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Products => Thermal Imaging => Topic started by: Fraser on March 07, 2020, 11:27:45 am

Title: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on March 07, 2020, 11:27:45 am
For those of us who buy elderly thermal imaging equipment it is not unusual to find Germanium lenses that are suffering various forms of age or abuse deterioration. A recent conversation with Ultrapurple on the topic made me think a forum post on buying used lenses is justified. Here it is.

When purchasing a used thermal imaging lens, no matter what age or the materials used, it is important to understand the ‘issues’ that can exist with such lenses. A lens that has been badly abused is likely to show clear visible signs of such so even when buying from eBay the buyer can often assess the serviceability of a lens.

I have previously warned about buying the correct lens type for a particular operating band. If you are using a LWIR Microbolomter based camera, there is no point in buying a lens designed for single band MWIR use, or as AGEMA used to call it SW ! That is just selection of the correct lens and not a defect issue.

I will list some known defects and issues that should be considered when buying a thermal imaging lens........

1. Buy a lens that has an Anti-Reflection (AR) coating for the band you are using (LWIR, MWIR or SWIR) The AR coating increases the transmission of a Germanium lens from around 45% to around 90% (depending upon lens specifics). It is either mono band or can be broad band/multi band. With multi band lenses, the transmission figure is normally different for each band.

2. Buy the correct type of lens. There are Primary lenses intended to illuminate a sensor array and Supplemental lenses designed to illuminate a primary lens as in an ‘add-on’ lens for wider or narrower Field Of View (FOV)

3. Check a lens for evidence of deterioration, even if new. A lens that has been stored incorrectly can deteriorate over time. Obvious signs of damp storage are corrosion on the lens barrel, spider web lines across the lens surface (corrosion of the Germanium) and delamination of the AR coating with grey-white corrosion beneath (serious corrosion of the Germanium). There is also loss of lens coating through delamination that does not appear to be corrosion related. This often appears as flaking away of a layer of coating on the lens but the layer beneath remains shiny, uncorroded and not the silver-grey colour of bare Germanium. More on this later. Scratches to the surface are often visible to the naked eye or when a light is directed at the lens surface. It is important to remember the relatively low resolution used in thermal imaging systems compared to visible light cameras. Such a low resolution system can be quite forgiving of scratches on a lens. You should assess scratches in terms of severity. Light scratches from incorrect cleaning are often tolerable but deep scratched that penetrate the AR coating or removal of the AR coating from incorrect cleaning practices can seriously degrade a lens’s performance. It is best to test a lens that has AR layer damage before purchase. Depending upon the severity of the damage, the lens may still be useable. Light scratches on industrial and fire fighting camera lenses are not uncommon.

4. All lenses are not born equal. Be aware that some lenses used on elderly thermal imaging systems may look pristine and impressive from their size but they were often designed for use with very different technology to that used in modern thermal cameras. The sensor system may have involved scanning mirrors or a staring array that used large pixels. A lens designed for use with relatively low resolution systems may not have adequate resolution performance when used on a modern microbolomter with far smaller pixels and possibly higher resolution. Some of the older Germanium lenses are truly beautiful pieces of optical engineering but they should be purchased with some caution. In the case of some Inframetrics and AGEMA lenses, part of the lens system is fixed within the cameras lens mount. Without that lens the removable lens is an incomplete optical system. Be warned and buy wisely !

5. Lens Field of View is very much dependant upon the physical dimensions of the sensor array. If an older primary lens is used on a modern camera that has a much smaller microbolometer die, the FOV will decrease in proportion to the die size differential. This could be an advantage to those wishing to source narrower FOV lenses for their modern cameras. There is, of course the issue of lens resolution to consider however. This may be offset somewhat by the fact that the modern, smaller imaging sensor will be using the central area of the lens that has higher performance than the periphery :)

6. Which is better.... Germanium, Chalcogenide IR Glass or Zinc Selenide (ZnSe) ?
There is no simple answer as much depends upon the age (resolution) of the lens, its condition, cost and intended use. All lens materials have their own little foibles that need to be considered.

Germanium is traditionally the best and most common material for use in high performance thermal camera optics. The lens element is cut from a Germanium mono crystal so is of superb ‘clarity’. It is however vulnerable to corrosion and is temperature sensitive in terms of its transmission.

