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Thermal imaging has been used since the 1930s with the discovery of electromagnetism radiation by William Herschel. The original device was insensitive and non scanning, known as an evaporagraph. Due to its limitations in contrast, sensitivity and response time it was found to be unsatisfactory for most thermal imaging tasks. The 1940's brought the development of 2 alternative methods to thermal imaging. With most development from earlier designs was the discrete-detector by mechanically scanning analogues of TV's and the secondly was infrared vidicon (non mechanical scanning device) with less success. In 1952 the American army developed the first thermal imager, known as a thermograph. It consisted of a single detector element; two dimensional, slow framing scanners which recorded the images onto photographic film, this was not a real time device. In thermography, a thermal map is form by measuring a large quantity of direct temperatures over an area of the target surface. Thermography has high spatial resolution; it is an excellent device for analysing and visualising targets with different temperature ranges (Gowen et al. 2010). The US Army developed on the original device in the 1950's and 1960's, who are now the leading developers of thermal imaging. The thermal imaging at the time where fast framing images which was infeasible and gave rise to the need for real time image sensor (Vadivambal, et al 2009).
Thermal imaging is a technique to convert the invisible radiation pattern of an object into visible images for feature extraction and analysis. Thermal imaging is a non-destructive, non-contact system of recording temperature by measuring infrared radiation emitted by a body surface (R. Vadivambal 2009). Thermal imaging is a non-invasive, non-contact, and non-destructive technique used to determine the temperature mapping of the material or process in a short period of time. All objects above 0°K (−273.15°C) expel infrared rays which are part of the electromagnetic spectrum. The major difference between each band in the spectrum is in their wavelength, which correlates to the amount of energy the waves carry. Electromagnetic spectrum comprises of radio waves (100 mm to 10,000 km), microwaves (1 mm to 100 mm), infrared rays (750 nm to 1 mm), visible light (380 nm to 750 nm), ultraviolet rays (10 nm to 380 nm), X-rays (10 pm to 10 nm), and gamma ray (10 pm or less) . The wavelength of infrared rays is in the range of 0.78 - 1,000 μm. The infrared region is split into 4 different regions: near infrared (0.75-3 μm), mid infrared (3-6 μm), far infrared (6-15 μm), and extreme infrared (15-1,000 μm). The intensity of radiation released by an object is a function of its surface temperature. Infrared radiation is energy radiated by the motion of atoms and molecules on the surface of object, where the temperature of the object is more than absolute zero. The higher the temperature of the body, the greater is the intensity of infrared radiation produced by the object. Thermal imaging is a method which converts the radiation released by an object into data without direct contact with the object (Vadivambal, et al 2009).
Thermal imaging system consists of thermal camera equipped with infrared detectors, a signal processing unit and an image acquisition system, usually a computer. The infrared detectors absorb the infrared energy released by the object and convert it into an electrical impulse. The electrical impulse is sent to the signal processing unit which translates the data into thermal image. Most of the thermal imaging devices scan at a rate of 30 times per second and can sense temperature ranging from −20 to 1,500°C, but the temperature range can still be increased by using filters. The detector is the vital component of thermal imaging system. It converts the radiant energy into electrical signals proportional to the quantity of radiation placed on the object. The main types of detectors are thermal and photon detectors. In thermal detectors, IR radiation heats the detector element resulting in temperature rise, which is taken as a measure of the radiation hitting on the object. The photon detectors occur when radiation interacts at a molecular level causing the detector to produce charges. This generates a voltage across the detector element or a change in its electrical resistance (Gowen et al. 2010, Vadivambal, et al 2009).
Thermal imaging devices can be classified into un-cooled and cooled. The most commonly used thermal imaging device it the un-cooled as they less expensive. The infrared detector elements are contained within a unit that operates at room temperature. Un-cooled TI devices resolution and image quality tend to be lower than the cooled device. Cooled thermal imaging device, occurs when the sensor elements are contained within a unit that is kept below 0°C. Even though they have a very high resolution and can detect temperature difference as low as 0.1°C they are not as widely used as un-cooled TI device due to their high cost. They are mainly used in military and aerospace applications (Vadivambal, et al 2009).
