As is well known, traditional incandescent lamps are provided with a tungsten (W) filament which is made incandescent by the passage of electric current. The efficiency of traditional incandescent lamps is limited by Planck's law, which describes the spectral intensity I (X) of the radiation emitted by the tungsten filament of the lamp at the equilibrium temperature T, and by heat losses through conduction and convection. The energy irradiated by the tungsten filament in the visible range of the electromagnetic spectrum is proportional to the integral of the curve I (X) between i=380 nm and B2=780 nm.

According to Kirchoff's law, whereon the present invention is based, under thermal equilibrium conditions the electromagnetic radiation absorbed by a body at a specific wavelength is equal to the electromagnetic radiation emitted.

A direct consequence of this law is that the spectral emittance"E"of a surface coincides with spectral absorbance"a". Spectral absorbance"a"in turn is linked to spectral reflectance"p"and to spectral transmittance", T" through the relationship a=l-T-p whence descends the relationship l-s=T+p. For an opaque material, T is substantially nil and spectral reflectance p coincides with (1-s) ; note, however, that any material, for sufficiently small thickness values, has a spectral transmittance T different from 0.

The relationship T+p=l-e implicitly states that, if

the surface of an opaque body has a low spectral reflectance at a given wavelength, the corresponding spectral emissivity will be very high ; vice versa, if spectral reflectance is high, the corresponding emissivity will be low.

Emissivity, absorbance, transmittance and reflectance are functions, not only of wavelength, but also of temperature T and of the angle of incidence/emission 0, but the above relationships hold true for any T, any wavelength and any angle, since they descend from pure thermodynamic considerations. In general, the relationship T+p=l-s can thus be rewritten as T r ( ?,, T, 0) + p (X, T, è) =1-s (2,, T, 0).

The curves of reflectance and spectral transmittance at a given temperature T, from which descend the values of absorbance and emissivity at that temperature, can be calculated a priori through the optical constants (always at temperature T) of the material or of the materials constituting the emitter for any geometry of the emitter and for any angle of incidence/emission.

The optical constants of the material are the real value n and the imaginary value k of the refraction index; the values of n and k for most known materials have been measured experimentally and are available in the literature. In general, there are no values of n and k available at the temperatures of interest for incandescent sources. The reflectance and transmittance calculation, presented in the remainder of the description and in the related figures, refer to optical constants measured at ambient temperature; however, the above considerations have general validity and can easily be transferred to the case of high temperatures.

In a traditional incandescent source, radiation is emitted by a tungsten filament, whose operating temperature is around 2800K ; the emitted radiation follows the law of the black body, whose corresponding spectrum is given by Planck's relationship. The filament can be considered, with good approximation, a grey body, i. e. with constant emissivity throughout the spectrum of interest. By definition, a black body is a grey body with emissivity £ (, T, 0) independent of k and of 9 and equal to 100% (maximum value). The emission spectrum of a grey body can be obtained multiplying the black body spectrum I (X) (given by Planck's relationship) for an emissivity value of £ (T).

For a non-grey body, Planck's curve Planck I () must instead be multiplied times a spectral emissivity curve s (k, T, 9).

The spectral emissivity of tungsten is generally a function of temperature ; it has been demonstrated empirically that the mean emissivity of tungsten follows the relationship Em (T) =-0.0434+1. 8524*10-4*T-1. 954*10-8*T2.

At low temperatures the spectral emissivity curve can easily be derived measuring the reflectance spectrum of tungsten and applying the relationship s (, T, 0) =l-p (,, T, 0) ; at incandescence temperatures, this type of measure becomes unfeasible, because the spectrum of reflectance and the spectrum of emission are obviously mixed. At the temperature of 2800K, the mean emissivity of tungsten is about 30%, which corresponds to a mean reflectance of about 70%. At 2800K, the peak in the emission spectrum is at a wavelength slightly greater than 1 micron, which presupposes that most of the radiation is emitted in the form of infrared. In particular for a grey body at a temperature of 2800K, slightly less than 10% of

radiation is emitted in the visible spectrum (380-780 nm), whilst over 20% is emitted in near infrared (780- 1100 nm). In fact, the tungsten filament is not an actual grey body, but it has a spectral emissivity that is more or less constant in the visible spectrum, and tends significantly to decrease in near infrared, as is readily apparent from the reflectance and spectral emissivity curves shown in Figure 1. In the graph of Figure 1, the curves CR and CE respectively represent the reflectance and the emissivity of tungsten at ambient temperature for different wavelengths in the visible and near infrared spectrum. This causes the efficiency of a tungsten filament, i. e. the ratio between visible radiation and total emitted radiation, is far greater than that of a grey body ; the advantage is still more significant when considering the spectral emissivity at ambient temperature.

