What is Light? Electromagnetic Wave Theory Light is just one portion of the various electromagnetic waves flying through space. The electromagnetic spectrum covers an extremely broad range, from radio waves with wavelengths of a meter or more, down to x-rays with wavelengths of less than a billionth of a meter. Optical radiation lies between radio waves and x-rays on the spectrum, exhibiting a unique mix of ray, wave, and quantum properties. At x-ray and shorter wavelengths, electromagnetic radiation tends to be quite particle like in its behavior, whereas toward the long wavelength end of the spectrum the behavior is mostly wavelike. The visible portion occupies an intermediate position, exhibiting both wave and particle properties in varying degrees. Like all electromagnetic waves, light waves can interfere with each other, become directionally polarized, and bend slightly when passing an edge. These properties allow light to be filtered by wavelength or amplified coherently as in a laser. In radiometry, light’s propagating wavefront is modeled as a ray traveling in a straight line. Lenses and mirrors redirect these rays along predictable paths. Wave effects are insignificant in an incoherent, large scale optical system because the light waves are randomly distributed and there are plenty of photons. Ultraviolet Light Short wavelength UV light exhibits more quantum properties than its visible and infrared counterparts. Ultraviolet light is arbitrarily broken down into three bands, according to its anecdotal effects. UV-A is the least harmful and most commonly found type of UV light, because it has the least energy. UV-A light is often called black light, and is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light - thus appearing to glow in the dark. Most phototherapy and tanning booths use UV-A lamps. UV-B is typically the most destructive form of UV light, because it has enough energy to damage biological tissues, yet not quite enough to be completely absorbed by the atmosphere. UV-B is known to cause skin cancer. Since most of the extraterrestrial UV-B light is blocked by the atmosphere, a small change in the ozone layer could dramatically increase the danger of skin cancer. Short wavelength UV-C is almost completely absorbed in air within a few hundred meters. When UVC photons collide with oxygen atoms, the energy exchange causes the formation of ozone. UV-C is almost never observed in nature, since it is absorbed so quickly. Germicidal UV-C lamps are often used to purify air and water, because of their ability to kill bacteria. The image below shows three different galaxies taken in visible light (bottom three images) and ultraviolet light (top row) taken by NASA's Ultraviolet Imaging Telescope (UIT) on the Astro-2 mission. Visible Light Photometry is concerned with the measurement of optical radiation as it is perceived by the human eye. The CIE 1931 Standard Observer established a standard based on the average human eye response under normal illumination with a 2° field of view. The human eye has two different kinds of photoreceptor cells, cones and rods. Cones are responsible for color perception (three different groups of cones for three different parts of the visible spectrum), while rods are more sensitive to light intensity so they are mostly activated during the night or in lowlight conditions. Rods are faster and more sensitive than cones. Color is further broken down into “hue” and “saturation.” The eye, as a photoreceptor, is sensitive to only a narrow band of the electromagnetic spectrum, this band corresponding to “visible light” that stretches from 360 (violet) to 780 nm (red). Figure 2.13 shows the sensitivity curves representing each of the three groups of color-sensing cells in the eyes (the cones). Cone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser toward the periphery of the retina. Cones are less sensitive to light than the rod cells in the retina (which support vision at lowlight levels), but allow the perception of color. Figure 2.14 shows the overall sensitivity of the human eye in different light level conditions. Infrared Light Infrared light contains the least amount of energy per photon of any other band. Because of this, an infrared photon often lacks the energy required to pass the detection threshold of a quantum detector. Infrared is usually measured using a thermal detector such as a thermopile, which measures temperature change due to absorbed energy. While these thermal detectors have a very flat spectral responsivity, they suffer from temperature sensitivity, and usually must be artificially cooled. Another strategy employed by thermal detectors is to modulate incident light with a chopper. This allows the detector to measure differentially between the dark (zero) and light states. Quantum type detectors are often used in the near infrared, especially below 1100 nm. Specialized detectors such as InGaAs offer