INFRARED WINDOWS IN INDUSTRIAL APPLICATIONS I a Andres E. Rozlosnik a SI Termografía Infrarroja -- Buenos Aires -- Argentina ABSTRACT This paper is a slightly modified version the one presented orally at THERMOSENSE XXVI April 13-15 2004 SPIE [5405-46] Session 8: Research & Development. Many industrial applications exist for which, due to the lack of access or for other reasons, thermal images or thermometry measurements cannot be generated. This happens especially when the target is inside an enclosure and cannot be viewed, for safety - process reasons, except through a window transparent to visible energy only. The traditional materials transparent to visible radiation ~ 380nm – ~780nm are not, in general, transparent to infrared radiation. It is clear there is a more specific exception in the NIR band. This lack of IR transmission in traditional glass / crystals has required the search for other alternatives in order to carry out measurements or generate images with instruments that work in the thermal bands (MWIR -LWIR) of the electromagnetic spectrum. This paper includes infrared windows basics and a study of attenuation with LWIR sensor of two windows two colors transmission (MWIR / LWIR) commonly installed in medium voltage electrical enclosures. Keywords: Infrared windows / Theory / Thermometry / Transmission / Attenuation 1 – INTRODUCTION Gas, liquid and solids can absorb / emit, transmit (+ scatter) and reflect electromagnetic radiations. For example the terrestrial atmosphere, which is a gas, is not transparent to all wavelengths and when it transmits, it does not transmit 100%. Some absorption, scattering and small reflections do exist. There are two basic atmospheric windows in the so-called thermal band, and they are approximately at λ = 3-5 µm (MWIR) and λ = 8-14µm. (LWIR). The level of transparency in these two bands depends mainly on: altitude, humidity, path length, pressure, soot/dust/aerosol concentration and temperature of the medium where we are carrying out the measurement. In other words: in which part of the planet are we doing the measurement? Is our target in the horizontal or vertical path? What is the distance to the target? What are the weather conditions? Is the atmosphere clean? etc. The two major constituents in the atmosphere that vary with temperature and altitude are ozone and water vapor. CO2 is part of the permanent composition of dry atmosphere. CO2 and water vapor (H2O) are the major IR absorbers. Figure N°1 shows atmospheric transmission curves for vertical and horizontal paths corresponding to the two thermal bands: MWIR (~ 2.5µm -- ~ 5µm) LWIR (~ 7µm - ~ 14µm). The vertical path length is 100 km and the horizontal path length is 1 km at sea level. (Source: Raytheon Vision Systems) The basic law that governs atmospheric transmission is based on the Beer –Lambert study: the exponential decrease in transmitted IR intensity. In the MWIR region it is an example of the problem of applying Beer’s law to an entity as complex as the earth’s atmosphere. Sometimes the extinction coefficient is very wavelength dependent. - γ R Φ = Φ0 . e Φ0 = radiant flux (watt) - γ R Atmospheric Transmission = τ = e (the negative sign indicate a loss of mechanism -- vacuum is the only perfectly transparent medium) Where: R = Path length γ = Extinction coefficient γ = σ (λ) + K (λ) ≡ (in the visible sometimes call turbidity) σ = Scattering component -- air molecules, aerosol, (suspended solid particles smoke dust) hydrometeors= suspended water droplets clouds and fog K = Absorptive component = gaseous lines /continues absorption (CO2 //H2O.etc) Normally we use infrared cameras in civil--industrial applications through relatively short media path lengths (camera-to-target through terrestrial atmosphere) and, depending on the camera we use, there is little or no correction necessary for atmospheric absorption (transmission is considered to be unity). Often environmental conditions can be neglected when they are constant and only qualitative measurements are required. Now, when we consider long path-length applications such as military or meteorological, atmospheric transmission becomes of greater importance and affects the SNR. (signal-to-noise-ratio) much more. A special case is when we look through the atmosphere in a furnace in order to see the walls and pipes. Inversely, in these cases, we need use filters to avoid undesirable radiation of the hot gases and flame. Horizontal 1 km path Horizontal Vertical 100 km path Vertical Wavelength (µm) Figure N°1—Atmospheric transmission MWIR–LWIR (Transmission values taken from Raytheon Vision Systems Chart) There is mathematical analogy (both exponential) between the atmospheric transmission and the transmission through a semitransparent solid like an infrared window: - α b The transmission through a semitransparent solid is: τ = e In which b is the thickness and α the absorption coefficient of the substrate respectively. Larger b and α means more attenuation, loss of transmission. On the other hand the absorption coefficient (α) is temperature dependant