Figure 2 compares the Planck's curve at 2800K, designated CP, with the spectral power emitted by a tungsten filament at 2800K ; for tungsten, the chart shows both the experimentally measured values (curve PM), and the values calculated using the optical constants of tungsten at ambient temperature (curve PC).

However, it should be observed that the overall efficiency of an incandescent source, i. e. the ratio between irradiated luminous power and dissipated electrical power, is in fact reduced by additional dissipation factors, in particular the convection losses from the filament to the bulb and the conduction losses along the reophores of the lamp.

Conduction losses can generally be reduced by increasing the ratio between filament length and section. Convection losses can be virtually eliminated by maintaining the filament in a vacuum; for practical

purposes, however, this solution is not feasible, because the rate of evaporation of tungsten in a vacuum is too high and it does not assure a sufficiently long mean life of the lamp. The effort to minimise heat losses has therefore led to the dual spiral shape, currently used in the overwhelming majority of incandescent sources. Halogen lamps instead exploit particular mechanisms which allow to bring the filament to higher temperatures, thus shifting the emission peak towards the visible region of the spectrum and increasing the light efficiency of the source.

Based on the above, the present invention aims to provide an emitter for incandescent sources, capable of being brought to incandescence by a passage of electric current, having a higher efficiency than filaments for incandescent lamps obtained with traditional techniques.

Briefly, the invention provides for the use of a an emitter made of conductor or semi-conductor material, and hence capable of being brought to incandescence by the passage of electric current, made in such a way as to have a value of spectral absorption a that is high in the visible region of the spectrum and low in the infrared region of the spectrum (wavelength greater than 780 nm), said absorption a being defined as a=l-p- , where p is the spectral reflectance and T is the spectral transmittance of the emitter.

The characteristic of spectral absorption of the emitter according to the invention can be obtained through the combination of the optical properties of the materials that constitute it and its particular geometry.

In a possible implementation, the objects of the present invention can be achieved by using a conductor material, for example a metal, that is opaque both. in

the visible and in the infrared regions (T=0), and providing said conductor with anti-reflection means operating to reduce reflectance p in the visible region of the spectrum, whilst maintaining high or even increasing reflectance? in the near infrared region.

Said anti-reflection means can be, for instance, dielectric coatings with one or more layers of the material which embodies the emitter; the anti- reflection properties can also be obtained through an appropriate micro-structure of the surface of the emitter, or else through a combination of micro- structuring and multi-layer dielectric coating.

Alternatively, the objects of the invention can be achieved by constructing the emitter with a thin layer of semi-conductor material, having low reflectance p both in the visible and in the infrared region, high transmittance T in the near infrared and low transmittance T in the visible ; as will be readily apparent hereafter, this characteristic at high temperature is typical of semiconductors with large prohibited band or"bandgap", such as silicon carbide (SiC).

Specific preferred characteristics of the invention are set out in the appended claims, which are understood to be an integral part of the present description.

Additional objects, characteristics and advantages of the invention shall become readily apparent from the description that follows, made with reference to the accompanying drawings, provided purely by way of non limiting examples, in which: Figure 1 is a chart which represents la reflectance (curve CR) and the emissivity (curve CE) of tungsten at ambient temperature for different wavelengths in the visible and near infrared spectrum.