Infrared energy emitted may also reflect, absorb and transmit infrared. When the temperature of the material equals that of its surroundings, the amount of thermal radiation absorbed by the object equals the amount emitted by the object. The quantity of radiation released by an object is dependent on its emissivity (ε) and temperature. Emissivity is the ratio of energy radiated from an object to that of a blackbody at the same temperature. A blackbody is a theoretical surface, which absorbs and re-radiates all the IR energy. It does not reflect or transmit any infrared energy. Blackbody's vary from 0 (perfect white body) to 1 (perfect black body), however, perfect blackbody surfaces do not exist in nature. Thermal imaging device uses the infrared energy released from a object is converted into an electrical signal via infrared detector in the camera and displayed as a colour or monochrome thermal image. As well as emitting infrared energy, materials also reflect infrared, absorb infrared and, in some cases, transmit infrared. When the temperature of the material equals that of its surroundings, the amount of thermal radiation absorbed by the object equals the amount emitted by the object. Thermal imaging has many prospective advantages in comparison to other more invasive sampling tests such as their high-speed, non-invasive analysis and prevention of cross contamination
Thermal imaging has many applications in a variety of areas such as agriculture, civil engineering, medicine, and veterinary (Lloyd 1975). Thermal imaging system has an extensive range of application in all areas where temperature differences could be used to assist in evaluation, diagnosis, or analysis of a process or product. Some of the potential use of thermal imaging in food industry evaluating the following:
Maturing of fruits bruises detection in fruits and vegetables,
Maturity Evaluation of Fruits
Quality of Ham
Detection of foreign bodies in food material,
Temperature distribution during cooking (Gowen et al. 2010, Vadivambal, et al 2009).
Thermal imaging is already plays an important part in monitoring product quality in the food manufacturing due to its wide variety of temperature controlled operations. Temperature control and monitoring of a food product during the manufacturing process is critical to ensure product is to the highest quality, in such areas of cooking, pasteurisation, sterilisation. Thermal imaging is also used to maintain the integrity of the cold chain during distribution of the product throughout production, transportation, storage and sales. As with much food temperature of the food affects the food safety and shelf life. Heating processing can often cause over heating resulting in loss of product quality such as in pasteurisation and sterilisation. Traditional temperature monitoring was carried out by using a direct contact processes such as thermocouples and thermometers. This method supply very limited data which minimise temperature mapping techniques. Food safety is the most important specification for any food. Thermal imaging methods in cooking lines would aid in quality control for compliance with food safety regulations for ready to-eat foods, while maintaining quality of the products which can be seen in Image 1. Thermal imaging cameras can continuously register the thermal image. Online process of monitoring can detect foods that are not the correct temperature, and consequently subject the product to a whole host of food borne bacteria. The estimation can be carried out continuously on the conveyor belt. ( Gowen et al. 2010, www.palmerwahl.com,
http://www.goinfrared.com/success/ir_image/1021/industry_id/1050/). Ibarra et al. (1999) found that the temperature of chicken meat immediately after cooking using thermal imaging to range between 3.4 to 5.0 μm. A statistical model was developed to convey the internal temperature of chicken breast. The external temperature and time observed should an accuracy of ± 1.22 °C for cooling times between 0 and 450 s, and an accuracy of ± 0.55 °C immediately after cooking.
Food Processing - non-thermal photo
Image 1: thermal image of chicken burgers being scanned by a FLIR thermal imager. (http://www.goinfrared.com/success/ir_image/1021/industry_id/1050/)
Bruise Detection in Fruits
Bruising of fruit is a major problem in food industry. Fruits are rejected during sorting because bruised fruits have been found to cause considerable damage to unbruised fruits during storage. Consumers are not willing to purchase fruits with bruises as it will shorten the shelf life at home.
