Anyone Can Build One Spectrometer…

Introduction

An optical spectrometer is a very well-known instrument that numerous spectroscopy engineers and scientists have designed and built at least once in their career. Hence, the title of this article claims that anyone (with some level of technical background) can build one spectrometer. However, if this spectrometer is going to be produced in larger quantities for integration into an end-user product, it becomes much more challenging to design the right spectrometer.

The difference between single unit and volume production

There are basically two different aspects that set the volume production of spectrometers apart from building a single unit.

  • The end-user might buy several instruments and expects all instruments to provide the same result when presented for the same sample. This is often referred to as transferability between instruments.
  • The instrument manufacturer wants a smooth production of spectrometer instruments with a high yield, no rework, and no field returns.

A lack of transferability between instruments can have a direct and severe consequence for the decisions made based on the measurements. As an example, consider a hospital with several instruments for diagnosing a specific type of cancer. If the different instruments provide different results for the same patient, the consequence might be that the diagnoses depend on which particular instrument was used the day the patient was investigated.

If the production of any type of product is not running smoothly it will naturally mean that the cost of producing the product will rise. If the yield is low, a lot of raw material is scrapped which leads to increased material costs relative to the final sales price. If the product has to go backward in the manufacturing process to be reworked iteratively it can cost a lot of extra labor. And finally, if the end-customers experience malfunctioning of your instrument and have to return it for repair, it costs you both money and (even worse) reputation as a quality supplier.

Consistency in performance by design or by alignment

A good way to ensure both transferability and smooth volume production is to design the spectrometer with a consistent performance from unit-to-unit. However, since all the optical and mechanical components used as raw material to assemble the spectrometer have tolerances on their physical parameters, you have to do something special to avoid that these tolerances stack up to a large unit-to-unit variation in performance.

This can be obtained by two significantly different methods:

  • By designing the spectrometer with compensators that can be used to actively adjust each spectrometer during product.
  • By designing the spectrometer such that the performance is not influenced by the various tolerances.

The animation below shows how the two designs would look like in a production. Both designs will end up with a product with consistent performance leaving the factory but, the consistency by alignment method is more costly because it requires extra labor time for the alignment.

How to ensure low unit-to-unit variation in volume production

As indicated above the consistency in performance from unit-to-unit must be built in already when the spectrometer is designed. This entails that the stack up of tolerances of all optical and mechanical parts are taken into account during the design such that a process tolerant spectrometer is obtained. In general, it is preferable with transmission rather than reflection optics and with supporting mechanics that are symmetric around the optical axis.

In relation to the raw materials, it is crucial that specifications are well-defined and that sub-suppliers do not change specifications with-out involving you as a customer. There must be a good process in place with the sub-suppliers to ensure that inevitable non-conformities are handled appropriately and corrective actions are taking.

Finally, the assembly of the spectrometers must follow standard works with clear and easy to follow work instructions. The whole process must naturally be monitored and audited both internally an externally.