- Figure 2 is a chart which compares Planck's curve at 2800K (curve CP) to the spectral power emitted by a tungsten filament at 2800K ; for tungsten, the chart shows both experimentally measured values (curve PM), and the values calculated using the optical constant of tungsten at ambient temperature (curve PC) ; Figures 3 and 4 are schematic sectioned representations of a portion of an emitter according to the invention provided, respectively, with a single- layer coating of yttrium and with a multi-layer coating, i. e. one having alternating dielectric or semiconductor layers ; - Figure 5 is a chart which represents the spectral emissivity at angle of incidence 0° at ambient temperature for a tungsten emitter coated according to the invention by a layer of yttrium or yttria layer (curve ER), compared with the spectral emissivity of an emitter in simple tungsten under the same conditions of temperature and angle of incidence (curve ET) ; - Figure 6 is a chart which shows the spectral power emitted at angle of incidence 0° by a tungsten emitter at 2000K coated according to the invention by a layer of 50 nm of yttria (curve PR), compared with Planck's curve (curve CP) and with the spectral power emitted by an emitter of simple tungsten under the same conditions of temperature and angle of incidence (curve PT); - Figure 7 is a chart which represents the spectral emissivity ad angle of incidence 60° at ambient temperature by a tungsten emitter coated according to the invention by a layer of 55 nm of yttrium oxide or yttria (curve ER), compared with the spectral emissivity of an emitter made of simple tungsten under the same conditions of temperature and angle of incidence (curve ET);

- Figure 8 is a chart which shows the spectral power emitted at angle of incidence 60° by an emitter made of tungsten coated according to the invention by a layer of 55 nm of yttria at 2000K (curve PR), compared with Planck's curve (curve CP) and with the spectral power emitted by an emitter made of simple tungsten under the same conditions of temperature and angle of incidence (curve PT); - Figure 9 is a chart which represents the spectral emissivity at angle of incidence 75° at ambient temperature for an emitter of tungsten coated according to the invention by a layer of 60 nm of yttrium oxide or yttria (curve ER), compared with the spectral emissivity of an emitter of simple tungsten under the same conditions of temperature and angle of incidence (curve ET) ; - Figure 10 is a chart which shows the spectral power emitted at angle of incidence 75° by an emitter of tungsten coated according to the invention by a layer of 60 nm of yttria at 2000K (curve PR), compared with Planck's curve (curve CP) and with the spectral power emitted by an emitter of simple tungsten under the same conditions of temperature and angle of incidence (curve PT); Figure 11 is a schematic perspective representation of a portion of an emitter superficially provided, according to the invention, by a one- dimensional diffraction grating, i. e. with periodic projections along a single direction ; -Figures 12 and 13 are schematic perspective representations of respective portions of two emitters according to the invention, superficially provided with a respective two-dimensional diffraction grating, i. e. with periodic projections along two orthogonal directions on the surface of the emitter;

- Figure 14 is a chart which shows the calculated spectral reflectance at ambient temperature for a filament of'tungsten with planar surface (curve RTc), the spectral reflectance measured at ambient temperature for a planar tungsten lamina (curve RTm), the calculated spectral reflectance for a tungsten filament micro-structured according to the invention with a one-dimensional grating (curve RM1) and that of a tungsten filament micro-structured according to the invention with a two-dimensional grating (curve RM2) ; all curves are for angle of incidence of 0°.

- Figure 15 is a chart which shows the profile of reflectance at a wavelength of 550 nm as a function of angle of incidence ; the different curves refers to calculated reflectance for a tungsten filament with planar surface (curve RT), calculated reflectance for a tungsten filament micro-structured according to the invention with two-dimensional grating (curve RM2), calculated reflectance for a tungsten filament micro- structured according to the invention with one- dimensional grating (curve RM1) for vibration plane perpendicular to the lines of the grating and lastly calculated reflectance for a tungsten filament micro- structured with one-dimensional grating (curve RM1') for vibration plane parallel to the lines of the grating (90°) ; - Figures 16,17, 18,19, 20 are charts which show the profile of reflectance (curve RM1) as a function of wavelength of a filament provided according to the invention with a one-dimensional grating of tungsten, with pitch 0.275 micron, depth of the projections 0.3 micron and filling factor (i. e. ratio between width of the projections and period of the grating) equal to 19% ; the Figures refer respectively to angle of incidence 0°, 50°, 60°, 70°, and 80° at ambient

