Approaches to high performance SWIR lens designTrevor Cook, 8 April 2016
Right now, the world is waking up to the advantages of short-wave infrared (SWIR) imaging for some very good reasons. Indium gallium arsenide (InGaAs) focal plane arrays (FPAs) are advancing technically all the time. Prices are also falling as volumes increase and fab processes start to mature. These and many other factors mean that high definition imaging, using a high performance SWIR lens, is now possible in affordable cameras that can be used for a variety of applications in fields as diverse as food quality control, waste recycling, industrial and border security, seeing through smoke or mist.
However, there is a problem. Currently, the pace of development of affordable high performance SWIR cameras is not being matched by the equivalent high performance SWIR lenses. The result is that the image quality is now more likely to be limited by the lens rather than the camera.
In this post, I explain the technical challenges in more detail and how these can be addressed effectively.
The Technical Challenge
The real challenge lies in the fact that a compatible lens must deliver higher image quality and transmission across the full extent of the SWIR band (wavelengths at or near the short-wave end of the infrared). Until recently, this was solved by re-coating and adapting existing visible lens designs to cover the SWIR band. This only really works for low resolution cameras however.
There are many other hurdles to overcome. While total transmission may be improved, the image quality metrics such as aberrations, MTF, field curvature and image distortion all suffer adversely. In addition to specified focal length and aperture requirements, the image circle size and MTF must match the most recent generation of larger high resolution InGaAs FPAs while keeping size, mass and cost of the lens module low.
Even just a few years ago, the sensitivity range of InGaAs detectors was restricted to the 900 nm – 1700 nm region. This was because the array was manufactured on an indium phosphide base layer which does not transmit in the visible spectrum. FPA manufacturers are now however delivering devices that have had most of this layer etched away which extends the sensitivity range to cover 400 nm – 1700 nm.
A camera / lens combination can now provide imagery continuously through the visible, NIR and SWIR bands. One advantage of extending down into the visible band is that though at dusk and in starlight conditions there are some SWIR photons available from the night sky irradiance effect, the VIS-SWIR sensor improves the night vision imagery compared with the SWIR only device by collecting the reflected light available from across the two bands. This maximizes the signal to noise ratio in conditions where photons are scarce.
There’s also the defense dimension to consider. For this application you need to maintain the designed performance over a wide temperature range and in harsh mechanical vibration / shock conditions together with immunity to total immersion in water.
All in all, this means we need a range of lenses which have high transmission from at least 500 nm – 1700 nm, around 50% contrast ratio at 50 lp/mm across the entire field, an image circle of around 20 mm, athermalized over -40°C – +71°C, sealed to IP68 and are mechanically extremely robust.
Moving Towards a Solution
To meet the challenges, optical designers started by examining the available choice of suitable glass types. The index and dispersion properties at SWIR wavelengths can differ significantly from their visible values, resulting in unusual glass combinations when matching elements for group lens construction. Choosing for these parameters and utilizing carefully selected co-efficient of thermal expansions (CTE) wisely in the elements of doublet or triplet components allows for accurate athermalization of the design.
It’s also important to be mindful of the fact that the SWIR band is sufficiently close to the visible (VIS) so the same constituent materials can be used for the deposition of thin film AR coatings. However, continuous AR high performance from the visible band out to 1700 nm is very demanding. Of course, you also want to keep the lens small, but this can result in high incidence angles on some of the steep surfaces, which imposes even tighter constraints on coatings. With this in mind, coating engineers have focused on developing techniques to achieve this continuous high transmission over the VIS-SWIR band for wide field angles whilst also keeping volume production in mind.
It’s also true to say that the required module performance forces very high tolerances on both individual lens form and centering of the elements within the lens groups. In order to achieve these extreme requirements whilst keeping the assembly process simple, component providers must be capable of extreme accuracy in these parameters for both spherical and aspheric curves by using state of the art machine processing. This must be combined with the availability of very high dimensional tolerances for the mechanical components, ultimately enabling simple volume assembly techniques that help to keep the cost low but the repeatable quality high.
Once all this has been achieved the optical components can then be bonded using mounting and adhesive assembly techniques without the need for adjustment on assembly. The finished product can therefore operate in extremely harsh mechanical and thermal environments with full performance.
Innovation in Practice
Although the task may seem daunting, progress has recently been made in the industry. Here at G&H we are currently in pilot production of a 25 mm high performance SWIR lens (model name, the Barle) which meets all the requirements listed above. We also have 12.5 mm fixed focus (the Avill) and 40 mm – 200 mm continuous zoom (the Isle) optimized optical designs at the mechanical development stage.
Our Glenrothes facility has the required capability of extreme accuracy in the optical component parameters for both spherical and aspheric curves using our state of the art optical lens machining, polishing and centering equipment. This in concert with our advanced thin film design, coating facility and optical assembly shop at Ilminster together with our extremely accurate automated machining partners gives us the ability to produce the high performance VIS-SWIR modules that the market needs and most importantly – at an affordable price.