Abstract DIP model - Part I

A comprehensive all encompassing abstract model for Digital Image Processing (DIP) come across my mind. Let me put forth my thoughts in a lengthy manner to span several posts. This post is the first part of the series.

Introduction

Normally DIP is studied as a stand alone subject. Learners misunderstand the subject and associate it with compression, compression and compression. I personally feel it should be studied as “part of a whole.” Then only the real face of DIP can be perceived. My abstract model is an outcome of 'part of a whole' philosophy. As it lacks the academic rigour, model is not suited for scholarly publication. But model may be helpful to gain insights and dispel myths about DIP.

Engineers are expected make products to improve the quality of life of human beings. They are expected to use scientific knowledge in product making. The product are made in industry and sold in the market. The required level of knowledge about industry and market is not taught in the curriculum. This severely hampers engineers' thinking. Hard liners may counter argue in following way. “Part of whole thinking” will dilute engineering. If a student wants to learn about market, let him do an MBA.

The abstract model contains four sections viz.; Acquire, Transfer, Display and Interpret. In practice images are captured and then either stored or transferred. Later they are either printed on paper or shown on a screen and the images are interpreted by human brain with the help of eyes. What is new in this model is human brain is brought to the fore and not human eye. One may wonder, why human eye is not given the due credit or put in other way, why human brain's role in seeing is given undue importance in this model. Whether it is a sensational article written to draw more visitors? Please read the article further and I assure you all your anxieties will tend to cease. 

Acquire

The responsibility of acquire section extends from acquiring reflected light from the subject that is shot till conversion of captured light into electrical signals. It has four subsections viz. Lens, Sensor, Read-out electronics and A-to-D converter. Lenses collect the light that is reflected from the subject and focus it on the sensor. Array of sensors ('n' rows x 'm' columns) are used to in camera to capture images. Number of sensors in the array and resolution of image is directly proportional. Sensors can be categorized into CMOS and CCD type. We all know numerous photons forms a light ray. Photon impinges on the sensor's photosite (i.e. light sensitive area), and the electrons in the valence band moves to conduction band. This causes flow of electrons and forms current in the sensor. This phenomenon is called photoelectric effect.  The stored charges in the sensors can be treated as tiny capacitors (we know junction capacitance in diode can be treated as capacitor).  In a sensor, only 40 % of area is covered by photosensitive material. Remaining area is filled with amplifiers and noise reduction circuits [1]. The charge stored in tiny capacitors (actually sensors are built using MOS transistor), has to be read out before they get discharged (similar to working of dynamic RAM). Faster reading-out is required for higher resolution images. Then read-out voltage signals are amplified and converted into digital signals (or data). I guess higher the resolution leads to lesser the A-to-D conversion time pixel.  For detail discussion refer [2], [3]. Figure 1 beautifully explains the concept of read-out [3]. Line sensor arrays (1 x m) are used in photocopying (Xerox) machines. Here a stick that contains row of sensors moves from top of the page to the bottom of the page to collect the pixel information. In thermal systems only single pixel sensors (1 x 1) are available.

Figure 1. Photon collection by photosite and read-out 

The above paragraph would have provided the functioning of light capturing in a superfluous way. Technical details are trimmed to minimum level so as to highlight the principle of light capture. Knowledge on optics and machining is very important to fabricate lenses. The power of DSLR camera hinges on powerful lenses. Good knowledge on micro electronics is absolutely essential to understand the functioning of sensor, read-out amplifier and A-to-D converter. To design and fabricate reasonable good resolution acquiring subsystem, a sound knowledge on Very Large Scale Integration (VLSI) and knowledge on related software tools are essential. In reality subjects like optics, microelectronics and VLSI are taught even without veiled referenced to camera or scanner systems. 

     The technology has reached to such a stage that even entry level camera (low priced camera) is capable of taking 10 Mega pixel resolution images. When film based camera reigned, photography was a costly hobby. So very few bought the camera. To acquire digital colour image requires three filters namely red, green, and blue. Use of three filters is costly and instead single sensors are used to cut down the cost. For that 'Bayer Patterns' are used. When Bedaprata Pain [4] and his team developed affordable CMOS active pixel sensor, digital camera become affordable and today every mobile phone is embedded with a camera.  