temperature; each figure shows the spectral reflectance of a filament made of planar tungsten (curve RT) under equal conditions of temperature and angle of incidence, for comparison purposes ; - Figure 21 is a chart which shows the percent variation of spectral emissivity of the one-dimensional grating of Figures 14-18 for angle of emission 0°, with respect to the planar tungsten filament; - Figure 22 is a chart which shows the percent variation of spectral emissivity of the same one- dimensional grating of Figures 14-18 for angle of emission 80° with respect to the planar tungsten filament ; - Figure 23 is a schematic representation which shows a possible solution for increasing the effective angle of emission in an emitter provided with anti- reflection micro-structured surface according to the invention ; - Figure 24 is a schematic sectioned representation of a portion of an emitter according to the invention, with micro-structured surface with a layer of oxide ; - Figure 25 is a chart which shows the spectral absorbance curve of a layer of crystalline silica with a thickness of 20 micron, at ambient temperature; Figure 26 is a chart which shows the transmittance, reflectance and spectral emissivity curves (respectively'curve TF, RF and EF) of an emitter provided, according to the invention, with a 5 micron coating film of amorphous carbon (a-C: H) with 25% of hydrogen at ambient temperature; - Figure 27 is a chart which shows the comparison between the emission spectrum of a black body, or Planck's curve, at 2000K (curve CP) with the emission spectrum of a material having optical characteristics at 2000K equal to those of hydrogenated amorphous

carbon (curve PCA) with angle of incidence 0°, and with the spectrum of a tungsten filament (curve PT) under the same conditions of temperature and angle of incidence; - Figure 28 is a chart which shows the spectral emissivity of the hydrogenated amorphous carbon at ambient temperature (curve ECA) and the spectral emissivity of graphite at ambient temperature (EG) ; - Figure 29 is a chart which represents the profile of the bandgap energy (Eg) as a function of temperature for germanium (curve Ge), silicon (curve Si), gallium arsenide (curve GaAs) and silicon carbide (curve SiC) ; - Figure 30, is a chart which shows the spectral emissivity of a layer of silicon carbide at ambient temperature (curve ESiC) and a hypothetical profile of the spectral emissivity of silicon carbide at 2000K (curve ECSiC) ; - Figure 31 is a chart which shows the profile of intrinsic concentration of carriers as a function of temperature per germanium (curve Ge), silicon (curve Si), gallium arsenide (curve GaAs) and silicon carbide (curve SiC) ; - Figure 32 is a schematic sectioned representation of a portion of an emitter according to the invention, constituted by a layer of high temperature semiconductor material coated with an anti-reflection layer of refractory oxide ; - Figure 33 is a chart which shows a comparison between the spectral emissivity of a silicon carbide emitter coated, according to the invention, by a layer of yttrium oxide (curve ESiCR) with that of a silicon carbide emitter of equal thickness (curve ESiC).

In a first possible embodiment, the spectral absorption characteristic of the filament or emitter according to the invention is obtained through the use

of a conductor material, for example a metal, which is opaque both in the visible and in the infrared regions (, r=O), and coating said conductor with anti-reflection means operating to reduce reflectance p in the visible region of the spectrum, while maintaining the reflectance p high in the near infrared (wavelength over 0.78 micron).

The aforesaid anti-reflection means can for instance be single-layer or multi-layer dielectric coatings; Figure 3 shows for this purpose a portion of a tungsten emitter or filament F provided, according to the invention, with a coating of yttrium oxide RO, whilst Figure 4 shows a portion of a tungsten emitter F provided, according to the invention, with a multi- layer coating, i. e. having alternating dielectric layers RP or semiconductors with different refraction index, in which the combination of thickness and of the refraction index of the materials constituting the different layers RP is such as to maximise reflectance in the infrared region of the spectrum and to minimise reflectance in the visible region of the spectrum.

Figures 5,7 and 9 instead show the spectral emissivity at different angles of an emitter of tungsten F coated by a layer RO of yttrium oxide or (yttria), represented by the curves ER, compared with that of simple tungsten, represented by the curves ET.

As shown for instance in Figure 5, the presence of a layer of 50 nm of yttria allows to significantly increase emissivity in the visible region (from 0.5 to 0.95 for a wavelength of 500 nm) in the presence of an undesired, though much smaller, increase, in the near infrared region (from 0.3 to 0.4 for a wavelength of 1.5 micron). The advantage is even more significant at large angles of emission, as is readily apparent for instance in Figure 7 ; in particular, at 60° relative to

the normal, a 55 nm thick layer of yttria leads to a smaller increase in emissivity in the near infrared (from 0.32 to 0.36 for a wavelength of 1.5 micron). At yet greater angles, emissivity in the near infrared decreases with an increase in the visible spectrum; this is clearly visible in Figure 9, which compares the spectral emissivity of the tungsten at 75° (curve ET) with the spectral emissivity, also at 75°, of a tungsten filament coated by 60 nanometres of yttria (curve ER).

