Regarding “close-up” lenses…….
We need to define what we mean by a “close-up” lens. They can be as follows:
1. A simple, single element, Bi-Convex, Plano-Convex or Positive Meniscus lens profile placed in front of a cameras built-in lens. The simple lens element acts like the lens in reading glasses to permit closer focus than the primary lens can offer. Such supplemental lenses are common in visible light photography where they are supplied in different Diopters fir Macro photography. The simple nature of this type of close-up lens makes them less than ideal and they can degrade the quality of the image produced by the camera. The cheapest and most easily available lens elements for thermal imaging in the LWIR band are those designed for CO2 10.6um laser engravers and cutters. The base ZnSe material and surface AR coatings are compatible with LWIR imaging. These laser focus lenses are not intended to be used for thermal cameras so they are not the quality that would be found in a thermal camera lens or an accessory close-up lens from a company like FLIR. That said, they get the job done and do not break the bank in the process ! I have used such ZnSe close-up lenses for decades so they are nothing new. Better quality single element supplemental close-up lenses are sometimes offered as accessories by some manufacturers of thermal cameras. Industrial camera makers will charge a small fortune for such a lens but it will be if excellent quality and characterised in terms of transmission. Professional close-up lenses used to be made from appropriately AR coated Germanium. These days it is common to find such lenses made from Chalcogenide IR glass or ZnSe but the lens is designed for thermal imaging rather than borrowed from the CO2 laser focus lens parts bin ! Commercial Close-up lenses if this single element type are usually of the positive Meniscus profile. These close-up lenses only permit a thermal camera to achieve good focus at close range (dictated by the focus distance of the supplemental lens) and they do not actually magnify the target of interest. Getting closer to the target enables the user to better fill the thermal scene of the camera though.
2. Macro lenses are a form of close-up lens but they are more capable and usually provide a better image than a simple, single element, close-up lens. There are two main types of Macro-lens, the complete primary lens or the supplemental lens that mounts on the front of a cameras primary lens.
a) The dedicated Macro focus primary lens is basically a standard, multi element lens format that has been designed to provide excellent focus at close range. This type of lens offers the best possible imaging performance but is specialist in nature and requires the camera to have a removable primary lens. The materials used fir the lens elements are the same as those used for common thermal camera lens assemblies, that is Germanium and Chalcogenide IR glass. Sometimes ZnSe lens elements are present in the lens stack.
This type of lens assembly may be designed to produce magnification if desired. The price of such specialist primary lenses can be eye-watering and they are the preserve of the Industrial thermal imaging camera manufacturers.
b) The “reversed primary lens” type of macro lens is common in visible light macro photography and is often achieved by literally mounting a reversed lens on the front if the cameras primary lens so it is a supplemental lens type that does not require the removal of the cameras primary lens. As a reversed lens it contains the same lens elements as found in a primary lens, but operated in reverse ! This design can produce good macro imagery but it has the drawback of multiple lens elements in the optical path with resultant loss of energy transmission to the thermal sensor array. If the reversed lens is the same specification as the cameras primary lens, the pixel size on target will be the same as that of the thermal cameras FPA. This is like a thermal microscope but the focus distance can be very close to the lens ! It is not common to see a reversed primary lens offered as an accessory for a thermal camera but such may be created by an end user. What is sometimes offered is a version of the reverse primary lens format that has been designed from scratch to offer best performance and focus distance. They looked nothing like a primary lens that had just been reversed.
3. There is also the option to insert spacer rings between a cameras primary lens and the Thermal FPA in order to provide a very close focus capability for the camera. Such spacers are only useable if the camera has a removable primary lens.
So there you have it, the term “close-up lens” includes some very different approaches to achieving the desired result.
There are optical solutions in the thermal imaging domain that would be frowned upon in the high resolution visible light photography community as poor practice. Yet we see very expensive thermal imaging lenses that appear very simple in design, yet also very expensive. Thermal imaging has traditionally used simple lens assemblies unless in a specialist role that requires multi fields of view or Zoom telephoto capabilities. In the thermal camera, the optical resolution is quite low and so forgiving of less than ideal lens designs. The cost of the specialist lens elements encouraged thermal camera lens designers to keep things simple with a low lens element count in the BoM. With the advent of cheaper Chalcogenide IR glass moulded lenses we may see more sophisticated lens assemblies becoming affordable and better suited to higher resolution Thermal FPA’s.
Looking at the above, is it any wonder that the single lens element close-up lens is so popular ?
Fraser
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