Product Market

The next level of innovation will be in improving usability of camera and not in cost cutting. As the cost comes down heavily quantum of profit will also comes down. To maintain the profit industries go in for volume. Let Camera Company named ABC sells 1000 camera for the price of Rs. 5000. Let the profit be Rs. 500. The net profit is Rs. 5,00,000 (1000 camera x Rs 500). If the same company sells 10000 camera for the price of Rs. 3000 then the net profit is Rs 30,00,000  (10000 camera x Rs 300 as profit). Profit has increased many folds. This logic go well until everyone acquires a camera. After that ABC has find innovative ways to keep the net profit same. 

The ultimate aim of the camera manufacturing companies can be put in this way “even a moron should take pictures like a professional photographer.” As we all know we have huge number of amateurs and very few good photographers. The improve the market size of costly DSLR (Digital Single Lens Reflex) camera, industries should target the huge amateur base. But general public neither have patience nor time to become like a professional. To bridge the skill gap lot of intelligence is added in the camera. 

Market need satisfying algorithms

Face detection algorithms are used to help to shoot proper pictures by amateurs. Earlier this feature was available in point-and-shoot cameras. Nowadays this feature is extended to professional models like DSLR cameras.  Most of us, are unable to set proper ISO, aperture and shutter speed for the required shot. That is why auto-focus and auto-lighting cameras sprung up. But there is a lot scope for improvement in these cameras. Next, amateurs' hands are not stable at the time of taking shot and invariably it results in shaky pictures. This can be corrected by using “image restoration” class of image processing algorithms. Sometimes enough lighting may not be available at the time of shooting or extraneous light may fall on the subject. These errors can be partially corrected using image editing softwares like Photoshop and GIMP. Photoshop is the most popular commercial image editing software and GIMP (GNU Image Manipulation Program) is a open and free software. Lot of image processing algorithms will be deployed in ensuing intelligent camera. 

Source
1. How Digital Cameras Work, [Available Online], http://www.astropix.com/HTML/I_ASTROP/HOW.HTM
2. Digital Processing Techniques, [Available Online], http://www.astropix.com/HTML/J_DIGIT/TOC_DIG.HTM
3. ZEISS Microscopy Online Campus | Microscopy Basics | Understanding Digital Imaging, [Available Online], http://zeiss-campus.magnet.fsu.edu/articles/basics/digitalimaging.html
4. Bedabrata Pain - Wikipedia, the free encyclopedia. [Available Online], http://en.wikipedia.org/wiki/Bedabrata_Pain

Television and Movies – Visual Fast Food?

Everyday we encounter lot of pictures. Pictures appear in television, cinema, newspaper, magazine or the Web.   Pictures are used to convey emotions and messages. Except a few, most of us take things for granted and spend our scarce resource (thinking) on odd or rare events. For example until Sir Isaac Newton, the falling of apple from a tree was considered the norm and people simply consumed the fallen apple.  Likewise viewing pictures are taken as a very usual thing and we skirt to think about it. In this post, the discussion will be on the “Role of pictures in our life.”

It will better to define first and then get into the essay. Pictures can be classified eye-captured pictures, device-captured pictures and synthetic picture. If I physically visit the Amazon jungle and enjoy the beauty through my own eyes then I call it eye-captured picture. If I see the Niagara Falls in a movie or television or magazine then I call it as device-captured picture. A picture that is created by artistic rendition with or without computers is called as synthetic picture. 