Chalcogenide IR Glass is a bit like a ‘wonder material’ for the thermal camera manufacturing industry. It is a special mix of Germanium fragments and IR transmissive material that forms a ‘glass’ that may be moulded rather than the lathe cutting required for Germanium. It is far cheaper to manufacture than a Germanium lens as a result. Early Chalcogenide IR glass lenses were relatively low resolution and high loss due to the high number of micro fractures within the material (it is not a mono crystal) The technology has improved since but this is still a relatively young technology. Performance is generally not quite as good as that of a quality Germanium lens but do bear in mind what I have said about older Germanium lenses and their resolution. Chalcogenide IR glass is generally not considered to be as thermally sensitive as Pure Germanium lenses. In short, this development offered manufacturers a far cheaper lens for their camera cores without sacrificing too much performance in the process.

Zinc Selenide (ZnSe) is a relatively soft broad spectrum transmissive material that provides coverage from visible light through to the LWIR spectrum. As such it may be used in a wide range of imaging systems at very different wavelengths. The AR coating dictates the passband for the particular lens element. It is not uncommon to find high performance lenses that contain a combination of Germanium and ZnSe lens elements. It is a high performance lens material and quality examples can be very expensive. ZnSe optics have become very common in CO2 laser applications. Lenses for such are now mass produced, reducing cost to the consumer. Be warned however, lenses made for use in a CO2 laser are often not rated or recommended for imaging applications by the manufacturer. The performance of CO2 ZnSe lenses is very variable and high quality types are very expensive. ZnSe lenses used in thermal imaging applications are of the highest quality of manufacture and are very expensive.

I will end this post here but will add to it as and when I think of anything useful to detail.

I will be detailing lens coating delamination soon.


Title: Re: Buying used Thermal imaging lenses - general buying guidance from Fraser
Post by: Vipitis on March 07, 2020, 04:12:58 pm
Will be a helpful guideline. I can provide some photos of different coatings, worn, pristine, broken and bubbling.

But I won't be home till late Monday
Title: Re: Buying used Thermal imaging lenses - general buying guidance from Fraser
Post by: Fraser on March 07, 2020, 05:30:44 pm
Thanks Vipitis, that would be really helpful.

I just want this post and others in the thread to assist people with their purchase choices when it comes to thermal imaging optics. There is not a great deal on Google about used Germanium lens issues and degradation for instance.

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on March 09, 2020, 12:39:21 pm
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: (

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



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 :)


Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on March 09, 2020, 03:57:41 pm
Supplemental Lenses.......

Supplemental lenses deserve a new post all to themselves as they are unusual beasts  ;D

A supplemental lens is designed to mount in front of the imaging systems primary lens as an addition to the optical block. It’s introduction does incur some losses in terms of transmission, but these are normally acceptable and some cameras even permit the entry of the additional lenses characteristics in order to maintain calibrated measurement accuracy.

A supplemental lens is normally Afocal. That is to say, it receives energy from the scene being viewed and converts it to the required output to be fed into a primary lens. Common uses for a supplemental lens are to increase or decrease field of view, provide close focus capability or to otherwise change the view that the cameras primary lens sees through it. In a supplemental lens block for, say, a X2 or X0.5 role, the energy leaving the rear of the supplemental lens is often parallel beams so the camera lens is set to infinity focus for best match. Single element close focus lenses do not always have parallel beam output but the camera primary lens is still a good match at the designed close focus point. Supplemental lenses are normally compatible with both fixed and adjustable focus primary lens systems.

A supplemental lens is usually located in close proximity to the cameras primary lens to avoid vignetting issues. It is important that the supplemental lens output pupil is the same, or larger than that of the primary lens input. It is acceptable to use a larger output pupil supplementary lens, but a smaller output pupil lens will likely cause vignetting for obvious reasons.

The mounting method for a supplemental lens can take many forms ranging from simple ring clamps to threw threads and bayonet mounts. These mounts can be quite exotic and manufacturer specific. No standard for such mounts exists. It is possible to adapt the mount on a supplemental lens to fit a primary lens with a different mount. Provided the output pupil is adequate the mount is purely a mechanical issue that may be resolved with an adapter of some description.

