When buying a lens assembly it is important to understand some of the physical properties of the lens and its compatibility with your intended use. I will detail a few such important details here......
1. Primary Lens output illumination circle size and intended compatibility.
A lens can be specified to illuminate a specific area at its back focus point. That is to say, it must fully illuminate the sensor array or optical system that it feeds into. If a lens is designed fir a larger sensor than it is used with, we see over illumination which results in a reduction of the true field of view provided. This is a permissible situation if it still meets the needs of the user. The opposite situation where a lens designed to illuminate a small sensor is used on a larger sensor is fraught with issues. The illumination circle behind the lens may not fully cover the sensors area and vignetting will occur. Even if the illumination circle does cover the sensors full area, the edge performance could be poor as areas of the lens elements are being used that were not intended.
2. Primary Lens Back focus distance.
A lens has a specified Back Focus Distance that is the distance between the rearmost lens element and the sensors detection surface. The Back focus must be correct for a well focussed image on the sensor. Back focus distances are not a standard and vary in distance. The user should consider whether the lens will suit the application. Some short back focus distance lenses would not be compatible with cameras that employ chopper wheels, filters or FFC shutters between the lens and the sensor. There would not be enough room for such components in the short back focus distance. A relatively long back focus distance is helpful as the lens mounting may be adapted to provide the required back focus no matter whether the camera is designed for short of longer back focus lenses
3. Manual or fixed focus ?
Lenses come in both manual and fixed focus types. There are also motorised focus types but I will not cover such exotics here
The fixed focus lens is designed to be set for a specific optimum focus point and locked into position. Depending upon the application, that focus point can be near to the camera for close-up operation or it can be set at the Hyperfocal focus point where the depth of field covers half that distance out to infinity. The user does not adjust the lens in a fixed focus system but the focus is not always optimal. A manual focus lens is designed to be mounted on a camera at a set back focus distance and the focus may be then adjusted by the user to meet the needs of the situation, close-focus all the way out to Infinity focus, as required. Such Manual focus lenses can provide sharper images but at the cost of user interaction required with the lens due to shorter depth of field. I some situations fixed focus is preferable to manual focus and vice versa.
4. Iris or no Iris ?
On a modern Microbolomter based thermal camera it would be unusual to find a mechanical iris in the lens assembly. They have been used in the past however. A mechanical Iris provides the camera or user with the ability to reduce the amount of thermal energy exiting the optical block and illuminating the sensor system. Sensor technologies such as Pyroelectric Vidicon’s and BST arrays had limited dynamic range. As such they could ‘overload’ and ‘white-out’ when presented with a scene containing too much thermal energy for the sensor to handle. An automatic mechanical IRIS was used to monitor the sensor output signal and reduce the aperture to keep the energy level striking the sensor with acceptable limits. The IRIS was common on cameras used for fire fighting for obvious reasons. The Iris could also provide a secondary feature to protect the sensor from damage, namely a closed Iris when the camera is off to prevent ‘burn-in’ on a sensitive sensor left viewing a high energy scene whilst switched off. This was more applicable to pyro-electric vidicon tube technology however.
An effect of using an Iris is to effectively recalibrate a cameras radiometric measurement system, if such is present. If the Iris is not a carefully calibrated design the sensor system no longer ‘knows’ exactly how its optical block is performing in terms of transmission. As such, it would be unusual to use an uncalibrated Iris in a radiometric thermal camera used for measurements. A lens block that contains an Iris may be repurposed simply by removing or disabling the Iris elements of the design.
5. Athermal or non-Athermal ?
It is a fact of physics that metals change their dimensions in response to a change in temperature. This must be understood when designing precision optics that are housed within a metal supporting structure. The optical elements within a lens assembly (system) are set at precise distances for optimum performance and focus. If those distances change with temperature as the supporting structure heats or cools, the image will be negatively impacted.