Two hundred years back pictures mean almost eye-captured pictures only. Rich people only had the opportunity to own synthetic pictures (paintings). Commoners who live in big cities like Rome would have enjoyed the Michelangelo paintings in the ceiling of the Sistine Chapel. Colour photograph started appearing after 1861. It helped to capture portrait of a person or natural landscape with less effort and time. Earlier times painters performed this task. Thus human painter was substituted by colour camera. But it was no way an easy task produce multiple copies. First colour illustrations appeared in newspaper in 1934 in UK. To have a glimpse of old colour photographs refer [1]. Colour television emerged in the year 1960 in USA. After 1980 World started seeing lot of device-captured pictures. I can fairly assume everyone see TV for two hours per day. The amount of pictures in print medium (newspaper, magazine) is relatively less. On the Web, picture is more than print medium but lesser than the TV.  Let us conclude in a day average device-captured pictures viewed is for two and half hours (two hour tv plus half an hour Web and print). Within a span of two hundred years, the time spent on device-captured pictures rose from near zero to 150 minutes.

A cursory glance of  “150 minutes of device-captured picture viewing” looks like a trivia. At most it may amuse people and make people feel proud of technological superiority. Broadcast media (TV and movie) is a medium that transcends the distance. For example seeing a war, seeing piranha fish present in Amazon rivers, and seeing skiing in Alps mountain range with naked eye is a rarity for a common man living in India. Thus one is able to have a near real experience in battle field, jungles and sky scrapers without moving from their physical place. 

A coin has two sides. Likewise the ability to “transcend distance and take part important events in world” has profound positive and negative effects. Without picture, visualizing Amazon jungle with textual description is nearly impossible. Our knowledge has tremendously increased with the rise of access to pictures. People in India know very well about US President Barack Obama, Osama bin Laden, Bruce Lee, Hollywood celebrities, Kangaroo, Niagara Falls, and Eiffel Tower all because of pictures. Learning medicine, architecture, archaeology and many fields become easier because of availability of pictures. Forensic experts are able to identify criminals without physically visiting the crime spot. Surveillance camera captured pictures which help us to prevent crime as well as to capture the criminals. 

The negative sides are “we are conditioned to see what we want to see” and the gap between device-captured and eye-captured pictures is very high. When viewing a TV and movie, we see the world through the eyes of content creator (director of movie). In one sense our freedom is lost. As watching movies acts as a medium of escape, we voluntarily subject our-self for loss of freedom. Thus it becomes easy to mass brain-wash the so called modern man than their ancestors. Next, a person in India via TV can live in America for few hours per day. So the distinction between real and reel shrinks and confuses person's thinking ability.

The important point is we see extremes in device-captured images. First principle of journalism states that “If a dog bites a man then it is not news and if a human bites a dog then it is news.” Mathematically it means lower the probability of occurrence, higher the probability to be published. That is why we see six-pack males like Arnold Schwarzenegger and Sylvester Stallone, handsome Leonardo DiCaprio and beautiful hour-glass females. Fig.1 contains the still from the movie Titanic and it is very romantic. Seeing this kind of romantic encounters with naked eye is almost impossibility. Thus for 150 minutes we see what is not possible to see with naked eye.

Figure 1. A romantic scene from the movie Titanic     Courtesy: Internet  

Studies have established long hours of TV viewing affects children's ability to learn, their retention capability and socializing skills. The impact of camera-captured pictures in humans has to be documented with scientific data. Seeing a picture is not an independent task of eye alone. It is an outcome of close coordination between eye and brain. Thus it will be better to say “We Perceive” than “We See.” When we encounter optical illusions our brain fails to interpret the incoming visual signal from the eye properly. When ever meditation or prayer is performed, we normally close our eyes. This helps us to cut down the distraction as well as reduce the work load of brain. Seeing something actually makes our brain to give priority to process on the incoming visual signal. That is why when we sit in a park or watching TV makes us to feel as if we are getting rid of our problems. Actually, the brain starts to process visual signals rather than pondering on the problem. 

We have a high intake of processed food (fast food) compared to our ancestors. Wide spread prevalence of life style diseases like diabetes, obesity are linked to processed food. Similar in lines we have high intake of camera-captured pictures compared to our ancestors. Will it creates any problem for us?

Before we wind up we will have quick recap of what we have discussed in this post.