By multiplying the spectral emissivity values of Figures 5,7 and 9 for Planck's curve at 2000K, one obtains the emitted spectral powers, shown in Figures 6,8 and 10 ; the operation would require the use of optical constants for tungsten and for yttria relating to the temperature of 2000K, but the difference with respect to the optical constants at ambient temperature T, though significant, is not relevant for the purposes of the present disclosure.

The presence of the layer RO of yttria leads to a relative increase in efficiency, defined as the ratio between the power emitted in the spectral interval 0. 38-0. 78 micron and the power emitted in the spectral interval 0.3-3 micron, equal to 25%, 22% and 26% respectively for angles of emission 0°, 60° and 75°.

More significant efficiency increases can theoretically be obtained using a multi-layer anti- reflection coating, but are practically limited by the lack of variety of refraction indices in refractory materials, to be used as layers of said coating.

The final efficiency value obtained with the single-layer coating of yttria RO for emission at p 0° is 6.5% equivalent to what can be obtained simply by increasing the temperature of the tungsten filament, in the absence of coating, by about 100K.

In a second'possible implementation of the present invention, the increase in efficiency of visible emission is obtained by means of an appropriate micro- structure of the surface of the emitter or filament for the incandescent lamp.

The desired anti-reflection behaviour can be obtained both with a one-dimensional grating, i. e. with periodic projections along a single direction on the surface of the filament, both with a two-dimensional diffraction grating, i. e. with periodic projections along two orthogonal directions on the surface of the filament. For this purpose, in Figure 11 the reference F designates a portion of an emitter according to the invention, which superficially has a diffraction grating R formed by periodic micro-projections R1 along a single direction ; in the case shown in Figures 12 and 13, instead, the portion F of emitter according to the invention superficially has a diffraction grating R formed by periodic micro-projections R2 along two orthogonal directions. In Figures 11-13, the reference h designates the depth or height of the projections R1, R2, the reference D designates the width of the projections and P the period of the grating R; the filling factor of the grating R is defined as the ratio 'D/P in the case of Figure 11, as the ratio D2/P2 in the case of Figure 12 and as the ratio 7cD 2/ (4p2) in the case of Figure 13.

The advantage of the grating R formed by the micro- projections R1, R2 with respect to multi-layer anti- reflection coatings is that the filament can be brought to higher temperatures, for example 2500K, without being limited by the melting temperature of the oxides used as layers of the coating.

In Figure 14, the curve RTc represents the calculated spectral reflectance at ambient temperature

for a tungsten filament with planar surface, the curve RTm represents the measured spectral reflectance at ambient temperature for a planar tungsten lamina, the curve RM1 represents the calculated spectral reflectance for a micro-structured tungsten filament with a one-dimensional grating R (of the type shown in Figure 11, with parameters D/P=0. 15, h=0.3 micron, P=0.25 micron) and the curve RM2 represents the calculated spectral reflectance of a micro-structured tungsten filament with a two-dimensional grating R (of the type shown in Figure 12, with parameters D2/P2=0. 15, h=O. 1 micron, P=0.25 micron). The data of Figure 14 refer to ambient temperature and to an angle of incidence of 0°.

From Figure 14, the different profile of reflectance between planar tungsten filaments and filaments structured according to the invention is readily apparent. Figure 15 instead shows the profile of reflectance at a wavelength of 550 nm as a function of the angle of incidence ; in particular: - the curve RT refers to the calculated reflectance for a traditional tungsten filament with planar surface, - the curve RM2 refers to the calculated reflectance for a first tungsten filament according to the invention, micro-structured with two-dimensional grating R (of the type shown in Figure 12, with parameters D 2/p2=0. 15, h=O. 1 micron, P=0.25 micron), - the curve RM1 refers to the calculated reflectance for a second tungsten filament according to the invention, micro-structured with one-dimensional grating R (of the type shown in Figure 11, with parameters D/P=0.15, h=0.3 micron, P=0.25 micron), for vibration plane perpendicular to the lines R1 of the grating, e

the curve RM1'refers to the calculated reflectance for the tungsten filament micro-structured with one-dimensional grating R, but for vibration plane parallel to the lines Rl of the grating (90°).