It should be noted that not all supplemental lenses are useable on other brands of thermal camera primary lens. The operating band has to be the same of course but some supplemental lenses invert the image at their output. If the camera system does not know to invert the image in its processing stages, the user sees an inverted image. Whether this can be dealt with in the user settings of the camera is camera specific so I can make no comment.
Some supplemental lenses, called “Telescopes” were designed to mount onto the front of scanning thermal imaging systems. These Telescopes are often Keplerian inverting types and can have unusual output beam paths. They are great lenses from the likes of Inframetrics but they do need some experimentation to get them working with modern Microbolomter FPA cameras.

The joy of owning decent supplemental thermal imaging lenses is that a user can often use them on different cameras by the use of simple 3D printed or CNC cut adapter rings. They can be a very useful addition to the thermographers or photographers tool kit :)

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Bill W on March 09, 2020, 04:29:14 pm
This link is a good review of materials: (
if missing out Ge Chalcogenides for which this explains: (

A couple of points to add:

2 - back focus
There are two distances quoted and sometimes mixed up.  One will measure from the lens metalwork 'mechanical working distance', while one uses the back of the lens elements often 'back focal distance'.  These can differ a lot and no guarantee which is the smaller, if there is a convex rear element.

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.  I have seeen athermal designs using 3 tubes of nylon (high expansion) and 3 sets of invar rods (anchors the nylon).  All extreme, and for the same costs you might as well just have motor focus.

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on March 09, 2020, 07:39:12 pm
Thanks for the input and correction Bill. Much appreciated and I have added a correction in the original text.

I see that Edmund Optics cover this issue well. I have provided a link in my previous post but reproduce parts of  the Edmund Optics page text here for ease of reference.........

Coefficient of Thermal Expansion

An athermal optical system should be developed for applications subjected to temperature fluctuations. Athermalizing optical systems involves balancing the coefficient of thermal expansion (CTE) and the change in index with temperature (dn/dT) of the materials used to make the system insensitive to the thermal change in their environment and the resulting system defocus. Developing an athermal design is especially critical in infrared applications.

CTE is a measure of the fractional change of a material's size due to a change in temperature.

Typically, as an object heats up it becomes larger due to the increased kinetic energy of its constituent molecules. However, there are some rare exceptions where there is an inverse relationship between temperature and length, such as water, where its CTE becomes negative below 3.983°C and causes it to expand as the temperature drops below 3.983°C.

Changes in temperature (∆T) lead to a change in the length of a material (∆L) based on the material’s coefficient of thermal expansion (CTE).

The CTE is given in units of 1/˚C. When selecting an optic for your application, CTE is important to consider because changes in the optic’s size may influence alignment and stresses on the component. In environments involving swings in temperature, users need to be cognizant that their optic will not expand when heated. An optic that is 25mm at room temperature may be 25.1mm at 300˚C, which could break the mounting or skew light in an unwanted direction, thereby affecting pointing stability or laser alignment; this is generally why a small CTE is desired.

Temperature Coefficient of Refractive Index

The temperature coefficient of refractive index (dn/dT) is a measure of the change in refractive index with respect to temperature. The dn/dT of most IR materials is orders of magnitude higher than those of visible glasses, creating large changes in the refractive index. The density of a substance is almost always inversely proportional to temperature, meaning that a material’s density will decrease as the temperature increases. Therefore, refractive index decreases as the temperature increases.

dn/dT is irrelevant for reflective optics, except for minor performance variations due to changes in the refractive index of the coating. However, dn/dT is an important property for transmissive optics as it helps determine their stability under temperature variations. There will always be some absorption with a high power laser beam incident on an optic, leading to an increase in temperature; dn/dT determines how much this affects performance

The change in an optical component’s refractive index with temperature (dn/dT) can lead to a shift in a lenses focal length (∆f), changing the focus position.

Thermal Conductivity

The thermal conductivity (k) of a material is a measure of the ability of the material to transfer heat via conduction It is commonly measured in W/(m⋅K) or Btu/(hr⋅ft⋅°F) and is used to define the rate of thermal conduction

The thermal conductivity of a material (k) defines its ability to transfer heat through a given thickness.

