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SWIR Processing Infrared Filters
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#1 Andy Perrin

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Posted 29 June 2019 - 19:26

Sticky :: IR Photography: Cams, Mods, Lenses, Lights, Links

by Andy Perrin for UltravioletPhotography.com
Started: 28 June 2019
Edited: 30 June 2019

Note from the author:
To paraphrase Andrea, "This is a joint effort by the members who enjoy [Shortwave] Infrared photography. Thanks to everyone for their suggestions, comments, proofreading, lists, links, measurements, experiments and all round good fellowship."

Please PM Andy Perrin on UltravioletPhotography.com with any corrections, additions or suggestions.

Abbreviations:
  • IR = infrared (taken here to mean the entire band from NIR-LWIR)
  • UV = ultraviolet
  • NIR = near infrared
  • SWIR = shortwave infrared
  • MWIR = mediumwave or midwave infrared
  • LWIR = longwave infrared
Quoted prices are in US dollars and are only meant to give a rough idea.



INTRODUCTION

As another well-known Guide once put it, the infrared is big. It is, in fact, so hugely mind-bogglingly big that it can't be properly treated as a single band for many purposes. It has thus been divided into a handful of sub-bands according to several different mutually inconsistent schema. Wikipedia (as of June 28, 2019) lists five different schemes to divide up the infrared, several of them with overlapping nomenclature for different wavelength cutoffs to add to the confusion.

For this guide, we will use the scheme termed "Sensor response division scheme" on Wikipedia, which starts with Near-Infrared (NIR) from 700nm-1000nm, which is where silicon sensors cut off, followed by Shortwave-Infrared (SWIR) from 1000nm to 3000nm, Midwave-Infrared (MWIR) from 3000-5000nm, Longwave-infrared (LWIR) from 8000-12000 microns or 7000-14000 microns, and then Very-long wave infrared from 12000-30000 microns. The interested reader can consult Wikipedia for the other schema, while the alert reader is left to ponder what became of the gap from 5000-8000nm.

Unfortunately, this mess has real consequences for those trying to purchase shortwave infrared camera equipment because it may not be listed as "shortwave infrared" on eBay or other sites. Equipment may be listed by the type of sensor it is compatible with, usually InGaAs, or as SWIR, shortwave, the generic "IR," or even NIR (which is sometimes considered to include as far out as 5 microns!) The searcher is advised to try variants on all of these or else miss out on deals.

SHORTWAVE INFRARED PHOTOGRAPHY

SWIR CAMERAS

Using the classification above, SWIR starts at 1000nm. Silicon still has some sensitivity in the 1000-1100nm region, and for this part of the band one can use an ordinary converted silicon camera. However, while some interesting SWIR effects start here, such as water becoming light-absorbent, most of the differences from NIR don't become significant until one is past 1100nm.

Silicon sensors can be made to exhibit some sensitivity to SWIR from 1460-1600nm by coating them in an up-converting phosphor material. These coated sensors use the anti-Stokes effect, in which two SWIR photons hit the material and a single NIR photon is emitted, which is then captured by the silicon sensor. Edmund Optics sells these. They are intended for calibrating telecom lasers, and have not been tested for imaging purposes. Typical new cost is the $2000-3000 range. The author cautiously warns against purchasing one, unless you find a really outstanding deal, for reasons described below.

A second (bad) option is to use an up-converting phosphor screen in conjunction with a converted silicon camera. The screens are available from Edmund and also under other brand names for much less than Edmund's. The author purchased one of these on eBay for ~$200 and did a series of experiments with them. In these experiments, it was found that the phosphor material is extremely weak, rendering the apparatus as a whole quite insensitive. Thus it was necessary to use a very intense light source, nearly to the point of setting fire to the scene. In addition, the screen itself was granular and did not provide good resolution or contrast. Because the same phosphor materials are involved in the coated sensors of the last paragraph, the author doubts that the coated sensors will work well for general purpose photography. However someone would need to acquire one to test the hypothesis — a lot of money for something expected not to work well.