Figure 15 also clearly shows the different reflectance profile between planar tungsten filaments and filaments structure according to the invention. As is readily apparent, the superficial structure of the tungsten which constitutes the emitter allows significantly to lower the value of reflectance in the visible region in the presence of a greatly lower reduction in reflectance in the near infrared.

In fact, to understand the entity of the efficiency increase it is necessary to consider the curves of percent variation in spectral emissivity ; there is an actual improvement in efficiency only when the relative variation in emissivity in the visible region is greater than the relative variation in emissivity in the near infrared region.

An interesting result, exhibited for the first time by the present invention, is the fact that the anti- reflection characteristics of the one-dimensional grating R are retained even when the vibration plane is parallel to the lines of the grating (curves RM1'of Figure 15).

Figures 16,17, 18,19, 20 show the profile of the reflectance curve RM1 as a function of wavelength for a one-dimensional tungsten grating R according to the invention, with pitch 0.275 micron, depth of the projections 0.3 micron and filling factor D/P (i. e. ratio between width of the projections and period of the grating) of 19%. In particolar, the figures respectively refer to angles of incidence of 0°, 50°, 60°, 70°, and 80° ; each figure shows the curve RT of spectral reflectance of the planar tungsten, for

comparison purposes.

It should be noted that, above 60°, the reflectance of the grating R in the near infrared becomes smaller than that of planar tungsten this entails an increase in efficiency that is greater than with normal incidence. Multiplying the values of relative emissivity at the different angles (obtained as the opposite of reflectance) for Planck's curve at 2500K, one obtains an efficiency variation respectively of-2%, +10%, +23%, +48% and +90%. The reduction in efficiency in the case of angle of emission 0° is due to the fact that, although the increase in emissivity in absolute terms is far greater in the visible part than in the infrared part of the spectrum, relative variation is greater in the infrared region (as is readily apparent in Figure 21). It is also readily apparent that the efficiency increase is particularly significant at large angles; on average, between 50° and 80°, efficiency increases by 35%. Efficiency nearly doubles at 80°, thanks to the strong relative decrease in spectral emissivity in the infrared region (see Figure 22).

Hence, this suggests the possibility of obtaining an incandescent source, in which the macroscopic shape of the emitter is characterised by a roughening whose purpose is to increase the mean angle of emission with respect to the normal direction to the surface of the emitter, whilst a microscopic anti-reflection grating on the surface of the emitter allows to increase emission efficiency at large angles; an example of this embodiment of the invention is shown in Figure 23, where the reference MA designates the surface of the emitter provided anti-reflection micro-structure.

The'roughening does not modify the emission lobe relative to the plane K, which remains substantially

Lambertian, but causes the radiation emitted in the cone [-y, +y] relative to the normal direction to the plane K to be in fact emitted with an angle relative to the direction normal to the surface of the emitter (or effective angle of emission) included within the range [p-y, ß+Y]. Since the emission lobe is Lambertian, maximum intensity is in correspondence with y=0, which corresponds to an effective angle of emission of P. For every y in the range [-90+ß, 90-ß] in general there are two effective angles of emission, P+y and ß-Y. For y>90- P the effective angle of emission is ß-y, whilst for y<-90+P the angle of emission is (3+y. For example for ß=60°, and y=40° there is an effective angle of emission of 20°, whilst for y=20° there are two effective angles of emission, one at 40° and one at 80°. The concept is represented in Figure 23 in the two-dimensional case, but it can easily be transferred to the three-dimensional case.

In general, the anti-reflection grating according to the invention can also be multi-level or with continuous profile, which allows to increase the degrees of freedom to optimise the grating and further enhance efficiency.

In accordance with an additional configuration, the diffraction grating can be coated with one or more layers of refractory oxide, for example yttrium oxide, in order further to increase the degrees of freedom in optimising efficiency. Such a case is schematically shown in Figure 24, where the reference F designates a filament provided with the diffraction grating formed by projections R1, the grating being coated by a layer of oxide RO. Moreover, the presence of an oxide coating can allow to operate the filament under conditions of less pronounced vacuum or, in principle, also in air without oxidation of the filament ; in any case, both

under vacuum conditions and in inert gas atmosphere, the presence of the oxide coating allows to reduce the evaporation rate of tungsten and thus increase the mean life of the source.

