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
AC 20-74 29 July 1971 'urves, a visual inspection through the entire area should be made to determine whether or not there are any noticeable shadows or areas where visual observation would indicate puestionable conformance. If any such questionable arcas die noted, further measurements should be made in those areas in demonstiate satisfactory coverage.
Redi Mile l'osition Light. A horizontal distribution from 70) degrees right to 70 degrees left " directly to the reur.
Three vertical distribution curve. from 90 degrees up to 90 degreus down through the following horizontal points: directly astern, i duprees lett of directly to the rear, and in degrees rishit of directly to the rear. Laboratory reports of such measurements should also include at least the followiug:
(1) A list of the test equipment and calibration dates for light standards which should show calibration against the lab working standard within the past 30 days, and the working staudard against the lab primary standard within the last 180 days. The laboratory primary standard should be traceable to the National Bureau of Standards.
(2) If a luminous-efficiency-correction filter is included, data adjustment for filter errors should be shown and substantiated.
(3) If transmittance measurements ate used, the data should show adjustment for the comparison in transmittance between any clear filter used during the intensity measurements and the transmittance of the colored filter. If a clear cover is used, it should have the identical shape as the colored cover.
Transmittance data should be shown on a computation sheet such as shown in Table ?.1. Also, íf a spectrophotometer is used it should be substantiated that the sample used in the measurements is representative of the actual light cover.
(4) When red glass is used, the data should show the temperature if light covers during measurements and data on the transmittance characteristics relative to temperature. Measured intensity values should be adjusted as follows:
(a) For measurements made with the red cover in place, adjust the intensity values to those corresponding to an ambient temperature of 100°F. If no actual measurement is made with an ambient temperature of 100°F, extrapolation should be supported by data.
(b) When transmittance data 1s used, values should be adjusted to correspond to a glass temperature equivalent to that of the outside surface of the light cover. Glass temperature measurements should be made after the glass has stabilized with an ambient temperature of at least 100°F. Optionally, a temperature of 130°F should be allowed in lieu of measured temperature. Chap 3 Page 22 Par "
29 July 1971 AC 20-74
(5) A description of the procedure used to obtain intensity and tranamittance data, including the calculations. A diagram of the test set-up 18 desirable. This description and diagram should show:
Intensity measurements made from a distance sufficient to give accurate results with the linear operating range of the photocell of prime consideration. The distance should always be at least 10 times the diameter of the light source and preferably greater. (See IES Lighting Handbook, 4th Edition, page 4-18)
(b) Voltage measurements made as near the light source as possible, using a suitable meter when considering accuracy and loading. Current readings should also be recorded and data supplied to show where the lamp falls with respect to the manufacturing tolerance limits.
b. Color. Position fight color measurements require equipment such as described in Chapter 2, paragraph J. Aviation green contormance should be show with data in chromaticity-coordinate form (x, y, and
z). Aviation red conformance should be shown by the same type data, or by the optional brightness meter and filter method. when spectrophotometric procedures are used, the data should include a computation sheet such as shown in Table 2.1. Laboratory reports which accompany the data should include at least the following:
(1) A list of the teat equipment with, when applicable, calibration dates.
(2) When red glass is used, the data should include the temperature of light covers during color measurements, and data on the color characteristics relative to temperature. Red glass data should also include the following:
(a) For measurements made with the red cover in place, substantiating data to show that the color will be within limits when the outside temperature of the glass 18 78°F.
If chromaticity-coordinate measurements are made, measured values should be adjusted to those corresponding to an outside cover glass temperature
(3) A description of the procedure used to obtain the color data.
A diagram of the test set-up 1s desirable.
Chap 3 Page 23 Par 9
AC 20-74 29 July 1971
3. ANTICOLLISION LIGHT DATA.
a. Intensity. Measurements for "effective intensity" require techniques as described in Chapter 2, paragraph 2.b. (3). The light may be white liane, on measured as a white light, and the intensity values adjusted according to the transmitlance of the red cover. The computation sheet used to determine the chromaticity-coordinates contains the data for determining the luminous transmittance of the red cover. If a clear cover is used in the intensity measurements, the ratio of the transmittances of the two covers me be used to correct the data.
To show field of coverage, a combination o! vertical and horizontal measurements is necessary. Figure 1.6 is a typical presentation of vertical elfective intensity distribution and is shown with the FAA minimum intensity requirements. Such curves are constructed from points representing separate effective intensity measurements.
To assure sufficient points to accurately draw the vertical distriburion curve, measurements are usuilly made at vertical angles of 430°, +20°; $10°, $5' and O. Ibree of nine such points are shown in Figure 3.6. To «btain the values for the three points, individual intensity vs time curves are recorded and processed. The processing for long duration type lights can be done from a paper recording, using a planimeter for area measurements. Horizontal coverage must also be substantiated. As the curve of Figure 3.6 represents a single horizontal direction, there should be enough measurements to assure complete field of coverage. In other words, it should be substantiated that any variations in vertical patterns around the light will not result in an out of tolerance condition at any horizontal position.