• Eye-captured, Camera-captured and Synthetic pictures
• Camera-captured images are different from eye-captured pictures
• Camera-captured picture from 18th century, transcends distance, captures extreme events
• Amount of camera-captured pictures is 150 minute per day and 200 years back almost zero minutes.
• With camera-captured images voluntary brain-washing is carried out.
• Cognitive load on brain due to camera-captured pictures is high.
• Camera-captured picture = Fast food 

Source
1.  Colour images from 1930s unveiled – Daily Record [Available Online] http://www.dailyrecord.co.uk/news/scottish-news/colour-images-from-1930s-unveiled-1276428

Acknowledgement
 Grammatical correction was carried out by a final year engineering student.

Second Avatar of Stereoscopy

     Recently I was watching a promotional video of palatial hotel from YouTube video server. That video clip had 3D viewing option. It stirred my curiosity and selected the option. All I was able to see was a blurred video as I did not possess required 3D glass. I searched in Google and I found that YouTube automatically converts short video clips that have a resolution of 1080p (1920 x 1080 progressive mode) from 2012 onwards [1]. If I had watched with 3D glass I would have virtually visited the hotel rather than seeing. Depth perceivable in 3D creates a new experience. The right technical word for 3D movie is stereographic movie.

   The YouTube video clip made me nostalgic. My first stereoscopic experience was way back in 1984. As a boy I had an opportunity to view “My Dear Kuttichaathan” (in Tamil language 'kutti' means small, 'chathan' means shaitan or genie) movie. Exhibitors collected extra fee for 3D glasses and at the end of the movie they got back the eye glass. I was really shocked when arrows from silver screen tried to poke my eyes and pleasantly surprised when bunch of roses and cone ice-cream popped out of screen. No doubt the movie was a block buster. After a gap of 25 years I watched a 3D movie. It was none other than James Cameron's Avatar movie and my children were my fellow viewers. My children enjoyed to the core. I liked the theme of the movie but the 3D effects did not create an 'awe' in me. I realized I have become old. 

    After 'My Dear Kuttichaathan' few 3D movies came to tap the emerging 3D market. I saw one or two. I was not impressed and public too shared my opinion. Slowly 3D popularity declined. Production cost of stereographic movies were high compared to normal movies and production work-flow has to be modified to suit 3D movies [2]. Stereographic movies required two projectors instead of one and both of them have to be synchronized. Directors were not able to effectively use the 'depth' to convey their story to audience. Consumers required to wear an eyeglass and safe return of the eyeglass was their duty. Whole lot of extra efforts among stake holders for few pop-ups was not worthy. After a lull period of 25 years Avatar movie created the frenzy. One may be perplexed why there was a long gap of quarter century and why there is a 3D frenzy now? Answers to these questions will come out when we dwell into past and do some reasoning.

     As for as India is concerned before the color TV penetration, 'movie going' was the prime pass time activity. Entire family went to movie halls and films are also produced to cater the needs of entire family. In late 1990s TV and satellite broadcasting glued the family to the drawing room. Youth (15 to 30 years of age) become major customer base for films. Automatically movie content were made to suit the audience. Youngsters like stunning visuals. Thus 3D become a apt to tool to make youth to come to theatres.

    The next threat for theatre came in the form of VCD (Video Compact Disk). Prior to VCD Video Home Systems (VHS) was the norm. It used magnetic tape to store the analogue signals. VHS player had lot of mechanical components and regular maintenance was required. Copying from Master tape to another was cumbersome. The quality of copied content quality were inferior to the Master. Thus piracy was kept at a bay. Thus VHS never challenged the dominance of theatres. In contrast, the VCD carried digital signals and VCD player had more electronics and less mechanical components. As VCD market expanded prices of VCD player started falling. Pirated VCD making was a simple task. Thus just released movies were available in pirated VCD and family watched in their television. This was death blow to theater owners and in turn to movie industry.