Materials with high thermal conductivities, like metals, are able to dissipate heat much quicker than materials with low thermal conductivities, such as glasses or plastics. Because one of the primary effects of transmitting laser radiation through an optic is the conversion of the radiative energy to thermal energy, knowing the thermal conductivity of a material is important in order to evaluate the energy balance around the optic in laser optics applications. Materials that do not reflect or transmit specific wavelengths will absorb more light and heat up more quickly; examples include colored glasses and absorptive filters. If non-steady state accumulation of heat occurs in the optic, damage will quickly ensue, especially without the addition of an effective cooling system. Even then, if optical components are non-homogenous their ability to conduct heat is non-uniform and hot spots in the material could quickly and more effectively cause damage to the component. Similarly to the temperature coefficient of refractive index, understanding thermal conductivity is important for modeling high power laser systems and understanding what optical performance effects to expect.


“TIE-19: Temperature Coefficient of the Refractive Index.” Schott, July 2016.
Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Vipitis on March 11, 2020, 03:37:14 pm
I am kinda late and this isn't all that I promised.

this is a picture of a 40mm Ge Window that is 2mm thick. It has a "diamond-like" hard carbon coating on one side and a AR coating on the rear side. It is severely worn and it looks like a few hundred tiny impacts. It was exposed for a few years to harsh conditions of open sea and store somewhere for another few years before it got to me.

looking through it with my cameras only drops the signal by 2-3% either direction.

While this is a window, I believe my phone camera is protected by a similar material and I also have a lens that uses this coating as front element, with equal signs of wear.

more to follow when I find my broken element - it wasn't at one of the three places I thought it would be so that is worrying-
Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on March 11, 2020, 04:36:47 pm
This is the window from a sea going FLIR thermal camera. The effects of Salt Water attack on the surface are clear to see. Even a marinised camera with Hard Carbon coating eventually succumbs to the assault of the sea.

The camera does not normally see this spot damage on the window however. It is cosmetic but the window would be replaced as part of a FLIR service.

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on March 11, 2020, 04:48:04 pm
The lens system on this Inframetrics 760 thermal camera is in a sorry state.

There appears to be damp atmosphere induced corrosion on both the lens element in the lens mount and the removable lens front element. Both are classic presentations of the deterioration in lens coatings that make this camera, and associated lens, a bad purchase for anything but spare parts ! The damage to the coating on the camera is clear and classic corrosion. The damage to the front element of the removable lens is not as clear. At some point in its life the lens coating started to flake away from the lens surface. The flakes are tiny pieces of AR coating material and any attempt to wipe that lens will just remove all the flakes. That is what has happened here. The flakes have been wiped away, along with the surface coating :( That lens is a bit of a mess. I have seen such coating damage on several occasions and there is no remedy. The lens is basically a parts donor only.

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Vipitis on April 27, 2020, 03:20:38 pm
okay, it may be a little late but I managed to find the day I dropped it (15th July last year) as well as the remains of it. My visible light camera is partially broken so this is the best image I get here.

this shows how the lens has an internal crystal structure, you can see how there is no coating on the edge but also the little debris where the lens was mounted. it looks like there are tiny chips in the coating on the surface, but I can't really tell. There are more pieces left of this lens element, but this one showed it well.

PS: sorry Bill.
Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Bill W on April 27, 2020, 07:34:12 pm
PS: sorry Bill.

.............. you MURDERER !!!!!!!!!!   

 :-DD   :-DD   :-DD

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Mr. Scram on April 27, 2020, 07:49:34 pm
A somewhat related question: how does one clean a thermal camera lense? I'm afraid to rub or brush dust off of regular camera lenses and thermal camera lenses scare me even more.
Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Fraser on April 27, 2020, 08:15:47 pm
Mr Scram,

This has been covered in the past if you search for “lens cleaning” you will find some comments.

This thread may be what you need :) (

I did write a more detailed lens cleaning guide. It may have been in my “E4 useful information” thread ?

Title: Re: Buying used Thermal imaging lenses - general buying guidance
Post by: Mr. Scram on April 27, 2020, 08:22:54 pm
Mr Scram,

This has been covered in the past if you search for “lens cleaning” you will find some comments.

This thread may be what you need :) (

I did write a more detailed lens cleaning guide. It may have been in my “E4 useful information” thread ?

Of course you were one step ahead of me. Thanks for all the very useful information.