. 8980 104 640 O. 6558 25 650 0. 4336 1637 660 2628 670 O. 1448 680 O. 804 690 404 700 0. 209 720 110 720 S7 730 O. 28 740 O. 14 750 o. -6 760 770 SUMS 109828 100000 35547 X- 8+8+2 1+7+2 2- XH3+2 Y, and z
COMPUTATION FORM Table 2.1 Chap 2 Par 6 Page 17
AC 20-74 29 July 1971 Summing the column for xEr would yield X which is the relative amount of red primary required for the match. Y and ¿ can be found with the other two columns. Then, the chromaticity coordinates for representing the sample on a chromaticity diagran, or evaluation against regulatory requirements is found as follows:
X = X+Y+Z The luminous transmittance T of the ware can be found from the same computation sheet as follows: The sum of the yEt column (Y) is divided by the sum of the yE column. This is possible because y and V(1) values (luminous-efficiencies) are identical.
For example, 1f the sum of the yEt column is 24407, dividing by 100,000 gives 24407 or 24.4%. The values given in this table have been adjusted so that the sum of the yE column is a power of ten to simplity computations. This means that oniy 24.4« or the light incident on the ware is being transmitted through 1c.
A more detailed explanation is given in the Appendix as "Tristimulus Colorimetry and Aviation Lights.
Spectroradiometer. Spectroradiometers are similar to spectrophotometers, but can be used to measure the spectral distribution of external sources over a wider range of wavelengths than just the visible spectrum.
Brightness Meters. When evaluating the color of ware with these instruments, a standard filter is used for comparison. The results are not chromaticity coordinates (x, y, and z), but are in or out oi tolerance indications. such procedures, due to ilter limitations, are used only on highly saturated red glas:
(sharp cut-off).
One method of determining whether a color meets a specific requirement, by using a brightness meter and a NBS filter, is given in MIL-L-25467C. The filter specified in this procedure has a yellow limit corresponding to instrument and panel lighting red (National Bureau of Standards Filter No. 3215). However, the Ted five te plea procedure has been used with an aviation red filter (NBS Filter No. 3647A). The procedure is a Go/No-Go measurement with the NBS filter used as a measurement standard. A brightness reading is taken of a colored aviation light at 10 ft. distance. A second reading is taken after a NBS filter is added in the light path.
The ratio of these 2 readings must approximate a similar ratio obtained using a white light source in place of the colored aviation light. In the second case, a first reading is made with Page 18 Chap 2 Par 6
29 July 1971 AC 20-74 one NBS filter and the second reading 1s made with two NBS filters. In this way, the aviation light color is compared to the NBS filter color. The two ratios must not differ over 3% (in the yellow direction) to qualify the test light color.
d. Tristimulus Colorimeters. If the response of three photocells could be adjusted by glass filters, so their responses follow the curves of the C.I.E. Standard Observer, x, y, and z (see Figure 1.4), they could be made to yield direct measurements of tristimulus values X, Y, and 2. Complete success of these colorimeters depends on the ability to duplicate the C.I.E.
Standard Observer System. Acceptable instruments could simplify the measurements and reduce the required data processing. Presently, such instruments have more application to production line measurements than for showing initial compliance.
TEMPERATURE EFFECTS. All glass filtering elements have their color and tranamittance affected in varying degrees by increased temperature. The change is more pronounced for red than for other colors. One manufacturer of glass has published data such that different red glasses are identified by their percent transmittance at room temperature. for example, red glasses sre identified as 12.6%, 25.3%, etc., at 78° F. For each number, the color and transmittance at any temperature up to 500° F is charted and plotted. Most red glasa colors are reversible up to the softening temperature(over 800°F). Reversibility means that after heating, when 25625 the temperature 1s returned to 78° F, the original color will return.
Glasses that have transmittances of 25% or more at 78° F have transmittance va temperature curves which are essentially linear up to 300° F, and transmittances change approximately 0.5% for each 10° F change in temperature, In application of these filters, proper evaluation of their color characteristic at elevated temperatures requires knowing the following:
a. The initial glaas temperature, color and transmission.
b. The range of glass temperatures found in operation.
c. The type glasa used in the filter and its characteristics. Generally, an increase in red glass temperature makes its color more red and decreases its transmission. The amount of such changes is determined by the temperature range of the glass in operation and the characteristics of the glass.
Chap 2 Page 19 (and 20) Par 6
29 July 1971 AC 20-74 CHAPTER 3
MEASUREMENT DATA GENERAL. Aircraft exterior lights, on which measurements are required, include position and anticollision lights. Measurements include intensity (coverage and overlap) and color.
Position Lights. The airworthiness requirements for aircraft position 11ghte are given in FAR 23.1385 through 23.1397, FAR 25.1385 through 25.1397, FAR 27.1385 through 27.1397 and FAR 29.1385 through 29.1397.
(1) Intensities.
Horizontal Coverage. Figure 3.1 shows the minimum intensity requirements for the horizontal plane at 0° vertical.
Overlap limits are not shown as they vary when intensities exceed 100 candelas.
(b) Vertical Coverage. Figure 3.2 shows the minimum intensities for any vertical plane. The value of "I" indicates the maximum required candelas for a given horizontal position.
(2) Color. A graphical presentation of the chromaticity coordinate limits for aviation red (left), aviation green (right), and aviation white (rear), 18 shown in figure 1.5.
b. Anticollision Lights. The airworthiness requirements for aircraft anticollision lights are given in FARs 23.1401, 25.1401, 27.1401 and 29.1401.
(1) Intensity & Coverage: Figure 3.3.
(2) Color: Each anticollision light must be either aviation red or aviation whice.
9. POSITION LIGHT DATA.
a. Intensity. Measurement of position light intensity requires photometric equipment such as that described in Chapter 2,paragraph 2. Pigure 3.4 and 3.5 show typical recordings as made in a photometric laboratory.
the data should include enough distribution plots to adequately substantiate coverage. The following data ahould be sufficient:
Forward Red and Green Position Lights. A horizontal diatribution curve in the zero degree vertical plane from directly forward outboard through 110 degrees (figure 3.4). Vertical distribution curves from 90 degrees up to 90 degrees down at 0, 10, 20; and 110 degree horizontal points (figure 3.5). In addition to these distribution Chap 3 Par 8 Page 21