ASTM G155 Xenon Arc Testing Standard Explained

In material weathering tests, “ASTM G155” is a commonly specified test requirement. However, simply describing it as “exposing materials to a xenon lamp” does not fully explain what the standard covers.

ASTM G155-25, the current edition, is officially titled Standard Practice for Operating Xenon Arc Lamp Apparatus for Exposure of Materials. It establishes procedures for creating, measuring, and controlling controlled exposure conditions using xenon arc apparatus, rather than defining one universal test cycle or fixed set of parameters for all materials.

To understand ASTM G155 correctly, the focus should therefore be on how spectrum, irradiance, temperature, moisture, and exposure cycles work together to create the test environment.

1. What Does ASTM G155 Actually Specify?

ASTM G155 is essentially a practice for operating xenon arc exposure equipment.

A xenon arc lamp can simulate solar radiation, while different optical filters can be used to reproduce different exposure environments, such as outdoor daylight or sunlight passing through window glass.

As a result, ASTM G155 does not provide one universal exposure condition for every material.

Depending on the applicable material standard, product specification, or test objective, the following conditions may need to be defined:

• Optical filter

• Irradiance

• Black panel temperature

• Relative humidity

• Water spray

• Exposure cycle

• Exposure duration

This is why a customer request that simply states “ASTM G155 testing” may still require clarification of the specific exposure conditions.

2. Why Do Optical Filters Matter?

A xenon arc lamp produces a broad spectrum, but the spectrum reaching the specimen depends on the optical filter system.

Different filters modify the distribution of ultraviolet, visible, and infrared radiation, which can affect the photochemical reactions occurring in the material.

For example, simulating outdoor sunlight and simulating sunlight passing through window glass involve different spectral conditions.

Xenon arc testing is therefore not simply a matter of comparing “light intensity.” The spectrum itself can influence the degradation mechanism.

This is also an important distinction between Xenon Arc Testing vs UV Weathering Testing: What's the Difference? The two methods use different light sources and spectral distributions; they are not simply “strong UV” versus “weak UV” tests.

3. Why Is Irradiance More Important Than Lamp Power?

Irradiance is one of the key control parameters in xenon arc weathering.

It represents the radiant power received by the specimen, while the rated power of the xenon lamp does not directly indicate the radiation intensity at the specimen surface.

Actual irradiance is affected by factors such as lamp condition, optical components, filters, sensors, and the control system.

This becomes particularly important during long-term testing. Xenon lamp output changes as the lamp ages. If the chamber cannot continuously monitor and control irradiance, the actual radiation received by the specimen may change even when the setpoint remains unchanged.

For this reason, when evaluating a xenon arc chamber, it is more meaningful to consider whether the equipment can measure and control irradiance accurately and consistently than simply asking how many watts the xenon lamp has.

4. Why Does Black Panel Temperature Matter?

Another important parameter in xenon arc testing is Black Panel Temperature (BPT).

Radiation from the xenon lamp is absorbed by the specimen and converted into heat. As a result, the specimen surface temperature may differ significantly from the air temperature inside the chamber.

BPT represents the thermal response of a black surface under irradiation. Therefore:

Black Panel Temperature ≠ Chamber Air Temperature

This distinction is not merely a difference in measurement method.

Temperature directly affects the rate of material degradation. Even when irradiance remains the same, different temperatures can change oxidation, diffusion, and other degradation processes.

Therefore, when specifying ASTM G155 conditions, it is important to identify which temperature is being controlled rather than simply stating a temperature value.

5. Why Is Moisture Part of the Exposure Condition?

Materials exposed outdoors are rarely subjected to sunlight in a continuously dry environment.

Rain, dew, and humidity can change the surface condition of a material. ASTM G155 therefore allows moisture to be introduced through water spray, humidity control, and light/dark cycles, depending on the selected exposure condition.

Water spray and high humidity do not produce exactly the same effects.

Water spray can rapidly wet and cool the specimen surface, while humidity control may keep the specimen in a high-moisture environment for a longer period.

Therefore, a test involving periodic water spray cannot automatically be considered equivalent to a test that only controls relative humidity, even when the total light exposure is the same.

6. ASTM G155 Does Not Define One Fixed “1,000-Hour Test”

“ASTM G155 for 1,000 hours” is a common specification in practice, but it is technically incomplete.

ASTM G155 allows different exposure conditions, so exposure duration needs to be interpreted together with the selected exposure cycle.

For example:

Therefore, two tests may both run for 1,000 hours while exposing the specimens to significantly different environmental conditions.

The meaningful comparison is:

Exposure Duration + Exposure Condition

rather than exposure time alone.

7. Why Can the Results Differ Even When Both Tests Follow ASTM G155?

This is one of the most important technical aspects of ASTM G155.

Suppose two laboratories both perform ASTM G155 testing, but one uses a Daylight filter while the other uses a Window Glass filter. If their irradiance, temperature, or water-spray cycles also differ, the resulting material degradation may be significantly different.

ASTM G155 recognizes that different exposure conditions can produce different results. Therefore, results from different chambers or laboratories should not automatically be considered directly comparable simply because both are labeled “ASTM G155.”

The key variables include:

Spectrum → Irradiance → Temperature → Moisture → Exposure Cycle → Equipment Stability

A chamber that can perform an ASTM G155 cycle and a chamber that can maintain that exposure condition consistently and reproducibly over long-term testing are not necessarily the same thing.

8. How Should ASTM G155 Test Results Be Evaluated?

ASTM G155 primarily addresses:

Under what exposure conditions should the material be exposed to xenon arc radiation?

It does not, by itself, define:

What performance must the material achieve after exposure?

After exposure, properties may be evaluated according to the applicable material or product specification, including:

• Color change

• Gloss retention

• Cracking

• Chalking

• Yellowing

• Tensile strength

• Other physical properties

A complete weathering test therefore has two distinct parts:

Exposure: Establishing the specified artificial weathering environment.

Evaluation: Measuring how the material changes after exposure.

This distinction is important because “ASTM G155 compliant” alone does not fully describe a material weathering test.

The core of ASTM G155 is not simply determining how many hours a xenon lamp should operate. It is about establishing a controlled and measurable xenon arc exposure environment.

The resulting material performance is influenced by the combination of:

Spectrum + Irradiance + Temperature + Moisture + Exposure Cycle

When a customer specifies ASTM G155 testing, the specific filter, irradiance, temperature, moisture conditions, and exposure cycle should therefore be clarified rather than relying on the standard number alone.

For a xenon arc weathering chamber, the operating range of individual parameters is only the starting point. What ultimately matters is whether the equipment can maintain these critical exposure conditions consistently over time and provide reproducible test results.

The key principle is simple:

ASTM G155 is not simply about using a xenon lamp. It is about controlling the exposure environment.