  The VCD threat was countered by producing movies with spectacular visuals (ex. Matrix movie fight scenes were talk of the down) and with surround sound systems like Dolby. This discouraged movie patrons to view movies on their Television sets. Stereoscopy produces stunning visuals to draw the crowd to the theatres and curtails piracy.
 

A still from the movie 'Avatar'

    Avatar movie grossed box office collection of two billion dollars, which is a huge sum in even in Hollywood [3]. Right technology and sizable market emerged in late 2000 and arrival of Avatar movie ushered a new chapter in stereoscopic movie industry. Cinema producers realized 3D is a untapped potential and started releasing animation movies. The number of 3D theaters started exploding after 2000. In the year 2007 it was 1300, in the year 2009 it reached 9000 and at present 45000. Most of the theaters are constructed in China. In the year 2005 Hollywood produced only five 3D movies, in 2009 it was 20 and in 2012 it almost doubled [4]. In the year 2012, out of 15 highest-grossing films nine were stereoscopic movies. Quarter of the revenue is generated from USA and three quarters comes from rest of the world. Rising economies like China, India contributes a lot. As movie industry falls under 'high risk – high reward' category and 3D technology becomes a safe bet.

Seven reasons for rise for 3D

1. It introduces a illusion of depth which produces a new experience. In normal movies shadow acts as surrogate for depth.
2. It suits well with youngsters, who prefer stunning visuals than a emotional roller-coaster. Next they are experience conscious. So price is not a barrier.
3. Amount of money grossed from a stereoscopic movie is huge. The failure rate is less. Thus 3D movie is a safe bet for film producers.
4. Cost of stereoscopic movie ticket is 30 percent higher than normal movies. This makes a hole in movie patrons but helps theatre owners to fill their coffers.
5. Stereoscopic movies curtails piracy.
6. Digital projection technology go well with 3D movies.
7. Digital production is very cost effective for  3D movies.

Problems with 3D

    Film is a visual art that helps to tell a story. A good story will make the audience get hooked to the characters of the movie. That is why classical movies like Ben Hur, Five Men Army, Mackenna's Gold and Bridge on the river Kwai are still touches our hearts. A good movie should have a judicious mix of stunning visuals and emotions (ex. valour, sacrifice). Yesteryear directors were not sure of 3D medium's effectiveness in story telling and simply avoided the medium.

     Sometimes the presence of depth of field may become a source of distraction. For example in nude photography, there are photographers who still use B&W (Black and White) film stock instead of colour film stock. They claim B&W medium helps to appreciate the shape of female body. The faithful reproduction of flesh tone by color film really distracts the viewers and photographers are unable to convey their intention. Once visual effects artist commented in a public meeting that “When Science gets in Art goes out (from movies)”. Over indulgence on technology may actually spoil story telling capability.

    3D movies favours themes that are based on mythology, magic (ex. Harry Potter), adult, horror and cartoons. Thus movie goers are transported away from reality for 90 minutes. Thus 3D can be regarded as entertainment medium than a visual art medium.

     Our eyes has to focus properly on screen to feel the depth. Those who fail to focus get head ache and other related ailments. Visual discomfort and visual fatigue are studied extensively by scientists to improve the 3D movie going experience [5].

Summary

1. It is a visual rich medium and toning down may be necessary to tell a story compelling way.
2. It is genre limited. It is well suited for mythology, magic, horror and cartoons.
3. Visual fatigue and visual discomfort has to be studied well for wide acceptance among public.  

Source
[1] Official Blog: How we’re making even more 3D video available on YouTube [Online] http://youtube-global.blogspot.in/2012/04/how-were-making-even-more-3d-video.html
[2] Casting a magic spell, [Online] http://www.thehindu.com/thehindu/mp/2003/05/15/stories /2003051500260100.htm
[3] Avatar (2009 film) - Wikipedia, the free encyclopedia, [Online] http://en.wikipedia.org/wiki/Avatar_(2009_film)
[4] 3 Signs That 3D Movies Are The Way Of The Future | Business Insider India [Online] http://readbi.in/st9nJY
[5] M. Lambooij and W. IJsselsteijn , “Visual Discomfort and Visual Fatigue of Stereoscopic Displays: A Review,” Journal of Imaging Science and Technology, vol. 53, no. 3 pp. 030201–030201-14, Mar. 2009. [Download]
http://www.cs.sfu.ca/CourseCentral/820/li/material/source/papers/Visual-discomfort-09.pdf