Image 2: Thermal imaging of a bruised of an apple.See full size image
The early detection of damages in apple fruits is very important in agriculture products processing due to a small number of injured fruits may cause infection by microbes. The whole batch can be infected, causing economic losses and affecting further operations, such as storage and sale. The most important post-harvest damage in fruit picking can occur during transport and storage. A mechanical bruise caused by external forces, which cause physical changes of texture, i.e. browning and softening of fruit tissue. Research has found that thermal imaging as an excellent method of detection of bruising in fruit. Vadivambal et al (2009) research should that pulsed-phase thermography was used to detect early bruising. The result obtained should a temperature difference between sound and bruised areas as well as between shallow and deeper bruises were close to 0°C, indicating that thermal imaging could not be used for early bruise detection. However, when the apple was heated for 1s, considerable temperature difference was noticed between bruised and good apples. The temperature of the bruised part was colder and variation ranged between 0.9 and 2.1°C. Pulsed-phase thermography was found to distinguish between bruised apples at the early stages and the bruises reaching various depths under the skin of the apple. The thermal images of bruised and unbruised fruits were found to have a difference in the temperature range of between 0.2- 1.0°C and the temperature of the bruised fruits were lower than the unbruised fruits which can be seen in Image 2 (Vadivambal et al 2009).
Detection of Foreign Bodies in Food
The presence of foreign bodies in food is a major safety concern. Visual inspection is generally used however; several factors affect the detection of foreign objects. Physical separation methods are used such as sieving, sedimentation, screening, filtering, and gravity systems. Depending on the product, more advanced methods being used such as metal detectors, X-ray machines, optical sensors, and ultrasonic. Studies have shown that the difference in the cooling behaviour of food and a foreign body. This give was for the use of thermal images to distinguish between the product and foreign body. Thermal imaging has been used to detect foreign materials (rotten nuts, hard shells, and stones) in hazelnuts. The studies showed that images with high contrast to differentiate foreign bodies could be made by using a flash light to heat the hazelnuts. The result obtained showed the distinction between foreign bodies and food material by thermal imaging is possible due to differences in their thermal properties. Three different image processing approaches were used for quality inspection of hazelnuts using thermal imaging
The former two were found to be suitable for the investigation of large quantities of hazelnuts carried on a conveyer belt (GOWAN , 2009). Hazelnuts and foreign bodies, used as a control, were passed on a conveyor belt and slightly heated. After a period of cooling time, thermal images were taken. Image processing methods were used: for example thresholding and texture analysis algorithms. The results obtained showed that thermal imaging could be used to detect foreign materials and determine the quality of individual hazelnuts such as the ones with insect stings or foul nuts which can be seen in Image 3.
Image 3: Thermal image of Hazelnuts and foreign bodies.
As no real object is defined a black body, many different fact have to be weighed up to adapt the model accordingly. Some are related to mechanical characteristics, such as density, geometric shape, and surface roughness, while others depend on environmental factors like temperature uniformity, lighting, and humidity. Even the relative position of different objects can affect the detection. Further studies have shown the potential of thermal imaging to detect foreign bodies in food products using a Thermosensorik CMT 384 thermal camera Ginesu et al. (2004). The emissivity or the different heat conductive capacities of the material were used to distinguish between a food material and a foreign body. Since difference in emissivities may not produce good contrast images, the difference in heat capacities of food and other materials to detect undesirable materials were used.
The food materials chosen were almonds and raisin and foreign bodies were wooden stick, stone, metal chip, and cardboard. They used a pulse thermography and the experimental procedure was that the object was placed (food material and foreign body) on a conveyor belt under the camera and a heat pulse was applied; and then the decrease in surface temperature was observed. Due to difference in heating capacities, different object will cool down with different speed and they recorded a long sequence (500 frames, 80 frames per second) and extracted the thermal images. They applied various image processing techniques such as binarization, statistical, and morphological analysis. They concluded that results are promising and thermal imaging has a potential to detect foreign bodies in food materials. Meinlschmidt and Märgner (2002) conducted two different studies to detect foreign substances in food using Thermosensorik CMT 384 thermal camera. The first one was to detect the presence of cherries in chocolate chunks by their emissivity coefficient without applying any heat impact. The second study was to detect the presence of leaves, stalks, pedicels, and thorns in a variety of different fruits by difference in the heat conductivity or capacity of different materials by allowing the materials to pass on the conveyor belt with a heat source and a thermal camera captures the image during the state of decreasing temperature (Fig. 2). Their results showed that thermography could be used to detect foreign substances in the food material but they suggested that these methods have to be tested on a larger scale material in real-time environment.