505.7-4 RADIATION SOURCES
4.1 Test Facility
- a. The facility requirements consist of a chamber or cabinet, auxiliary instrumentation, and a solar lamp bank. This apparatus must be capable of maintaining and monitoring (see Part One, paragraph 5.18) the required conditions of temperature, airflow, and irradiation.
- b. Full spectrum lamps are recommended for both procedures, however Procedure I can be performed using lamps that do not meet the spectral energy distribution of Table 505.7-I, provided the guidance in paragraph 4.1.2 is followed.
- c. For both procedures consider the possible cooling effects of airflow over the test specimens. Caution is advised on the use of the low airspeed; rarely do high solar and high temperature events occur in nature without accompanying winds.
- (1) Procedure I: Unless otherwise justified, use an airspeed between 1.5 to 3.0 m/sec (300 to 600 ft/min). See paragraph 2.3.7 for additional details.
- (2) Procedure II: Unless otherwise justified, use an airflow rate sufficient enough to maintain the test item response temperature that was either determined from Procedure I or obtained from field data.
- d. To minimize or eliminate re-radiation from chamber surfaces, experience has shown that the best method is when the volume of the test chamber is a minimum of 10 times that of the envelope volume of the test item. (Consider the beam angles of the light source hitting the walls of the test chamber.)
- e. It is recommended that the solar radiation source area be such that the length and width of the target area of the test item are no more than one-half the dimensions of the lamp bank.
505.7-4.1 General
The radiation source may comprise one or more lamps and their associated optical components, e.g., reflectors, filters, etc., to provide the required spectral distribution and irradiance. The high pressure xenon arc lamp with filters can provide the best spectral match. Mercury vapor and xenon-mercury lamps have considerable deficiencies in matching that would lead to error. If not already covered in test method characteristics of these sources, features of filters, optical arrangements, etc., are covered in the following paragraphs. The following general information about several light sources may be helpful.
- a. Xenon lamps. The configuration and size of the lamp(s) used will depend on the test required. The relative spectral distribution of the xenon arc radiation has been found to be substantially independent of lamp power. However, variation of lamp power will change the temperature of the electrodes and hence the spectral distribution of their radiation. With long arc lamps, it is relatively simple to mask off the electrode radiation. The form of construction of the short arc lamp leads to considerably wider manufacturing variation compared with the long arc, a point particularly important when replacement becomes necessary. Routine replacement of either type of lamp will be needed, since the emission will change continuously with life, and there may be wide variations of the life characteristic from lamp to lamp.
- b. Metal Halide (HMI). Metal Halide lamps that are properly filtered and using proper electrical power supply to the lamp can meet the defined spectral requirements. Care must be taken regarding lamp age and lamp power adjustment as spectral shifting can occur leading to changes in spectrum (critical for Procedure II testing).
MIL-STD-810H
METHOD 505.7, ANNEX A
4.1.1 Substrate
Mount the test item either on raised supports or on a substrate of specified properties, e.g., a layer of concrete of specified thickness or a sand bed of a thermal conductivity and reflectivity representative of actual deployment, as provided in the requirements documents.
MIL-STD-810H METHOD 505.7
4.1.2 Solar Radiation Source
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a. Full spectrum lamps are recommended for both procedures.
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b. Use a maximum irradiance intensity of 1120 W/m2 (±4 percent or 15 W/m2, whichever is greater) and ensure the radiation across the upper surface of the test item area of concern is uniform to within 10 percent of the desired value.
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c. The diurnal variation in solar energy may be applied continuously (see Figure 505.7-I) or incrementally (see Figures 505.7C-5 and -6), with a minimum of eight levels, provided that the total energy of the cycle is maintained.
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d. Where only thermal effects, Procedure I, are being assessed, it is essential to maintain at least the visible and infrared portions of the spectrum as in Table 505.7-I. However, if not feasible, deviate from the spectral distribution as necessary, but adjust the irradiance to give an equivalent heating effect. Document any deviation from the solar power distribution (Table 505.7-I), and record it in the test report. If using infrared lamps, exercise caution because infrared-reflecting/absorbing coatings will reflect or absorb energy based on spectrum and an infrared light system may not produce realistic thermal effects when various material colors and structures are under evaluation. If a glazing system is incorporated in the materiel (see paragraph 2.1.1.1), verify that the infrared transmission is not affected when using an infrared source. Use a full spectrum source if attenuating coatings, glazing, or other systems that may affect spectral reflection/absorption are used in/on the test item. In order to determine the amount of adjustment necessary, employ either of two methods below, and document it in the test report:
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(1) Mathematically calculate the adjustment using the spectral reflectance or transmittance of the irradiated surfaces, and the spectral energy distribution of the particular lamps being used (and also the effect of any associated reflectors or glasses).