Status of DIP in India and possible remedies

        In this post, problems faced by Digital Image Processing (DIP) field in Indian academia is discussed.  Potential of DIP is highlighted and  underlying strength is discussed. Measures that has to be taken to achieve the potential is presented.

I. Problems

1. First and foremost weakness is prevailing misconceptions about the field.  They are listed as follows
• Bible for DIP is written by Gonzalez (Rafael C. Gonzalez and Richard E. Woods, Digital Image Processing, 2nd edition, 2002) and the book had more emphasis on mathematics. In recent editions, maths content was gradually reduced to make it convenient to computer science students also. Thus  phrase like “DIP is nothing but two dimensional spacial signal processing,” makes most of the people amused.
• Most of the faculty and students have a notion that DIP is all about compression, compression and compression. I don't know how this notion crept into academic community. If some one say I am doing my research on image processing then next question will be “whether you are working on image compression?”
• Academia has no qualms to associate Digital Signal Processing (DSP) subject (In American parlance - course) with mathematics. It is adjudged as a core electrical engineering subject. But 2D signal processing ie. DIP is not associated with maths and offered as elective course. What a pity.
2. The next important weakness is, there are only handful people who really know DIP. To score good marks in examination with minimal effort, DIP is the best option. But one has to struggle more hard to become an expert.  The reason is very simple. One has to get out of the  prevailing misconceptions trap.
3. There is a small pool of DIP experts only exist. Thus they nearly face no competition and there is no incentive for them to  improve their knowledge.
4.  Most of the professors in engineering colleges are from communication, networks, and vlsi background.  Almost all bright students prefer to work under a professor for their project work. Thus best talent are not available to DIP field.
5. In most of the engineering colleges in India, DIP is offered as an elective subject (In American parlance - course). It is studied in last semester B.E. or B.Tech. programme. Thus students loose an opportunity to do project in DIP. Even if they do a project, they lack the theoretical foundation. It results in low grades or marks and DIP field is falsely branded “unfit."

II. Potential

• There are really very few experts are there. So there is enough vacuum is there, that needs to be filled. Put it poetically “DIP is less trodden path.” It is difficult to travel but rich dividend is waiting makes worthwhile to walk alone.
• DIP applications are innumerable. So one can establish their expertise on any one of the applications.

III. Strengths

1. A Internet enabled laptop with MATLAB can act as a Lab. So every electronics engineering student can own a DIP lab.
2. Good amount of information is available in Internet. So one can download codes and run on their systems to understand the concepts.
3. Outcome of DIP is an image. So one can implement their methods / algorithms on computers. As one can have actual result, it really improves the confidence as well as self esteem. In areas like communication, networks and vlsi they have to be contented with simulations. They are supposed to work in labs of  premier institutes to come out with actual result (ex.  a integrated chip).
4. International journal “Transactions on Image Processing” is published from IEEE (Institute of  Electrical and Electronics Engineers) association. The USA based IEEE association is the largest and most respected electrical engineering association in the world. Impact factor of transactions on image processing is more than three (3.199). This is a very high score for a engineering journal.  This means there is huge scope and impact on society in doing research in image processing.
5. The cost of cameras, scanners, computing hardware and displays are coming down. Thus  inspection systems (Biscuit, rice etc), activity detection systems  can be built with low cost. This will create a huge market for new products and work flows.

