Environmental test chambers provide a controlled environment for testing and stress-testing components, systems, and products. Chambers range in size from small "bench top" boxes to huge computer-controlled "walk in" rooms, and can provide an amazing array of capabilities, depending on the desired tests. A few of the various environments that are tested by environmental chambers are listed in the environmental conditions table available in MIL-HDBK-217.Tests are usually divided into two categories: Climatic tests, such as air pressure, humidity, temperature and light, and dynamic tests, i.e., shock, vibration, acceleration, rotation etc. Environmental chambers can be used to monitor and analyze the performance of a product under specific environmental conditions. However, they can also be used to predict the lifetime of a product using accelerated testing, where the product is exposed to extreme conditions as a way to accelerate the aging process. Similar techniques can also be used to hasten the manifestation of defects which might otherwise only show up in the field, and thus improve the quality of the product by understanding and eliminating its weak spots. Various methodologies are used for stress testing: ESS (Environmental Stress Screening, HASS/HAST: Highly Accelerated Stress Screening/Testing, and HALT (Highly Accelerated Lifetime Testing). ESS is the "original" test system, and is used to test a product within its operating specifications in order to identify faults and prevent the product from reaching the consumer. In this respect, it can be part of an acceptance test. The basic assumption is that ESS is not designed to damage the product. HASS/HAST and HALT, on the other hand, are designed to "age" (and even destroy) the system or component in order to understand and predict failures. HALT is used during the design process and might help in improving the design and manufacturing, whereas HASS/HAST is used in production as a way to improve outgoing quality.