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(2) Empirically determine the adjustment by conducting a pre-test on samples that are representative of the materiel (the most important characteristics are material composition, color, and surface roughness). Measure the temperature rise above ambient air temperature of test samples under natural solar radiation conditions (the climatic category identified in the LCEP as the most extreme), and compare the results with the temperature rise above ambient (chamber) air temperature of test samples under simulated solar radiation. Gather an adequate amount of data under the natural condition portion of the test to account for the cooling effects of airflow over the samples (i.e., outdoor conditions rarely provide zero wind), and extrapolate the temperature rise at zero wind conditions to be comparable to results from chamber samples. This process requires the use of extensive multi-year stable data sets to establish a statistically viable analysis.
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e. Where actinic effects are to be assessed, (Procedure II), ensure the spectral distribution of the light source adheres to the distribution given in Table 505.7-I (within the given tolerances).
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f. Direct the radiation onto the test item and irradiate the entire surface of the test item facing the solar radiation source. To provide the highest degree of confidence in the measurements, the value of 1120 W/m2 theoretically includes all radiation received by the test item, including any radiation reflected from the chamber walls and any long-wave infrared radiation (but not greater than 3 µm) emitted by the chamber walls. Radiation reflected or emitted from the chamber walls is generally substantially lower than the radiation emitted directly from the light source, and a measurement device that has a measurement range of 285- 2800 nm should be sufficient to measure direct and reflected radiation. Additionally, if the intent of the test is to determine thermal heat loading (see paragraph 4.1.2h(2)), use a radiation measuring device that has the capability to measure infrared energy, and calibrate the radiation measuring device in the full wavelength range it is designed to measure.
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g. To prevent localized effects such as unintentional heating from individual bulbs, locate the radiation source at least 76 cm (30 inches) away from any surface of the test item. Spot lamps (as opposed to flood lamps) may produce a non-uniform exposure.
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h. Light source. The following lists are not intended to exclude new lamps made available by advanced technology. It may be necessary to use filters to make the spectrum comply with that specified in Table 505.7-I. Further guidance is given in Annex A.
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(1) Tests conducted for degradation and deterioration of materials due to actinic effects as well as heat buildup within the test items must satisfy the full spectrum of Table 505.7-I and may use one of the following acceptable radiation sources:
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(a) Metal halide lamps (designed for full spectrum application).
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(b) Xenon arc or mercury xenon arc (used singularly) with suitable reflector.
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(c) Combination of high pressure sodium vapor and improved mercury vapor with suitable reflectors.
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(d) High-intensity multi-vapor, mercury vapor (with suitable reflectors), and incandescent spot lamps.
NOTE: Use other combinations of the lamps listed above and below if it is proven that the combination produces the spectrum of Table 505.7-I.
- (2) Use the appropriate lamps from the following list for tests conducted to assess heating effects alone (and not actinic effects).
- (a) Mercury vapor lamps (internal reflector type only).
- (b) Combination of incandescent spot and tubular-type mercury vapor lamps w/ external reflectors.
- (c) Combination of incandescent spot lamps and mercury vapor lamps with internal reflectors.
- (d) Metal halide.
- (e) Xenon arc or mercury xenon arc lamps with suitable reflectors.
- (f) Multi-vapor (clear or coated bulb) with suitable reflectors.
- (g) Tungsten filament lamps.
- (h) Any other heat producing lamp.
4.2 Controls
- a. Temperature. Maintain the chamber air temperature (as specified in the test plan) in accordance with Part One, paragraph 5.2a. In order to adequately measure the temperature of the air surrounding the test item, measure it (with adequate shielding from radiated heat - see Annex A, paragraph 5.2) at a point or points in a horizontal reference plane at the approximate elevation of the upper surface of the test item, and as close as possible to the test item, making adequate provision for shielding from the effects of radiant heat from the test item. This is one way to ensure reasonable control of the envelope of air surrounding the test item. The temperature sensors used to measure the thermal response of the test item will also be affected by direct radiation of the light source. When practical, mount these sensors to the inside surface of the external case (upper surface) of the test item.
