Environmental qualification of spectrometers for industrial and harsh environments

Optical instruments based on spectrometers are used in industrial, portable, and field applications where the surrounding environment can be unpredictable. In addition, instruments are typically manufactured in a single location and then transported to customers by land, sea, or air, exposing them to further environmental stresses during shipping.

Common environmental influences include:

• variations in ambient temperature,
• variations in relative humidity
• vibrations
• mechanical shocks/impacts

These factors can temporarily or permanently alter the performance of a spectrometer or—in the worst case—cause complete instrument failure.

Environmental qualification of spectrometers verifies that the instrument can withstand defined external influences while maintaining performance within acceptable limits. In this way, environmental qualification reduces the risk of field failures and associated RMA costs.

Environmental qualification of a spectrometer involves a series of tests performed on a limited number of units, typically during the final stage of product development. Some tests evaluate the spectrometer’s performance while it is operating under various environmental influences, whereas others are designed as accelerated lifetime tests to verify that the spectrometer can withstand specific external stresses over its expected service lifetime.

Typical parameters that are checked during and after environmental tests are:

• visible mechanical changes like detached or loose elements
• Shift in wavelength calibration
• Shift in spectral resolution
• Shift in sensitivity/ throughput

Environmental qualification of spectrometers is typically performed with reference to established international standards that define how instruments should be tested under temperature, humidity, vibration, shock, and other environmental stresses. The relevant standards vary by application area—from general industrial and laboratory use to handheld instruments, transportation robustness, and the more demanding requirements found in military, aerospace, and space flight environments. The table below summarizes the most commonly referenced standards across these domains.

The standards marked with bold are the ones, Ibsen is testing against.

Although many environmental standards exist across industrial, handheld, military, and aerospace applications, most of them inherit test concepts from one another. As a result, the underlying test methods—such as temperature cycling, damp‑heat exposure, sinusoidal and random vibration, or mechanical shock—are often very similar. What differs between standards is typically the severity of the test conditions, the required documentation, and the environmental assumptions for the intended application. This means that while IEC 60068, ISO 9022, MIL‑STD‑810, and DO‑160 originate from different domains, they share a common foundation of environmental test principles.

Most environmental standards do not define specific performance limits for spectrometers. Instead, they describe how environmental tests must be performed—including temperature ranges, humidity levels, vibration profiles, shock pulses, and test durations. The manufacturer is responsible for defining acceptable performance criteria during and after testing, such as wavelength stability, resolution stability, or throughput. Standards like IEC 60068, ISO 9022, MIL‑STD‑810, and DO‑160 therefore provide the test conditions, while the spectrometer’s required performance under those conditions is determined by the intended application.

Spectrometers are built by combining optical, electronic, and mechanical components using adhesives and/or mechanical mounting techniques. Because the materials used in these assemblies expand and contract differently with temperature, the joints between them can experience mechanical stress during temperature cycling. Similar stresses can accumulate during repeated vibration or shock exposure. Over time, these stresses may lead to gradual misalignment of internal components or, in severe cases, complete joint failure. Adhesives are polymers, and their chemical structure can change under elevated temperature or high relative humidity, which may further contribute to misalignment or weakened joints.

Misalignment of internal components typically causes the spectrometer to lose wavelength calibration, alter its spectral resolution, or reduce its sensitivity. It is important to distinguish between reversible and irreversible changes. Reversible changes are sometimes less critical because they can be controlled, for example, by maintaining the spectrometer at a stable operating temperature. Irreversible changes are more severe and often indicate that a component or joint inside the spectrometer is degrading and may soon fail.

The most sensitive electronic component in a spectrometer is often the detector, which is a semiconductor chip mounted on a ceramic substrate and encapsulated in a housing made from plastic, ceramic, or metal. The detector’s signal and noise performance depend strongly on temperature, making it particularly important to manage thermal conditions during operation.

Spectrometer environmental robustness begins in the design phase by selecting appropriate materials and mounting methods. Optical glasses typically have a much lower coefficient of thermal expansion than most plastics and metals. This mismatch can be reduced by using low‑expansion alloys such as INVAR or KOVAR, which more closely match the thermal behavior of glass. However, these materials are significantly more expensive and more difficult to machine than common metals such as aluminium.

An alternative approach is to design the opto‑mechanical structure so that materials are allowed to move relative to each other in a controlled way. When adhesives are used, it is important to consider whether soft, compliant adhesives or hard, rigid adhesives are appropriate. Soft adhesives can absorb differential expansion and reduce stress transfer, while hard adhesives provide higher stiffness but may transmit more stress to the optical components.

The optical design itself also influences how sensitive the spectrometer is to misalignment. In general, telecentric and circularly symmetric optical architectures are less prone to performance degradation when components shift slightly. A crucial part of any spectrometer development process is the tolerance analysis, where wavelength calibration, spectral resolution, and throughput are evaluated under a wide range of axial, lateral, and angular misalignments of the optical components.

Finally, it is important to select a detector that is specified to withstand the environmental conditions relevant for the spectrometer. Condensation on the detector’s surface and cover glass can occur if the spectrometer is operated below the ambient dew point, which can lead to temporary loss of signal or long‑term damage. This risk can be mitigated by choosing a sealed detector package filled with an inert gas such as argon, or by using a detector with active temperature control. Maintaining the detector at a fixed temperature ensures stable responsivity and noise performance during operation.

The environmental qualification requirements for a spectrometer are derived from its intended use case. It is important to avoid over‑specifying these requirements, as unnecessarily harsh environmental tests will almost certainly increase the cost of the spectrometer without providing meaningful benefit.
Even when the spectrometer is designed for robustness, it is crucial that production follows well‑defined manufacturing processes under a quality‑controlled framework such as ISO‑9001. Adhesive mixing, curing, and mechanical assembly are examples of processes that can strongly influence the final robustness of the product.

Environmental qualification is typically performed as one of the final stages of product development. It is essential that the units used for qualification are identical to normal production units, including materials, adhesives, assembly methods, and calibration procedures.