History of infrared detectors the study of infrared was caused by the lack of sensitive and eade of the 1century.Thomas Johann Se eck began to 821 the ons are light were different phenomena,and the dis of th Seebeck's junctions.some uv/K.the measuren nent of ver 182 her momete hased on the thermoelectric effect dise eck in 1826.Four years later. Melloni introduce Fig.1.Hers measurable output voltage.It was at least 40 times n the put voltage of such structure l low)-see Fig.1(b)[2].In April 1800 he reported it to the Fig.2(a).It consists of twelve large bismuth and antimony Royal S dark heat (Ref.1.pp.0 ents were pla ght in a bra n disk wita15 entral a ture.I plet back the stand. or the hown in Fig.2(b).This instrument was much more sens And here the thermometer No. idely ras of the sun.Now.as before we had a risin a of 9 de century grees.and here the difference is al d the he wo thin ribbons of platinum foil connected so as to forn while,at the same time,the experiment sufficie wo arms of a W ridge (see Fig.3)[15 h infrared region and to measure the intensity of solar radia Making further experim nts on what Herschel called the tion at various wavelengths 9.16.17].The bolometer's sen- ed beyo he early history of IR wasre and in more r 10 elopme infrared physics and technology [11.12). (a) (b) 2.Historical perspective Fig.2.The Nobili-Meloni ther opiles:(themmopile' For thirty years following Herschel's discovery.very little t laws of Museum of the H Opto-Electron.Rev.0.no.3.2012 2012 SEP.Warsawlow) – see Fig. 1(b) [2]. In April 1800 he reported it to the Royal Society as dark heat (Ref. 1, pp. 288–290): Here the thermometer No. 1 rose 7 degrees, in 10 minu− tes, by an exposure to the full red coloured rays. I drew back the stand, till the centre of the ball of No. 1 was just at the vanishing of the red colour, so that half its ball was within, and half without, the visible rays of the sun. And here the thermometer No. 1 rose, in 16 minutes, 83 4 degrees, when its centre was 1 2 inch out of the visible rays of the sun. Now, as before we had a rising of 9 de− grees, and here 83 4 the difference is almost too trifling to suppose, that this latter situation of the thermometer was much beyond the maximum of the heating power; while, at the same time, the experiment sufficiently indi− cates, that the place inquired after need not be looked for at a greater distance. Making further experiments on what Herschel called the ‘calorific rays’ that existed beyond the red part of the spec− trum, he found that they were reflected, refracted, absorbed and transmitted just like visible light [1,3,4]. The early history of IR was reviewed about 50 years ago in three well−known monographs [5–7]. Many historical information can be also found in four papers published by Barr [3,4,8,9] and in more recently published monograph [10]. Table 1 summarises the historical development of infrared physics and technology [11,12]. 2. Historical perspective For thirty years following Herschel’s discovery, very little progress was made beyond establishing that the infrared ra− diation obeyed the simplest laws of optics. Slow progress in the study of infrared was caused by the lack of sensitive and accurate detectors – the experimenters were handicapped by the ordinary thermometer. However, towards the second de− cade of the 19th century, Thomas Johann Seebeck began to examine the junction behaviour of electrically conductive materials. In 1821 he discovered that a small electric current will flow in a closed circuit of two dissimilar metallic con− ductors, when their junctions are kept at different tempera− tures [13]. During that time, most physicists thought that ra− diant heat and light were different phenomena, and the dis− covery of Seebeck indirectly contributed to a revival of the debate on the nature of heat. Due to small output vol− tage of Seebeck’s junctions, some μV/K, the measurement of very small temperature differences were prevented. In 1829 L. Nobili made the first thermocouple and improved electrical thermometer based on the thermoelectric effect discovered by Seebeck in 1826. Four years later, M. Melloni introduced the idea of connecting several bismuth−copper thermocouples in series, generating a higher and, therefore, measurable output voltage. It was at least 40 times more sensitive than the best thermometer available and could de− tect the heat from a person at a distance of 30 ft [8]. The out− put voltage of such a thermopile structure linearly increases with the number of connected thermocouples. An example of thermopile’s prototype invented by Nobili is shown in Fig. 2(a). It consists of twelve large bismuth and antimony elements. The elements were placed upright in a brass ring secured to an adjustable support, and were screened by a wooden disk with a 15−mm central aperture. Incomplete version of the Nobili−Melloni thermopile originally fitted with the brass cone−shaped tubes to collect ra− diant heat is shown in Fig. 2(b). This instrument was much more sensi− tive than the thermometers previously used and became the most widely used detector of IR radiation for the next half century. The third member of the trio, Langley’s bolometer appea− red in 1880 [7]. Samuel Pierpont Langley (1834–1906) used two thin ribbons of platinum foil connected so as to form two arms of a Wheatstone bridge (see Fig. 3) [15]. This instrument enabled him to study solar irradiance far into its infrared region and to measure the intensity of solar radia− tion at various wavelengths [9,16,17]. The bolometer’s sen− History of infrared detectors 280 Opto−Electron. Rev., 20, no. 3, 2012 © 2012 SEP, Warsaw Fig. 1. Herschel’s first experiment: A,B – the small stand, 1,2,3 – the thermometers upon it, C,D – the prism at the window, E – the spec− trum thrown upon the table, so as to bring the last quarter of an inch of the read colour upon the stand (after Ref. 1). Inside Sir Frederick William Herschel (1738–1822) measures infrared light from the sun – artist’s impression (after Ref. 2). Fig. 2 . The Nobili−Meloni thermopiles: (a) thermopile’s prototype invented by Nobili (ca. 1829), (b) incomplete version of the Nobili− −Melloni thermopile (ca. 1831). Museo Galileo – Institute and Museum of the History of Science, Piazza dei Giudici 1, 50122 Florence, Italy (after Ref. 14)