- b. Test Sensors and Measurements. Use a pyranometer, pyrtheliometer or other suitable device to measure the total radiated energy imposed on the test item. Use a pyranometer with suitable filters or a spectroradiometer to measure the spectral distribution of the radiation imposed on the test item. A filtered pyranometer can only provide an approximate measurement of the spectral distribution. However, a spectroradiometer, although more delicate to employ, can provide a precise measurement of the spectral distribution. Use other measuring instruments only if they can satisfy the required specifications. See Annex B for the required measurement accuracy of these commonly used instruments. Test parameter tolerances appear in Table 505.7-II.
MIL-STD-810H
METHOD 505.7
| Test Parameter | Description | Tolerance |
|---|---|---|
| Total Spectral Irradiance | Sum of energy for all spectral bandwidths at a target irradiance level on the test profile (diurnal curve or constant irradiance). | ±4 percent or ±15 W/m² (whichever is greater) of the target irradiance. |
| Spectral energy Distribution | Energy within each spectral bandwidth at a single measurement point. | See Table 505.7-I tolerance for each bandwidth. |
| Irradiance Uniformity | Deviation between the measured and target irradiance at all measurement points on the target plane. The number of points shall be adequate to define irradiance variation across the target plane(s) to meet the specific test requirement. | ±10 percent deviation from the target irradiance |
| Instrumentation | Pyranometer, pyrheliometer, or radiometer | See Annex B |
| Temperature | Chamber control | ±2 °C (±3.6 °F) |
| Air Speed | Across the test item | 1.5 to 3.0 m/sec (300 to 600 ft/min) |
Note: When performing Procedure I, it is not required to provide a total irradiance energy level below a minimum level of 55 W/m² (280-3000 nm). If the diurnal curve calculated energy value is below 55 W/m² (280-3000 nm), a target value of 55 W/m² (280-3000 nm) may be used for the specified time period.
- c. Calibration of chamber. Because of the variety of permissible lamps and chamber designs, it is particularly important that the chamber be calibrated to assure the proper levels of radiant infrared energy are impacting the test area when heat alone is of concern, and that the proper intensity and spectral distribution of solar radiation are impacting the test area when actinic effects are of concern. See Table 505.7-I for spectral distribution and permitted tolerances.
If the test item is not available at the time the chamber is being calibrated, ensure the radiation uniformity is within 10 percent of the desired value when measured over the area covered by the test item, at a horizontal reference plane at the approximate elevation of the upper surface position of the test item. If the test item is available at the time the chamber is being calibrated, ensure the radiation uniformity is within 10 percent of the desired value when measured over the upper surface of the test item. As most types of lamps age, their spectral outputs change. To ensure that solar radiation chambers meet established specifications, perform a thorough check on spectral distribution, intensity, and uniformity at intervals not exceeding 500 hours of operation. Conduct a check of the overall intensity and uniformity before and after every test.
- d. Record chamber temperature, solar radiation intensity, spectral distribution, wind speed, and humidity (if required) at a sufficient rate to capture data necessary for post-test analysis (see Part One, paragraph 5.18).
505.7-4.2 Filters
Liquid filters have certain disadvantages such as the possibility of boiling, the temperature coefficient of spectral transmission, and long term drift in spectral characteristics. The present preference is for glass filters to be used, although the characteristics of glass filters are not as accurately reproduced as those of a chemical solution filter. Some trial and error may be necessary to compensate for different optical densities by using different plate thicknesses. Glass filters are proprietary articles and manufacturers should be consulted concerning the choice of filters suitable for particular purposes. The choice will depend on the source and its methods of use. For example, a xenon source may be test-compensated by a combination of infrared and ultraviolet absorbing filters. Some glass infrared filters may be prone to rapid changes in spectral characteristics when exposed to excessive ultraviolet radiation. This deterioration may be largely prevented by interposing the ultraviolet filter between the source and the infrared filter. Interference type filters, that function by reflecting instead of absorbing the unwanted radiation, (thus resulting in reduced heating of the glass), are generally more stable than absorption filters.
