Solar Testing Explained: MIL-STD-810 and Commercial

The first question a customer always asks is: "Why should I do solar testing and which test should I use?"The answer to the first part is relatively simple. You should perform solar testing if your product will be exposed to sunlight. This could be in front of a window located indoors, beneath a transparent canopy, or permanently stationed outside. The second part of the question gets a bit more complicated. There are several different types of solar testing. It can incorporate halogen, full spectrum or UV only lamps and can include temperature, humidity and water spray.First let’s focus on MIL-STD-810. It and other compliance standards require solar testing as part of product acceptance in which two different types of testing can be performed: Procedure 1 and Procedure 2.Procedure 1 is primarily a heating effect test and is usually preformed with halogen lamps following a diurnal cycle profile. The purpose is to determine the highest maximum temperature the test unit will reach with repeated cycles in a controlled environment. The lamp intensity is varied from 0 W/m2 to 1120 W/m2 over a 24 hour cycle with the lamps and chamber temperature following a profile that simulates a natural day/night cycle.Procedure 1 will reveal temperature related issues with the test unit and establishes the target test unit temperature for Procedure 2. This test can run 3 to 7 days in length with the equipment under test either powered on not. The airflow across the test articles is controlled to be the equivalent of a light breeze (300 to 600 feet per minute). MIL-STD-810 requires 3 days of stable and equal unit/chamber temperatures out of 7 days of testing. Full spectrum lamps can be used, but the difficulties controlling the intensity of full spectrum arc discharge lamps can sometimes be cost prohibitive.Procedure 2 is a combination actinic and heating effects test using full spectrum lamps. The solar aging properties combined with heating effects can degrade items such as LCD or LED displays as well as coatings and seals causing deterioration, fading and discoloration. The lamp intensity is fixed at 1120 W/m2 by varying the distance of the test unit to lamps and the cycle normally runs in one of two variants. The first is 20 hours on and 4 off and the second is with the lamps continuously on. The purpose of the on and off lamp cycle is to expose issues related to rapid temperature changes caused by solar loading while continuous exposure will find the maximum actinic effects.MIL-STD-810 calls for 20 hours on 4 hours off with exposure durations of 10 to 56 days or longer. Normally 10 days would be used for units that are primarily inside with some outdoor exposure while 56 day or longer would be for articles left outside such as transparent armor. With the 20 hour lights on cycle, solar aging is 2.5 times normal solar exposure. 10 days are the equivalent of 25 days outside in the sun and 56 days will be equivalent to 140.It is important to note that Procedure 2 uses the maximum part temperature established in Procedure 1 as the target part temperature and the airflow across the test unit is controlled to maintain target temperature with the lights on. If Procedure 2 testing is to be performed, Procedure 1 should be run first to establish the maximum temperature.Most commercial standards like ASTM or ANSI use similar lamp intensities and color spectrum as MIL-STD-810 (simulations with varying exposure times and environmental conditions), however some tests require a different type of simulator entirely.One example would be fluorescent UVA or UVB lamps with a combination of temperature, humidity and water spray. This type of testing is usually performed on small samples and can run anywhere from 3 days to several hundred days. A specification such as this is written around a specific type of tester, one of which is a QUV environmental simulator. They do not incorporate significant heating effects and are mostly actinic UV and weathering tests.Lastly, a common type of solar testing is performed on solar panels using lamps that provide high levels of UV and white light but little infrared and heating effects. This is typically done to establish power output and life cycle stability.NTS Tempe performs fully accredited MIL-STD-810 Procedure 1 and Procedure 2 testing along with UV testing using a QUV environmental test unit. Our specialty is customizing solar testing to meet special customer requirements. For more information on how we can help qualify your products please call the lab directly at 480.966.5517 or email our technical specialist Harold.Sibert@nts.com