A relatively low-cost option that would provide SWIR coverage up to 1550nm is the Find-R-Scope vidicon tubes. These are analog devices, but could potentially be coupled to an ordinary camera. They are relatively easy to find on eBay for prices under $1000. Buyer beware — not all of them go out to 1550nm. Read listings carefully, and if it does not say it is 1550nm-capable, assume it is not. Vidicon tubes are rated only for a certain number of hours before they wear out, so the age of the device may also be a consideration. The author has not tested any of these, but this may be the most cost effective entry into SWIR.

The next step up in quality from the vidicon tube imagers is the most traditional means of SWIR imaging: Indium Gallium Arsenide (InGaAs) cameras. These are usually digital cameras with resolution of 320x240 or 640x480 (and higher now, but at a cost) with excellent sensitivity from 900nm-1700nm. The cameras are generally machine vision cameras for industrial, medical, art conservation, or espionage use, so they need to be attached to a computer to take and store the photos. Conceivably some kind of portable apparatus could be rigged up by using a tablet as the computer. eBay price is generally $5000-$10000, but I have seen as low as $3000. Typical brand names are Goodrich Sensors Unlimited, FLIR Tau SWIR, Allied Vision, and Princeton Instruments (far from an exhaustive list). Good questions to ask of a seller include whether the camera has a lens, whether it has the driver software needed to run, and if it comes with a power supply.

A rarely seen alternative to InGaAs is the Germanium-on-CMOS imagers made by the defunct NoblePeak Technologies company. These cameras, collectively called the TriWave, ranged from 320x240 up to some higher resolution limit unknown to this author. The cameras are sensitive from 300-1600nm. A high resolution example was demonstrated by Nick Spiker on this forum here. The author owns an analog-output 640x480 version and paid $3000 for the camera in "new-unused-but-opened" condition (still in original plastic bags) with all accessories except the lens. The TriWave cameras were in active development at the time of the company's demise, and therefore it is likely that all the TriWave cameras have slightly different capabilities, depending on what point in the development cycle they were sold at. Later cameras probably had digital output via a USB port, as shown in TriWave datasheets. These cameras are cooled by a built-in thermoelectric cooling system and require about 60 seconds to "boot up" while you wait for the chip to reach -80C. The camera will not produce an image until it reaches -70C or so, with optimal results at -80C or lower. As with InGaAs, when buying used ask whether the camera has its original lens, whether it has the driver software needed to run, and if it comes with a power supply.

A new option by the company SWIR Vision Systems are the Acuros quantum dot cameras, which are quite high resolution, ranging from 640x512 up to 1920x1080 pixels as of 6/28/2019. The company claims the price is lower than InGaAs, but the author does not know the actual prices, which are not posted. Because the technology is new there are no used cameras available yet. The sensor seems to be particularly sensitive on the blue end of the visible spectrum (with unknown but likely high UV sensitivity) based on their published quantum efficiency chart. The sensitivity goes to zero by 1700nm. The cameras are digital machine vision cameras, so need a computer to operate like the InGaAs and Ge-CMOS cameras.

SWIR FILTERS

For all of the above cameras, appropriate filtration is necessary to block non-SWIR wavelengths and to narrow the piece of the SWIR spectrum the photographer wishes to look at. It is worth noting that some material properties can change significantly with wavelength in SWIR, so the 1000-1100nm band is different from 1200-1300nm, which is different from 1500-1600nm in terms of what one will see. In particular, sugar and water both have rapid variation across SWIR, so objects of biological origin like flowers or people are likely to show interesting effects.

Depending on the type of imager technology, different levels of blocking will be needed depending on sensor. InGaAs, in particular, is not sensitive below 900nm, so does not require any visible light blocking or UV blocking. The Triwave Ge-CMOS camera, on the other hand, is sensitive from 300nm-1600nm, which means 300nm-1100nm (or higher) needs to be blocked well to see any SWIR. The photographer will need to evaluate their blocking needs based on what camera they are using, and what light source.

The two best-priced options that the author has identified so far are Thorlabs and Omega Optical (in particular Omega's eBay site for out-of-spec filters is full of good deals). The author owns two SWIR Thorlabs filters, an FEL1500 (1500nm long pass), and an FELH1200 (1200nm premium long pass) and has noticed no out-of-band signals using the light sources available for testing with. In particular, visible contamination is a worry with the author's TriWave camera, but using the FEL1500, objects that reflect visible and NIR light extremely well but 1500nm SWIR poorly (e.g. human skin) show as black, indicating no contamination. For the 1400-1600nm band, the author feels the "skin test" is an easy way to check for poor filter blocking, much as dandelions are used to check for NIR leaks in UV. Note that skin is not dark in SWIR until 1400nm or so, so this is not a useful test in the 1000-1400nm range.