IV. Improvements

1. People working in Image processing should come together and they have to work in tandem for the welfare of DIP. This in turn will help them to grow. One may think why cant start an association? Yes, nice idea. But due to Indian mindset, office bearers and members will spend much time on playing politics than doing something for DIP.
2. In the universities DIP has to be given due importance.
3. DIP awareness camps has to be organized by institutions. So as to make popular among students.
4. Motivated youngsters are the need of the hour to build research capability and  to involve in DIP promoting activity. The prime objective of young researchers should be doing research and not acquiring PhD degree.
5. Free access to quality content. For example a wikipedia for DIP (dIPedia).
6. A trainer kit has to developed or popularized, which permits to do image processing applications on embedded systems.
7. If smart phone image processing trainer kits (Android, Symbian, and Windows mobile based) made available then students will come out with beautiful applications. For example a smart phone app (application or executable program) is written to detect presence of diseases in agriculture farms and suggest remedies.  Next, If someone hovers the smart phone on unknown plant then the app  can come out with the botanical name and local name. These apps can be  greatly helpful to public as well as DIP researchers.

After reading this post, some may wonder what is my contribution to DIP?   Answer is 'not much'. This post is culmination my thoughts and few failed activities. Earlier I started a website especially for image processing. Due to various reasons I was not able to carry out regular updates and finally it  failed.  Then I started this blog to bring out information that are not covered in the typical textbooks.

Inside Thermal Imaging Camera

     This post deals about components of infrared camera system. Infrared region ranges from 0.7 μm to 1000 μm. For convenience infrared is split into shortwave (3 to 5 μm) and longwave (8 to 12 μm) bands [1].  Few types of thermal camera are shown in Figure 1. Their appearance will be similar to conventional cameras.                                 

Figure 1. Various types of Thermal Imaging Cameras Courtesy ref. [3]

In ambient temperature, radiant flux emitted by is more in longwave than shortwave. But shortwave is more sensitive to longwave at ambient temperature conditions. Sun radiation is small but significant in shortwave range. It is helpful to increase image contrast in some cases and in cases like measuring temperature of an object, sun radiation is unwanted. Longer wavelength can penetrate mist and smoke better than shortwave. So longwave always preferred in surveillance applications.

            At cursory level, architecture of thermal cameras will be similar to conventional cameras. The main differences are presence of scanning subsystem and cooling subsystem.  The schematic diagram is shown in Figure 2. The IR rays from object are focused on detector or detector array through IR lens. In detector IR to electrical signal conversion takes place. Readout circuits transfer the electrical signals to storage devices and display devices.

(i) Lens: Functionally Infrared and visible range lenses are same i.e. they perform same duty irrespective of the spectrum in which they operate. But Infrared lens materials have relatively higher refractive index than visible spectrum lens. For example germanium has refractive index of 4.01 at 10 mm compared glass which is having a refractive index of 1.5 at optical spectrum.

(ii) Scanning system:  This subsystem is missing in conventional cameras. Purpose of the scanning is to transfer image formed in the lens to the detector or detector array in well controlled fashion. The types of scanning systems are as follows; object space scanner, image space scanner, and afocal scanner. For detailed information refer [2].

(iii) Detector: It is the most important component in the entire thermal camera. Thermal detector and photon detector are the two types of detectors. In thermal detector, IR radiation heats the detector element and this in turn change in physical characteristics (for example, electrical polarization or resistance). This change in characteristics is measured.

In photon detector, electron absorbs the photon that comes from IR radiation and the electron jumps form one quantum level to another. It is more sensitive than thermal detectors. To have better performance quantum detector has to be cooled. This facilitates photons–electron interaction. Photon detectors are constructed using semiconductors. Either they act as photoconductors or photovoltaic devices. In photoconductor absorbed photons moves a valence band electron into conduction band and there by increasing the conductivity of device. As name suggests absorbed photon generates voltage in the p-n junction.  Shortwave detectors need to be cooled up to 195K but long wave has to be cooled up to 77 K.  This is the temperature of liquid nitrogen. (I strongly believe liquid nitrogen it is flammable)

They are mainly three types of photon detectors are available.

