Miniature Spectral Imaging Camera

The use of Spectral Imaging to improve image analysis and allow discrimination between visually identical objects is increasing. Gooch & Housego has applied high speed acousto-optic tunable filters (AOTF) to develop the HSi-440C Hyper-Spectral Imaging microscope system for biomedical research which allows video rate processing of high resolution images. But while AOTFs offer high speed and digital flexibility there are applications where a simpler and more compact design may be preferable. To explore the potential for a miniature spectral filter we have prototyped a simple tunable Fabry Perot etalon and as a demonstration applied it directly to the front of a cell phone camera.

A simple Fabry Perot tunable filter (FPTF) can be formed with an air gap etalon made with a pair of flat high reflecting mirrors. The resonances of the etalon are narrow spectral pass-bands and adjusting the air gap thickness tunes the filter wavelength. The wavelength separation of adjacent resonances is inversely proportional to the spacing between the mirrors and the widest single-peak tuning range occurs when the separation is just half the wavelength of the transmitted light. When the optical coating thickness is considered, the required gap between the mirrors becomes even smaller. A typical digital camera imaging system requires an aperture of a few mm so a uniform spectral filter that can be placed directly in front of a camera requires two very flat (<λ/200) mirrors placed very close together (100-300nm) in precise parallelism (<0.02 arc seconds). Tuning such a filter then requires careful adjustment of the spacing between the plates, usually by just a few nm, while maintaining the precise parallelism. FPTF designs to date have achieved these requirements by using 3 or more independent PZT or MEMs transducers to move a plate while measuring the separation of the plates with 3 or more capacitive or optical sensors, thereby controlling each transducer independently. The complexity of such a multiple transducer and sensor control system is daunting and adds to the size and cost of such a device. The simple FPTF solution we have devised uses an existing “tweeter” optic already manufactured at our facility at Moorpark, CA, for avionic navigational Ring Laser Gyros (RLGs). RLGs use high reflectivity super-polished dielectric mirrors to create a HeNe ring laser cavity and monitor the optical phase very precisely. To stabilize the RLG cavity length, one of the mirrors in the ring is generally designed to be moved stably back and forth without misalignment. Such a tweeter mirror is constructed with a central mirror supported at both ends by two thin glass membranes that constrain the central mirror to move only in a highly linear motion. Membrane mirrors for RLGs as described in Patent US 3,581,227 date back to the 1960s. The use of an RLG tweeter simplifies the construction of a FPTF so that only one actuator and one sensor is required. The picture below is a simple schematic of such a FPTF.

Simple PZT driven Fabry Perot tunable filter

Simple PZT driven Fabry Perot tunable filter

G&H have assembled simple FPTFs using tweeter mirrors with thin silver and silica coatings and driven by PZTs at only a few 10s of volts. These filters are 2.5cm in diameter and 4 cm in total thickness and have a clear aperture of 5 mm so they fit comfortably on the front of the camera on a cell phone camera. With a narrow air gap spacing, the filter passes a single color that can be tuned in wavelength from 450nm to longer than 700nm. The video below illustrates the performance when a simple image is analyzed by scanning the filter in front of a red-green-blue camera. On the left is a construction of 5 pieces of differently painted surfaces and on the right is a photocopy image of the same construction. As the filter is scanned in wavelength the different spectral content of the original and the photocopy become obvious.

Spectral imaging of a colored paper next to a photocopy image. The spectral camera displays differences not easily visible to the eye

We believe such a simple PZT driven tunable filter could have a wide range of applications such as counterfeit detection and medical diagnostics. The technology is easily extendable to narrower spectral widths and longer wavelengths and simple enough to allow the construction of double filters with more complex spectral shapes and higher background rejection. If you are interested in G&H’s spectral filter capabilities please contact G&H CTO Murray Reed.