An additional possible implementation of the present invention consists of constructing the emitter with a thin layer of semi-conductor material, having low reflectance p both in the visible and in the infrared regions, high transmittance T in the near infrared and low transmittance T in the visible region.

This characteristic at ambient T is typical, for example, of silicon or of other semiconductors having bandgap Eg whose value is proximate to the visible transitions (1.6-3. 2eV) ; on sufficiently low values of thickness, a semi-conductor tends to transmit the longer wavelengths, associated to photon energy lower than the bandgap energy Eg. For example, crystalline silicon has a bandgap Eg at ambient temperature of l. leV and tends therefore to transmit photons with energy lower than l. leV, i. e. with wavelength exceeding 1.13 micron, defined by the relationship kg=1. 2469/Eg.

In the case of silicon, photons whose energy far exceeds the bandgap are reflected, which results in a spectral absorbance that is high in the visible and low both in the infrared and in the ultraviolet; for this purpose, Figure 25 represents the spectral absorbance curve of a layer of crystalline silicon with thickness 20 micron ; this is the reason why silicon has historically always been used as a detector of visible radiation, in CCD sensors, or as a material for photovoltaic cells.

Note that if silicon were able to withstand the high temperature of an incandescent filament and maintained its optical characteristics at such temperatures, it would be a nearly ideal emitter,

thanks to the sharp discontinuity in its spectral absorbance in correspondence with 1.1 micron. In speculative terms, a material having a similar spectral absorbance curve would, at 2000K, have a radiating efficiency of over 30% in the visible region.

Another material with optical characteristics that are very similar to silicon is hydrogenated amorphous carbon a-C: H, typically used in hardening coatings known as"diamond-like-carbon"or DLC. Optical constants of a-C: H vary greatly according to hydrogen percentage Figure 26 represents the curves of transmittance TF, reflectance RF and spectral emissivity EF of a 5 micron film of a-C : H with 25% of hydrogen at ambient temperature ; it can be observed that, like silicon, the film of a-C : H has high transmittance in the near infrared and high absorption in the visible, which would make it, too, a nearly ideal emitter. The oscillations in the reflectance and transmittance curves are due to phenomena of interference between the waves reflected by the first and by the second interface of the film; the frequency of said oscillations tends to grow with the thickness of the layer.

Figure 27 compares Planck's curve CP, i. e. the emission spectrum of a black body, at 2000K with the emission spectrum of a filament of a material having optical characteristics at 2000K equal to those of hydrogenated amorphous carbon (curve PCA) and with a tungsten filament (curve PT); the efficiency of said hypothetical material would be 14%, against the 5% of tungsten.

Carbon, unlike silicon, has a very high melting point and hence is. able to reach incandescent temperatures; unfortunately, at temperatures of a few hundred degrees a-C: H graphitises and its optical

constants become those of graphite, which unfortunately has a high value of absorption even in the infrared ; this fact is visible in Figure 28, which compares the spectral emissivity of hydrogenated amorphous carbon (curve ECA) and of graphite (curve EG).

It is well known that temperatures have a significant influence on the optical constants of materials; in the case of semiconductor materials, the main effects of a temperature increase are the decrease in bandgap energy Eg and the increase in electron concentration in the conduction band and of gaps in valence band, due to the increased thermal energy of the charge carriers. Figure 29 shows the profile of Eg as a function of temperature for germanium (curve Ge), silicon (curve Si), gallium arsenide (curve GaAs) and silicon carbide (curve SiC); the profile is nearly linear, with its slope slightly depending on the bandgap value at ambient temperature. From the point of view of absorption, the lowering of the bandgap leads to a shift of the spectrum towards the longer wavelengths ; for example, at 1000k the bandgap of silicon is about 0.9eV, which means that silicon at 1000K can absorb radiation of wavelengths up to kg=1. 2469/Eg=1. 39 micron and for sufficiently small thickness values it will transmit the radiation of wavelength exceeding kg. The bandgap of gallium arsenide at 1000K instead shifts from 1. 4eV (ambient temperature) at l. leV and it is therefore legitimate to expect a absorption spectrum that is similar, though obviously not in absolute terms, to that of silicon at ambient temperature.