Unfortunately, the author does not know of any SWIR absorption glass filters available for sale except the 1000nm longpass kind, which leak a bit of NIR.

SWIR LENSES

The lens situation in SWIR is similar to UV and the issues are the same. Most ordinary lenses will pass at least a bit of SWIR, sometimes as much as 50%. Multicoated lenses and lenses with many elements are bad news. Chromatic aberration and focal shift can cause problems. Because of the similarity in the underlying issues, UV-capable lenses probably make good candidates for SWIR testing also. One catch is that most SWIR cameras use C-mount lenses, so it will be necessary to use an adapter if one can be found. Another issue to keep in mind is that focal lengths are given in absolute numbers, but most SWIR cameras have tiny "cropped" sensors, so a 50mm lens on a TriWave with a 1/2" sensor, which has a crop factor of 5.4 relative to a 35mm sensor, will behave with an effective focal length of 50*5.4 = 270mm!

An alternative to accidental SWIR lenses is to buy a lens intended for SWIR imaging. These use special glass types that pass SWIR better than ordinary lens glass, and they have broadband anti-reflective (BBAR) coatings designed for the SWIR region. The author has tried two of these.

The first is a simple 50mm achromatic doublet from Thorlabs, which was purchased on eBay for less than the list price. The photographer is advised to always check eBay for Thorlabs equipment first, by entering the desired model number directly into the search box, because many bargains are available. Thorlabs also sells SM1 to C-mount adapters.

Another lens tried by the author is a 12.5mm/F1.4 Kowa SWIR lens, also purchased on eBay, which was found to perform significantly better than the doublet in sharpness and contrast, even on a 640x480 sensor. While the Thorlabs lens was much cheaper, the Kowa's performance was so much better that it is the recommended lens of the two. On the TriWave, this 12.5mm lens has an effective focal length of 67.5mm, so it performs as a telephoto, not a wide-angle as one might naively expect.

SWIR LIGHTING

First, there is natural lighting. The sun produces abundant SWIR light. A very interesting effect is that the clear night sky also produces SWIR, via an effect called airglow, which is emission by chemical reactions in the atmosphere. The author has not been able to detect this effect yet, possibly due to light pollution.

For artificial lighting, any incandescent source should work. Halogen lights are well-known to produce abundant SWIR. Unfortunately for SWIR photographers, while halogen room lights and desk lamps are still abundant at the time of writing (mid-2019), already it is clear that LED lights, which are more energy efficient and less likely to start fires, are dominating the market. In several more years, halogen lamps may become specialist items with expected price increases.

SWIR LEDs and laser diodes exist but are expensive. Thorlabs sells the parts. The author has no experience with these, nor is he aware of any torches for sale.

SWIR LINKS

CAMERAS

Phosphor/Silicon:
Phosphor-coated CMOS camera from Edmund (not recommended, quite insensitive)
Another phosphor-coated CMOS camera from Edmund (not recommended, quite insensitive)

Vidicon Tube:
Find-R-Scope from Edmund (less pricey than InGaAs or Ge-CMOS but has limited life)
These can generally be found much more cheaply on eBay. Be careful, not all go to 1550nm.

InGaAs:
The traditional SWIR imager, excellent sensitivity, pricey, requires lens, power supply and software driver.
Goodrich Sensors Unlimited
FLIR Tau SWIR
Allied Vision
Princeton Instruments

Ge-CMOS:
Excellent sensitivity, expensive, might be difficult to find.
TriWave

Quantum Dot:
The most recent SWIR imager, high resolution, not much known yet about these.
SWIR Vision Systems

SWIR FILTERS

Thorlabs filters
Omega Optical Filters
Omega eBay site for out-of-spec or batch over-run filters

SWIR LENSES

- Check eBay first for used copies, always.
- Remember to account for the crop factor of your sensor when choosing focal length.
- Keep in mind that it can be hard to find step rings for certain filter diameters.