excellent responsivity from 850 to 1700 nm. Typical silicon photodiodes are not sensitive above 1100 nm. These types of detectors are typically employed to measure a known artificial near-IR source without including long wavelength background ambient. Since heat is a form of infrared light, far infrared detectors are sensitive to environmental changes such as a person moving in the field of view. Night vision equipment takes advantage of this effect, amplifying infrared to distinguish people and machinery that are concealed in the darkness. Infrared is unique in that it exhibits primarily wave properties. This can make it much more difficult to manipulate than ultraviolet and visible light. Infrared is more difficult to focus with lenses, refracts less, diffracts more, and is difficult to diffuse. Most radiometric IR measurements are made without lenses, filters, or diffusers, relying on just the bare detector to measure incident irradiance. The Power of Light Quantum Theory The watt (W), the fundamental unit of optical power, is defined as a rate of energy of one joule (J) per second. Optical power is a function of both the number of photons and the wavelength. Each photon carries an energy that is described by Planck’s equation: Q = hc / λ where Q is the photon energy (joules), h is Planck’s constant (6.623 x 10-34 J s), c is the speed of light (2.998 x 108 m s-1), and λ is the wavelength of radiation (meters). All light measurement units are spectral, spatial, or temporal distributions of optical energy. As you can see in figure 2.1, short wavelength ultraviolet light has much more energy per photon than either visible or long wavelength infrared. Flat Response Since silicon photodiodes are more sensitive to light at the red end of the spectrum than to light at the blue end, radiometric detectors filter the incoming light to even out the responsivity, producing a “flat response”. This is important for accurate radiometric measurements, because the spectrum of a light source may be unknown, or may be dependent on operating conditions such as input voltage. Most sources are continuums, emitting over a broad band of the spectrum. Incandescent lamps are a good example. The color temperature and output of these lamps vary significantly with input voltage. Flat response detectors measure only output power in watts, taking into consideration light at every wavelength. Visible Light The lumen (lm) is the photometric equivalent of the watt, weighted to match the eye response of the “standard observer”. Yellowish-green light receives the greatest weight because it stimulates the eye more than blue or red light of equal radiometric power: 1 watt at 555 nm = 683.0 lumens To put this into perspective: the human eye can detect a flux of about 10 photons per second at a wavelength of 555 nm; this corresponds to a radiant power of 3.58 x 10-18 W (or J s-1). Similarly, the eye can detect a minimum flux of 214 and 126 photons per second at 450 and 650 nm, respectively. Use of a photopic correction filter is important when measuring the perceived brightness of a source to a human. The filter weights incoming light in proportion to the effect it would produce in the human eye. Regardless of the color or spectral distribution of the source, the photopic detector can deliver accurate illuminance and luminance measurements in a single reading. Scotopic vision refers to the eye’s dark-adapted sensitivity (night vision). How Light Behaves Reflection Light reflecting off of a polished or mirrored surface obeys the law of reflection: the angle between the incident ray and the normal to the surface is equal to the angle between the reflected ray and the normal. Precision optical systems use first surface mirrors that are aluminized on the outer surface to avoid refraction, absorption, and scatter from light passing through the transparent substrate found in second surface mirrors. When light obeys the law of reflection, it is termed a specular reflection. Most hard polished (shiny) surfaces are primarily specular in nature. Even transparent glass specularly reflects a portion of incoming light. Diffuse reflection is typical of particulate substances like powders. If you shine a light on baking flour, for example, you will not see a directionally shiny component. The powder will appear uniformly bright from every direction. Many reflections are a combination of both diffuse and specular components. One manifestation of this is a spread reflection, which has a dominant directional component that is partially diffused by surface irregularities. Refraction: Snell’s Law When light passes between dissimilar materials, the rays bend and change velocity slightly, an effect called refraction. Refraction is dependent on two factors: the incident angle, θ, and the refractive index, n of the material, as given by Snell’s law of refraction: n sin(θ) = n’ sin(θ) For a typical air-glass boundary, (air n = 