• (a) Cadmium mercury telluride (CMT): By changing the composition it can be made to operate in longwave or shortwave band.
• (b) Indium antimonide (InSb): It is used in shortwave band only. It can be constructed to operate as photovoltaic device or photoconductive device. For get best performance it has to be operated in liquid nitrogen temperature.
• (c) Platinum Silicide (PtSi): This detector functions as Schottky barrier photodiode. It works in shortwave but with 77K temperature. It has very poor quantum efficiency. Only two percentage of incident photons are converted. It is well suited for arrays only. This material is well compatible with the readout circuit.

(iv) Detector Arrays:  Development of thermal arrays is relatively recent and possible as a result of MEMS(micro-electromechanical systems) technology.

• (a) Pyrovidicon: This can effectively function in longwave band. Now this imaging device has become an historical interest. It functions similar vidicon video camera. But the detector is infrared sensitive.
• (b) Resistive bolometer arrays: This is very popular array structure. Here temperature is proportional to change in resistance. It is made of top temperature sensitive resistive layer and bottom layer that contains read-out circuit. These two layers are connected by pillars. The air gap between two layers acts as a thermal insulator. These types of bolometers incorporate coolers to maintain an optimum temperature that is conducive to resistive element and the readout electronics.
• (c) Ferroelectric/pyroelectric arrays: Construction-wise it is similar to resistive bolometer arrays, except the detection is based the pyroelectric or ferroelectric effect. Pyroelectric material become electrically polarized when there is change in temperature. This in turn generates charges and resistance of the top layer (resistive layer) goes down. A typical example for pyroelectic material is trigycine sulfate (TGS).
• (d) Bimetallic Cantilever Arrays:  It has two layers and both of them hinges on capacitance measuring circuit as in Figure 2. Top layer is made up of gold bimetal and consists of IR sensitive film. This layer is supported by SiO substrate. Top layer and bottom layer is separated by air gap. As both layers are metal and in between air is insulator it forms a capacitor. When infrared falls on the film, top layer bends towards the bottom layer. This change of physical arrangement causes capacitance to vary. This variation is measured.

• Staring array configuration: Thermal sensitivity of an imager can be improved if the effective imaging areas of the detector elements are increased. (Please note entire area of the detector is not occupied by imaging section of detector element. For example, in the bimetallic cantilever array only small portion of top cantilever is filled with IR sensitive film). Size of the detector elements must be limited to maximize spatial resolution of detector arrays. Various configurations are proposed and adopted in cameras, out of which staring array is very popular and deployed extensively [2].

(v) Cooling subsystems:

Most of thermal cameras employ cooling or temperature control for optimal performance. There are four types of cooling methods are used widely.

• (a) Bulk cooling: It is the simplest cooling method. Here detector is placed inside the Dewar (Flasks ?) and later is filled with liquefied nitrogen. It is well suited for laboratory purposes.
• (b) Thermoelectic cooler (Peltier cooler):  It is based on Seeback or Peltier effect. Current flowing through a junction of two dissimilar materials produces a change in temperature at junction. Direction of current flow decides whether the junction to be cool or hot. With this arrangement temperature up to -40K is achieved.
• Other two coolers are Joule –Thomson cooler and Stirling engine coolers.

(vi) Processing Electronics and Display:

The generated voltage or current has to be read out, conditioned (amplified and A to D converted) and stored in a memory. Major problem is making an electrical connection to each and every detector element. To reduce number of external connections, on-chip signal processing has to be carried out. This strategy is already employed in CMOS and CCD arrays.  Maximum resolution of thermal cameras will be 300x300. So LCD displays will be very sufficient.

Source:

1. IR Thermography Primer, FLIR Systems Co. Ltd, http://www.termogram.cz/pdf/thermography_primer.pdf, (749 KB, PDF)
2.  Thomas L. Williams, Thermal Imaging Cameras: Characteristics and Performance. CRC Press, 2009.
3. Thermal imaging guidebook for industrial applications, FLIR Systems AB,  http://www.flirmedia.com/MMC/THG/Brochures/T820264/T820264_EN.pdf  (2014 KB, PDF).