Particularly interesting in this regard is silicon carbide, or SiC, thanks to the high value of its melting temperature which makes it a material potentially usable in incandescent sources. SiC has an

ambient temperature bandgap of around 3.2eV, which means that for sufficiently low thickness values the material transmits above kg=1. 2469/Eg=386 nm.

The bandgap decreases significantly as temperature increases, bringing the value of? g=1. 2469/Eg at 2000K to about 540 nm. This would assure an effective emission of radiation in the visible up to 540 nm and very low emission in the red/infrared spectrum. This situation is represented in Figure 30, which shows the spectral emissivity of a 20 micron layer of silicon carbide (curve EsiC) at ambient temperature and a hypothetical shift of the curves, as temperature grows, towards the longer wavelengths (curve ECSiC).

Obviously, spectral absorbance values are also influenced by the variation in the intrinsic concentration of carriers and by modifications to the crystalline structure. In general, as temperature grows the concentration of free carrier grows and hence absorption rises. This increase also involves the infrared spectral region, with the consequent reduction in emitter efficiency. However, if the thickness of the layer is sufficiently small, absorption in the near infrared remains low and the emitter maintains a high efficiency.

Figure 31 shows the profile of intrinsic concentration of carriers as a function of temperature for germanium (curve Ge), silicon (curve Si), gallium arsenide (curve GaAs) and silicon carbide (curve SiC) ; as is evident, concentration decreases exponentially as the ratio 1/T increases. As is readily apparent from the chart, intrinsic concentration (in cm~3) in SiC at 2000K is in the order of 1016, typical value of a weakly doped silicon.

In an additional configuration according to the invention, schematically shown in Figure 32, the

characteristics relating to the two previously mentioned configurations are combined in such a way as to obtain an emitter F constituted by a layer of high temperature semiconductor material, such as silicon carbide, having high transmittance in the near infrared and good absorbance in the visible region, coated with an anti-reflection layer RO, for instance a single layer of refractory oxide, such as to maximise the absorption of the emitter in the visible region without increasing absorption in the near infrared.

By way of example, this can be obtained by coating a 20 micron layer of SiC (which constitutes the emitter F), with a 20 nm layer RO of yttria ; Figure 33 compares the spectral emissivity of such an emitter (curve EsiCR) with that of an emitter of only silicon carbide with equal thickness (curve EsiC). Alternatively, the anti-reflection behaviour in the visible region can be obtained through a superficial micro-structuring of the semiconductor, according to any of the techniques described above with reference to Figures 11-13.

From the above description it is therefore readily apparent that, in the various possible implementations of the invention, defining a (S, T) to be the spectral absorbance of the emitter at the operating temperature T, which absorbance is linked to spectral reflectance p (S, T) and to spectral transmittance T (k, T) by the relationship the composition and/or structure of the emitter according to the invention can be optimise in such a way as to minimise a (,) for belonging to the visible region of the spectrum'and to maximize a (X) for k belonging to the infrared region of the spectrum.

In this way, for equal operating temperature T, the ratio between the radiation emitted in the visible region of the spectrum and the radiation emitted in the

infrared region of the spectrum for an emitter according to the invention is greater than the same ratio with respect to the case of a traditional incandescent filament, with obvious advantages in terms of light source efficiency.

As shown, the means that allow to reach the intended objects are constituted by superficial structuring of the emitter, or by particular materials, having a high transmittance c (X, T) at infrared radiation and high absorbance a (S, T) in the visible region of the spectrum; said materials are also usable in combination, in such a way that the emitter has high transmittance T (, T) at. infrared radiation and high absorbance a (S, T) in the visible region of the spectrum.

Naturally, without altering the principle of the invention, the construction details and the embodiments may vary widely relative to what is described and illustrated, purely by way of example, herein, without thereby departing from the scope of the present invention.

In the various implementations, yttrium oxide could be replaced by the oxide of a metal belonging to the rare earth group.

The surface diffraction grating of the emitter according to the invention could be three-dimensional, i. e. periodic along two directions substantially parallel to each other and contained in the plane of the grating and along a third direction, substantially perpendicular to the surface of the grating. As mentioned above, the diffraction grating could be multi-layer, for instance with two or more levels, or have a projection that is variable in continuous fashion within the period.

The anti-reflection means could be in the form of

volume diffraction grating, having a configuration similar to the one shown in Figure 24, with a substantially flat outer surface and a periodic modulation of the complex refraction index within the volume of the emitter F.