Designed for SWIR:
Thorlabs Achromatic Doublets
Kowa SWIR lenses

Accidental SWIR Lenses:
Wollensak 1 inch 1.5 Cine Velostigmat C mount (suggested by dabateman; tested by Andy Perrin)

#2 Avalon

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Posted 30 June 2019 - 05:44

Thanks, great post. 1460-1600nm infrared photography could incredibly useful in painting documentation unfortunatelly IR reflectography InGaAs aparatuses are very expensive and have their limitations. But I tough that phosphors can convert light only from higher energy to lower, in other words visible to infrared but not opposite. Although if that is possible may I ask if there is way to convert ordinary camera to detect SWIR rays?

#3 Andy Perrin

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Posted 30 June 2019 - 06:00

Quote

But I tough that phosphors can convert light only from higher energy to lower, in other words visible to infrared but not opposite.
So, the situation is this: ordinary fluorescence always goes from higher energy to lower. This is the Stokes effect. The anti-Stokes effect is a different effect in which two photons go in, and one higher energy photon leaves. Because having two photons hit the same place at the same time is a rare occurrence, anti-Stokes effect is very weak. Unfortunately naming conventions in this area of physics are also a complicated mess, and the words "fluorescence," "phosphorescence" and so on regularly are misused or used in confusing ways.

Quote

Although if that is possible may I ask if there is way to convert ordinary camera to detect SWIR rays?
There is! And I did it and linked it in the thread above! Look here:
https://www.ultravio...-and-some-pics/

But notice the low quality of the results. This is why I do not recommend the approach. In my opinion, the best low cost option is probably the vidicon Find-R-Scope coupled to a camera.

Edited by Andy Perrin, 30 June 2019 - 06:04.


#4 dabateman

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Posted 01 July 2019 - 04:44

The SWIR vision Acuros cameras look very promising, possibly even for UV detection based on their quantum efficiency plot.
I doubt it falls off like a rock at 400nm. The bump at 1400 to 1600 may also be useful for specific SWIR detection.

Attached Images

  • Attached Image: quantum-efficiency.jpg

Edited by dabateman, 01 July 2019 - 04:45.


#5 Andy Perrin

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Posted 01 July 2019 - 05:05

It will work across the board from the look of it? I mean yes 10% QE is not great but given you probably did your UVB images with much less, it’s usable.

#6 dabateman

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Posted 01 July 2019 - 06:27

In regards to SWIR, what I ment was the sensor is more sensitive at 1400 to 1600 than 1000 to 1400. So a low cost RG1000 or similar filter, mine is a long pass with cut off at 950nm might work well for longer SWIR range.
I would love to see this sensors true uv range. As the plot indicates it may out perform our cameras at least in the UVA range.

#7 Andy Perrin

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Posted 01 July 2019 - 06:42

Yeah. Me too. Which reminds me I want to test the triwave in UV. It is a monochrome sensor with no Bayer, so would work for those true-color UV experiments we were discussing in the other thread awhile ago.

#8 Avalon

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Posted 14 July 2019 - 08:44

Yes quality is low but that might be enough. InGaAs camera's also have low resolution and they make up for it by using scanning method to created by resolution image. Could not find SWIR adapters on Ebay. Are they expensive?

#9 Andy Perrin

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Posted 14 July 2019 - 15:30

SWIR adapters on eBay are very rare. Should look for several brands not just edmunds but you probably have to set up a search and wait a while. But they don’t work that well anyhow. It’s better to get one of the Find-R-Scopes which seem plentiful.

#10 Avalon

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Posted 23 July 2019 - 04:40

When reading about Sony Exmor sensors I stumbled upon it's 4th generation Exmor sensors claiming to have enhanced infrared sensitivity because of deeper pixel well depth. In fact there are for sale infrared CMOS camera's that can detect in range of 400-1700nm although SWIR sensiivity is low but it's not at least some exotic technology.

#11 Andy Perrin

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Posted 23 July 2019 - 05:02

Link? (Also, I’m reserving judgement on whether that can be considered exotic!)

I just spent ten minutes searching for this tech and can’t find it. All the EXMOR and EXMOR R literature shows the quantum efficiency dropping like a rock at 1100nm, just like every other CMOS that doesn’t have another material on top (like the germanium in the TriWave, or quantum dots).

Edited by Andy Perrin, 23 July 2019 - 05:27.