When using the Blondel-Rey equation, the maximum value of effective intensity is obtained when t2 and t1 are choser so that the effective intensity is equal to the instantaneous intensity at t2 and t1- Io compute the highest possible effective intensity value for a given curve, estimates should be made until the proper values for t2 and t1 have been determined. When the instantaneous intensity at the t2 and [1 points approximate the effective intensity value computed from the Blondel-Rey equation, the maximum computed value has been found. Figures_3.7 and 3.8 have examples of data and the mechanics for determining maximum computable intensity for points A, B, and C of Figure 3.6. The heat correction factors used in Figures 3.7 and 3.8 are used because the light cover is at a higher temperature in the laboratory than in actual operation. For rotating beacon measurements, the motor is stopped and the light is concentrated on a particular area of the glass. As mentioned before, red glass decreases in transmittance with heat.
Page 24 Chap 3 Par 10
29 July 1971 AC 20-74 For short time flashes such as those produced by flashtube units, an integrating type photometer is generally used.
These instruments integrate the whole flash rather than should be followed. For typical flashtube measurements, the value or t2 - t, is negligible compared to u.¿ seconds.
Computed effective intensity for these lights is therefore maxibum when the entire flash 18 included. Effective intensity for short time flashes can be found by the following relation:
Footcandle-seconds (meter reading) x distance squared : 0.2 seconds 1250 To improve the accuracy, several flashes are integrated and an average 18 taken. Figure 3.9 is an example of typical flashtube measurement data. Information furnished usually includes the charge voltage, flash capacity, electrical energy stored (watt-sec), flash rate, and the plane of measurement.
In the example, the 15.6 watt-seconds results from the equation:
Energy • 1/2 E°C = 1/2 (420)2 (177X10-6) The 15.6 watt-seconds of electrical energy is partially converted to light energy. If reflectors and gas conversion efficiency are considered, the watt-second number can be used to estimate possible candelas. The test distance is given so that the meter reading 1s convertible to candelas. The multiplier
(125) in the example results fron D2/0.2. In the tabled data, therefore, the single flash footcandle-second reading tines 125 gives effective intensity.
Laboratory reports of such measurements should include at least the following:
(1) A list of test equipment and calibration dates as discussed in paragraph 2.2. (1).
(2) Luminous-efficiency-correction filter data as discussed in paragraph 2.a. (2).
(3) Iransmittance data as discussed in paragraph 2.a. (3).
(4) Temperature corrections as discussed in paragraph 2.a. (4). If a heat correction factor is used, it should be substantiated by data.
(5) Procedure information as discussed in paragraph 2.a. (5).
For anticollision lights, this information should also include the method used to determine "effective intensity."
Chap 3 Par 10 Page 25 443-651 0-71-3
AC 20-74 29 July 197!.
(6) For strobe sources, the spectral distribution data used in transmittance computations should be substantiated by data showing spectroradiometer measurements, or published data which can be shown to be applicable.
Color. The measurement for color of an anticollision light red rover is usually made with a spectrophotometer. In this measurement, the computation sheet (Table 2.1) should contain values for spectral distriburion which are representative of the light source being used. Pre-computed values for Ex. By.
and Ez are available for many type sources and are identified by color temperature. NBS Monograph 104 has several :amples.
The color temperature and the values for Ex, Ey, aud Iz must be accurately known if the results are to be dependable.
Laboratory reports which accompany the data should show at least the following:
(1) A. list of the test equipment with, when applicable, calibration dates.
(2) Temperature data as discussed in paragraph 2.b. (2).
(3) Procedure information as discussed in paragraph 2.b. (3).
chap :
Page 26 car 10
AC-20-71 29 July 1971 20° 20• 1=40 CANDELAS — Is 40 CANDELAS — 110• 30
LEFT RIGHT REAR IS UDINAL 140
POSITION LIGHTS, MINIMUM INTENSITY IN THE HORIZONTAL PLANE Chap 3 Par 10 Page 27
AC 20-74 29 July 1971
0.90 1 1.001- ABOVE 0.8012 HORIZONTAL 0.70 1€
0.50 IL 40
0.05 1 0.30 14 20° 30°
0.10 I T 15° 10°
HORIZONTAL 5' PLANE * 10° 15°
0.05 I 120° 30° 20• BELOW
HORIZONTAL
POSITION LIGHTS, MINUMUM INTENSITIES IN VERTICAL PLANES Figure 3.2 Chap 3 Page 28 Par 10
29 July 1971 AC 20-74
ANTICOLLISION LIGHTS, MINIMUM INTENSITY IN VERTICAL PLANE Figure 3.3 Date of I. C. Application:
Prior to 8/11/71 100 On or after 8/11/71 400 Chap 3 Par 10 Page 29