1, glass n’ = 1.5), a light ray entering the glass at 30° from normal travels though the glass at 19.5° and straightens out to 30° when it exits out the parallel side. Note that since sin(0°) = 0, light entering or exiting normal to a boundary does not bend. Also, at the internal glass-air boundary, total internal reflection occurs when n’sin(θ) = 1 (at θ’ = 41.8° for n’ = 1.5 glass. The index of refraction itself is also dependent on wavelength. This angular dispersion causes blue light to refract more than red, causing rainbows and allowing prisms to separate the spectrum. Diffraction Diffraction is another wave phenomenon that is dependent on wavelength. Light waves bend as they pass by the edge of a narrow aperture or slit. This effect is approximated by: θ=λ/D where θ is the diffraction angle, λ the wavelength of radiant energy, and D the aperture diameter. This effect is negligible in most optical systems, but is exploited in monochromators. A diffraction grating uses the interference of waves caused by diffraction to separate light angularly by wavelength. Narrow slits then select the portion of the spectrum to be measured. The narrower the slit, the narrower the bandwidth that can be measured. However, diffraction in the slit itself limits the resolution that can ultimately be achieved. Most monochromators use gratings to disperse light into the spectrum. Gratings rely on interference between wavefronts caused by microscopically ruled diffraction lines on a mirrored surface. The wavelength of reflected light varies with angle, as defined by the grating equation, where m is the order of the spectrum (an integer). Refraction Refraction underlies the optical properties of lenses. The Fresnel design allows the construction of large lenses by reducing the volume and weight that would be required for the construction of conventional lenses. Refraction is also responsible for the effect of the rainbow and the splitting of white light into component colors through a prism. The different frequencies of different colors travel at different speeds when they enter a new medium, leading to different directions for each wave of different color. This is also called chromatic aberration and can be seen in lenses of inferior quality. Prisms use glass with a high index of refraction to exploit the variation of refraction with wavelength. Blue light refracts more than red, providing a spectrum that can be isolated using a narrow slit. Internal prisms can be used to simply reflect light. Since total internal reflection is dependent on a difference in refractive index between materials, any dirt on the outer surface will reduce the reflective properties, a property that is exploited in finger print readers. Interference When wave fronts overlap in phase with each other, the magnitude of the wave increases. When the wave fronts are out of phase, however, they cancel each other out. Interference filters use this effect to selectively filter light by wavelength. Thin metal or dielectric reflective layers separated by an optical distance of n’d = λ/2, or half the desired wavelength provide in phase transmission. Wave interference. The wave interference can be (left) constructive or (right) destructive, depending on the phase difference of the waves. Electrical Incandescent Light Sources Incandescent lamps, as the name implies, are based on the phenomenon of incandescence. The principle does not differ from that of the blackbody, which radiates as it is being heated, starting from the infrared part of the spectrum and covering more and more of the visible spectrum as the temperature increases. Luminescent Sources Sunlight Solid-State Light Sources During the flow of electricity through such a solid-state diode, electrons are combined in the semiconductor junction with the positive holes, and this combination puts the electrons in a lowerenergy state. The energy state difference can be released as electromagnetic radiation (not always, as it can also be lost as heat in the crystal) with a wavelength that depends on the materials of the semiconductor. Such a light source is known as LED (light-emitting diode). LEDs emit radiation of narrow bandwidth (a range of a few tens of nanometers). If the material is an organic compound, then we have an OLED (organic light-emitting diode), and in case of a polymer compound the acronym used is POLED. The efficiency of LEDs is defined by several factors such as 1. The electric efficiency, which has to do with the number of charges in the material (>90% achieved); 2. The internal quantum efficiency, which is the number of photons per number of electrons (this depends on the material and construction of layers; heat and reabsorption are the main problems); 3. The extraction efficiency, which is the number of emitted photons per total number of photons (the geometry of the material and capsule plays an important role); 4. The spectral or optical