Correction by forum member Bill W:
The main problem with lenses over temperature is the optical material changing, rather than the metalwork. In particular with germanium, it has dn/dT of 400 ppm which is far more than simple housing expansion can cancel out.In most cases the operator of the lens mat adjust its focus to compensate for such changes in a lens. Other lenses with large depth of field cope without adjustment by the user. in an application where fixed focus or manual adjustment of the lens focus is not convenient or viable, an Athermal lens may be employed. An Athermal lens is designed to cope with a defined range of temperatures. It employs either a mix of specially selected metals or a thermometer responsive bellows system. Both approaches act to maintain the correct inter element distances over the specified temperature range. These lenses can be complex and so expensive. The bellows design often contains liquid or waxes that respond to temperature change and mechanically counteract the changes in the metal lens supporting structure. such Athermal lenses are not common in everyday thermal imaging systems and tend to be deployed in specialist scenarios. For most applications a non-Athermal lens works well enough. Remember, the lens casing is reacting to the ambient temperature around it and not the scene temperature, though in extreme cases such a furnace observation direct heating of lens elements needs to be considered !
Cooling jackets and heat reflectors are countermeasures to the undesirable heating of a lens or camera system in high thermal energy environments such as Steel foundries etc.
Edit: This Edmund Optics page details the issues with lens temperature change:
https://www.edmundoptics.co.uk/knowledge-center/application-notes/optics/thermal-properties-of-optical-substrates/Worthy of note whilst on this subject is the behaviour of Germanium lens elements when heated. Germanium is thermo reactive and it’s transmission figure reduces with an increase in its temperature. This effect is not noticeable at everyday ambient temperatures but is noted at lens element temperatures approaching 60C. By 100C the effect is significant. The change in transmission through the ‘hot’ lens elements effectively Decalibrates a radiometric Camera unless it is designed to cope with such changes in the lens system illuminating the sensor. if the behaviour of the lens at differing temperatures is tested and plotted in an offset table, the camera can monitor the lens temperature dynamically and compensate within its measurement routines.
6. Lens mount - a standard ?
It pains me to say that lens mounts used on thermal imaging cameras are anything but standardised,except in terms of Metric Vs Imperial Vs Bayonet etc ;
I list below some common lens mounting threads:
M10 x 0.5
M16 x 0.5
M18 x 0.5
M19 x 0.5
M24 x 0.5
M34 x 0.5
M34 x 1.0
M46 x 0.5
M58 x 1.0
USA
1.1875-24UNS
1.3125-32UNS
Wreathall screw thread course (flat top)
Wreathall screw thread fine
Bayonet (cooled cameras)
Supplemental lens mounts
AGEMA (Cooled) Bayonet
AGEMA (Uncooled) Bayonet
Fluke (Uncooled) Bayonet
FLIR (Uncooled) Bayonet
NEC AVIO M58 x 1.0
As can be seen, there are many thread diameters and pitches in use. It is not uncommon for a manufacturer to use a common diameter of thread but an uncommon pitch ! That is to say, instead of using a M34 x 0.5 metric thread, they use a M34 x0.75 or M34 x 1.0 thread
The cynical may believe that this is done deliberately to force users to buy their lenses from the OEM rather than a generic product at lower cost. When it comes to supplemental lenses used as ‘add-ons’ to a thermal cameras Primary lens, each OEM appears to design their own non-standard mounting system. Some have used the uncommon M58 x 1.0 thread (NEC AVIO) whilst others opted for various ‘bayonet’ mounts of unusual dimensions. FLUKE use a bayonet mount very similar to the Micro 4/3 visible light camera mount but the dimensions differ slightly. FLIR have their own bayonet mount design that is common across several of their camera series. That is not to say all such bayonet mount equipped lenses are interchangeable between those cameras however ! Data pins are also used in the mount.
Note: Some thermal cameras do not use a threaded or bayonet mount. They use a sleeve or flange mount and the lens is either ring clamped or retained in place with screws.
Well that is enough for now
Fraser