4.3 Test Interruption
Test interruptions can result from two or more situations, one being from failure or malfunction of test chambers or associated test laboratory equipment. The second type of test interruption results from failure or malfunction of the test item itself during required or optional performance checks.
505.7-4.3 Uniformity of Irradiance
Owing to the distance of the sun from the Earth, solar radiation appears at the Earth's surface as an essentially parallel beam. Artificial sources are relatively close to the working surface and means of directing and focusing the beam must be provided with the aim of achieving a uniform irradiance at the measurement plane within specification limits (i.e., 1120 W/m² (see Table 505.7-I)). This is difficult to achieve with a short-arc xenon lamp with a parabolic reflector because of shadows from the lamp electrodes and supports. Also, the incandescence of the anode can produce considerable radiation at a much lower color temperature, slightly displaced from the main beam, if only the arc itself is at the focus of the reflector. Uniform irradiation is more readily achieved with a long arc lamp mounted in a parabolic 'trough' type reflector. However, by employing very elaborate mounting techniques, it is possible to irradiate, with some degree of uniformity, a large surface by a number of short arc xenon lamps. It is generally advisable to locate radiation source(s) outside the test enclosure or chamber. This avoids possible degradation of the optical components, e.g., by high humidity conditions, and contamination of test items by ozone that has been generated by xenon and other types of arc lamps. Precise collimation of the radiation beam is not normally required except for testing special materiel such as solar cells, solar tracking devices, etc. However, some of the simulation techniques developed for space research purposes could be adapted for Earth surface solar radiation studies.
4.3.1 Interruption Due to Chamber Malfunction
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a. General. See Part One, paragraph 5.11, of this Standard.
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b. Specific to this Method.
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(1) Undertest interruption.
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(a) Procedures I and II. The test rationale is based on the total cumulative effect of the solar environment. Except as noted in (b) below, follow any undertest interruption by re-stabilization at the identified levels and continuation of the test from the point of the interruption.
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(b) Procedure I. The test is considered complete if an interruption occurs after 19 hours of the last cycle of Procedure I. (At least 92 percent of the test would have been completed, and the probability of a failure is low during the remaining reduced levels of temperature and solar radiation.)
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(2) Overtest interruption. Follow any overtest conditions by a thorough examination and checkout of the test item to verify the effect of the overtest. Since any failure following continuation of testing will be difficult to defend as unrelated to the overtest, use a new test item and restart the test at the beginning.
4.3.2 Interruption Due to Test Item Operation Failure
Failure of the test item(s) to function as required during mandatory or optional performance checks during testing presents a situation with several possible options.
- a. The preferable option is to replace the test item with a “new” one and restart from Step 1.
- b. A second option is to replace / repair the failed or non-functioning component or assembly with one that functions as intended, and restart the entire test from Step 1.
NOTE: When evaluating failure interruptions, consider prior testing on the same test item and consequences of such.
4.4 Test Execution
The following steps, alone or in combination, provide the basis for collecting necessary information concerning the test item in a solar radiation environment.
4.4.1 Preparation for Test
4.4.1.1 Preliminary Steps
Before starting the test, review pretest information in the test plan to determine test details (e.g., procedures, item configuration, cycles, durations, parameter levels for storage/operation, etc.). (See paragraph 3.1 above.)
- a. The required test procedures.
- b. The diurnal cycle to be used.
- c. Other variables, such as number of cycles, etc.
- d. Degree of removal of surface contamination necessary (see paragraph 4.2b). If the effects of surface contamination are to be assessed, include in the relevant requirements document the necessary information on preparation of surfaces.
- e. Comparative information. For eventual comparison between pre- and post-test items, photograph the test item and take material samples (if required).
4.4.1.2 Pretest Standard Ambient Checkout
All items require a pretest standard ambient checkout to provide baseline data. Conduct the checkout as follows:
- Step 1. In order to determine thermal response (paragraph 3.3b(2)), install temperature sensors in, on, or around the test item as described in the test plan.
