Bespoke Spectrophotometer System
AEON has completed design, manufacture, integration, and commissioning of a multi-position spectrophotometer, used to characterise the transmittance of large optics.
As part of an ongoing programme with a world-class space research and technology developer, AEON proposed a campaign of optical transmissibility tests on Optical Ground Support Equipment (OGSE) previously designed and manufactured by AEON. The purpose of the testing was to ensure the as-manufactured optical glass performed to the customer’s requirements and to provide data on how transmissibility varies both over the wavelength spectrum and across the surface of the optic.
While commercially available spectrophotometer systems exist, none are currently able to perform over the required wavelength spectrum and typically will only accept small optics up to (approx.) 25mm diameter. By comparison, AEON’s system can fully characterise optics up to 300mm in diameter and up to 30mm in thickness.
To provide good coverage over the surface of the optic, measurements were to be taken at approximately 100 discrete locations with measurements taken from 250nm to 2700nm at 1nm increments. In total over 230,000 discrete data points for each window were captured. For a person to perform this task, it would be extremely time consuming, AEON therefore decided to develop an automated system to conduct the test end-to-end in a semi-automated manner.
The spectrophotometer, developed by AEON, can be considered as a combination of two separate sub-systems:
- The optical system, used to generate and select a specific wavelength of light and to measure power.
- The mechanical positioning system used to manipulate the optic, allowing characterisation of any point across its surface.
To have as many measurements taken without human intervention, a bespoke system to position the optic autonomously and accurately (relative to the light source and detector) was required. To keep the working envelope of the positioning system to a minimum, a novel solution using a combination of linear and rotational motion was used to allow the light source and detector to be aligned to any position on the surface of the optic. To maintain optical alignment between the light source and detector, they were mounted independently from the positioning system which is allowed to translate in and out of the optical path.
Each optic is loaded into a bespoke retaining assembly, where it is held securely in place, sandwiching the glass between two o-rings, and therefore preventing contact with the metal housing. The retaining assembly incorporates a tensioned belt and groove to provide torque transfer. The retaining assembly also runs on specially profiled rollers providing a self-centring effect that ensures the window remains perpendicular to the emitter and detector. By eliminating mechanical swash and ensuring perpendicular alignment (between optical path and the optic) reduces the parasitic losses due to substrate reflection and refraction which would otherwise contribute to transmittance measurement errors.
Rotary motion is achieved using a stepper motor coupled to the drive belt by a small drive pulley. This gearing provides the required resolution needed to perform transmittance measurement anywhere on the surface of the glass to less than 1mm. As this system is capable of rotating the glass continuously through 360°, it reduces the linear travel distance required and thus minimising the overall size of the spectrophotometer system.
By developing each of the sub-systems in-house, AEON has the unique capability of performing transmissibility testing on large optics over a broad wavelength spectrum. The completed spectrophotometer is capable of:
- Accommodating optics up to 300mm in diameter and 30mm in thick (but could be modified for larger sizes if needed).
- Wavelength range of 250-3000nm (ultra-violet to near infrared).
- Performing point measurements anywhere on the optics surface to within 1mm (there is no limit to the number of points that can be measured, and measurements can be overlapping).