and varies with the purity of the material. (See Figure N° 2) When do we need to install or use an IR Window? When, for some reason (security… environment) a medium or specific atmosphere needs to be isolated, or it is already isolated and we want to know the thermal status inside it. The IR window should be transparent at wavelengths compatible with those of the IR sensor we will use. Additionally, we would use a window (UV-Visible-Infrared) when two different media or environments exist and electromagnetic radiation at specified wavelengths pass between the two. The attenuation effect of the window itself should be added to the atmospheric transmission phenomenon. (sometimes this is negligible) As an analogy, windows that we use in daily life (houses, offices, industrial buildings, cars etc) permit the visible & NIR infrared radiation go in, which allows us to view the external world and at the same time keep us isolated from the outside (temperature, winds, snow… dust etc). The Cambridge Dictionary defines a glass window in this way: “space usually filled with glass in the wall of a building or in a vehicle, to allow light and air in and to allow people inside the building to see out.” So IR window are used to isolate, or maintain the isolation between two media with different parameters: Medium 1 ≠ Medium 2. Between the two any of these situations can occur: -- Pressures differential (example vacuum & high pressure differential/survive flight environment) -- Temperature differential -- Different atmosphere - gas constituent -- Medium safety banned (example high voltage electrical &-nuclear applications) -- Protect systems from dust or other particles like salt spray, raindrops or effluents and excessive moisture.(In the infrared for example: IR windows in surveillance cameras enclosures-missiles domes, etc) Why can’t we use standard or reinforced glass in IR Inspections or IR Systems? Glass does not transmit MWIR or LWIR radiation! The cutoff wavelengths are around 0.32µm (UV) --2.5 µm (IR) (depending in thickness & type) Glass consists mostly of molecules of silicon dioxide, (SiO2) although there are usually also impurities present, even a certain amounts of water. This means that there are several different ways in which light passing through the glass can give up energy to the glass. Infrared photons have energies of about 0.4 ev⇒ 3 µ-- ~90 mev.⇒ 14µ (E=h.ν) This is approximately the energy corresponding to a rotational or vibrational state of a silicon dioxide (SiO2) molecule or a water molecule. When infrared photons pass through the glass, they tend to cause the molecules to rotate and vibrate. If one molecule starts to rotate or vibrate, it can share its extra 2 IR Window - α b Fresnel reflection losses equation: Internal Transmission = e = X2 / X1 = τi good approximation for tilt angles up to 45° b =:Thickness 2 The emitted (εw) flux its is External Transmission = X / X0 (total) n1 - n2 determined by a series of R = expression accounting for the n1 + n2 multiple reflections and transmittance losses. The total α = α absorption + α scatter = X2 - X1 / X0 α is temperature dependant! 2R radiance from surface I given as infinite series (1) 4πk Total reflectance (TR) = K= Extinction coefficient 1 + R α = X0 X1 X2 X n. λ 2 n n2 If an Infrared windows reach high T = 1 - R = n 2 + 1 temperature depending the material it can oxidize and dramatically n1 Refraction at interface: n1 2 increased the emissivity. (εw) In - α b Do not change radiation (1 – R ) e frequency (ν) order to recover the transmission properties probably the substrate Ta = Surfaces finish: should be polish again. 2 - 2 αb α = Absorption 1 - R e Machining R = One surfaces reflection (Fresnel) The thickness variation Grinding εw (1) = (1-ρ w) (1-τ w) / (1- τ w ρ w) = TR = Total reflection Polishing produce curvature into T = Transmission non abs. IR Window emerging wavefront - α b Ta = Transmission abs. IR Window (could lose of focus if not (1-R) . ( 1 - e ) εw = Emissivity IR Window optically flat) = τi = Internal transmission - α b n = Refraction index Acceptable 1 – R . e -1 Fourier Transform IR (FTIR) spectrometers can be used to determine α (cm ) λ/4=thickness variations FTIR spectrometers typically can achieve transmission accuracies of better than 0.1 %. for IR applications Figure N° 2 --- Transmission of radiation through substrates -classical formulas - energy with nearby molecules, and soon the initial energy of the photon is distributed amongst many molecules. The result is that the photon has been absorbed, and the glass is a little warmer. Another characteristics of glass that is that it is an amorphous material (vitreous). This feature it is not directly related to the transmission itself. There are some amorphous materials that transmit in the infrared. (AMTIR: Amorphous Material Transmitting Infrared). AMTIR-1, for example, is an amorphous IR transmission material. It was originally produced for night vision systems, but it has other applications including optical elements and optical sensors for remote temperature sensing.