- Step 2. Install the test item in the chamber (Part One, paragraph 5.8) and stabilize it at standard ambient conditions (Part One, paragraph 5.1) and in a manner that will simulate service use, unless the storage configuration is specified (see paragraph 2.3.4). Position the test item in accordance with the following:
- a. As near the center of the test chamber as practical and so that the surface of the item is not closer than 30 cm (12 in.) to any wall or 76 cm (30 in.) to the radiation source when the source is adjusted to the closest position it will assume during the test
- b. Oriented, within realistic limits, to expose its most vulnerable parts to the solar radiation, unless a prescribed orientation sequence is to be followed.
- c. Separated from other items that are being tested simultaneously, to ensure that there is no mutual shading or blocking of airflow unless this, also, is representative of the materiel's field use.
MIL-STD-810H
METHOD 505.7
- Step 3. Conduct a visual examination of the test item with special attention to stress areas, such as corners of molded cases, and document the findings.
Step 4. Conduct an operational checkout in accordance with the test plan and record the results.
- Step 5. If the test item operates satisfactorily, place it in its test configuration (if other than operational) and proceed to the first test as identified in the test plan. If not, resolve the problem and restart the checkout procedure.
4.4.2 Procedure I - Cycling
- Step 1. Adjust the chamber temperature and stabilize the test item to the conditions shown in the appropriate climatic category (zone A1 or A2) for time 0000.
- Step 2. Expose the test item to continuous 24-hour cycles of controlled simulated solar radiation and dry-bulb temperature as indicated on Figure 505.7-1 or as identified in the requirements document, measuring and recording test item temperatures throughout the exposure period. If the test facility is unable to perform the continuous curve of Figure 505.7-1, increase and decrease the solar radiation intensity in a minimum of eight levels (see Annex C, Figures 505.7C-5 and C-6 for the stepped levels) for each side of the cycle, provided the total energy of the cycle as well as the spectral power distribution is maintained. Perform at least three continuous cycles. If the maximum of the peak response temperature of the previous 24-hour cycle is not reached (±2 °C (±3.6 °F)) during three cycles, continue the cycles until repeated peak temperatures are reached, or for seven cycles, whichever comes first.
- Step 3. Based on the requirements document, the test item may or may not be operated continuously throughout the test. If operation is required, operate the test item when the peak response temperature occurs. For some single-use items (e.g., rockets), use thermocouples affixed to critical portions of the test item to determine the time and value of peak temperature. Conduct the operational checkout of the test item as in paragraph 4.4.1.2, Step 5. Document the results as well as the peak temperature. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
- Step 4. Adjust the chamber air temperature to standard ambient conditions and maintain until temperature stabilization of the test item has been achieved.
- Step 5. Conduct a complete visual examination of the test item and document the results. For comparison between pre- and post test items, photograph the test item and take material samples (if required).
- Step 6. Conduct an operational checkout of the test item as in paragraph 4.4.1.2, Step 5. See paragraph 5 for analysis of results.
Step 7. Compare these data with the pretest data.
4.4.3 Procedure II - Steady State
NOTE: If Procedure I has not been previously performed and no field/fleet data are available, conduct a preliminary test carried out in accordance with Procedure I (absolute minimum of three complete cycles) to determine the approximate maximum response temperature of the test item.
- Step 1. Adjust the chamber air temperature to the max temperature shown in the appropriate climatic zone (zone A1 or A2) as indicated on Figure 505.7-2 or the temperature identified in the test plan.
- Step 2. Adjust the solar radiation source to a radiant energy rate of 1120 ±47 W/m2 or as identified in the test plan. Use sufficient air speed to maintain the test item temperature to the peak response temperature obtained in procedure I or obtained from field data.
- Step 3. Maintain these conditions for 20 hours, measuring and recording the test item temperatures. If required, conduct operational checks during the last four hours of each 20-hour exposure when test temperatures are maximized. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
Step 4. Turn off the solar radiation source for four hours.
Step 5. Repeat Steps 1 through 4 for the number of cycles identified in the test plan.
MIL-STD-810H
METHOD 505.7
Step 6. At the end of the last radiation cycle, allow the test item to return to standard ambient conditions.
Step 7. Conduct a visual examination and an operational check as in Steps 3 and 5 of paragraph 4.4.1.2, and document the results. Take photographs of the test item and material samples (if required) for comparison between pre- and post-test items. See paragraph 5 for analysis of results.