efficiency, which is related to the eye sensitivity curve (this factor is not taken into account for an LED emitting at the limits of the curve). LED White Light Emissions White light can be created with different-colored LEDs (red, green, blue or yellow and blue or four different colors) or by using a phosphor on a UV or blue LED (UV LED with a trichromatic powder or a blue LED with a yellow powder—YAG:Ce). With three or more primary LEDs, all colors can be created. Red LEDs are the most sensitive to temperature and, therefore, corrections need to be made as the LEDs heat up. Moreover, the light intensity and angle of incidence of each LED must match and mix appropriately in order to create the white light correctly. The combination of blue and yellow light also gives the impression of white light since the yellow light stimulates the sensors of the eye that are sensitive to red and green, but the resulting white light will be of low color rendering index. The other method of creating white light without using more than one LED, is to convert ultraviolet or blue LED light into different colors by using a phosphor. The use of phosphor lowers efficiency due to Stokes losses and other losses on the powder, but it still remains the easiest and cheapest way of creating white light, while the color rendering is usually better due to the larger spectral range of the powder. This method allows us to create white light with a color temperature of up to about 5500 K, but with the addition of another powder that emits in the red part of the spectrum we can also create a warm white light temperature of 3200 K and better color rendering, at the cost of reducing the source efficiency. The use of powder on a blue LED is the most economical way to create white light. Finally, a method that is being developed rapidly is the use of quantum dots: nanocrystalline semiconductor materials with dimensions equal to a few dozen atoms that emit light (fluoresce) with high efficiency under electrical or optical stimulation. The wavelength of radiation can be controlled by controlling the size of the nanocrystals, and this method is in the experimental stage. Detectors Thermal Detectors In a thermal detector the incident radiation is absorbed to change the material temperature, and the resultant change in some physical property is used to generate an electrical output. The detector is suspended on lags, which are connected to the heat sink. The signal does not depend upon the photonic nature of the incident radiation. Thus, thermal effects are generally wavelength independent, the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. Since the radiation can be absorbed in a black surface coating, the spectral response can be very broad. Attention is directed toward three approaches that have found the greatest utility in infrared technology; namely, bolometers, pyroelectric, and thermoelectric effects. In pyroelectric detectors a change in the internal electrical polarization is measured, whereas in the case of thermistor bolometers a change in the electrical resistance is measured. In contrast to photon detectors, thermal detectors are typically operated at room temperature. They are usually characterized by modest sensitivity and slow response (because heating and cooling of a detector element is a relatively slow process), but they are cheap and easy to use. They have found widespread use in low cost applications, which do not require high performance and speed. Thermopiles The thermocouple was discovered in 1821 by the physicist, J. Seebeck. He discovered that at the junction of two dissimilar conductors a voltage could be generated by a change in temperature. The thermopile is one of the oldest IR detectors, and is a collection of thermocouples connected in series in order to achieve better temperature sensitivity. Although thermopiles are not as sensitive as bolometers and pyroelectric detectors, they will replace these in many applications due to their reliable characteristics and good cost/performance ratio. The internal voltage responsible for current flow in a thermocouple is directly proportional to the temperature difference between the two junctions ΔV = αs ΔT where αs is the Seebeck coefficient commonly expressed in μV/K. Bolometers Another widely used thermal detector is the bolometer. The bolometer is a resistive element constructed from a material with a very small thermal capacity and large temperature coefficient so that the absorbed radiation produces a large change in resistance. In contrast to the thermocouple, the device is operated by passing an accurately controlled bias current through the detector and monitoring the output voltage. The first bolometer designed in 1880 by American astronomer S. P. Langley for solar observations. The change of voltage of a constant current-biased bolometer is ΔV=IΔR Pyroelectric Detectors Whenever a pyroelectric crystal undergoes a change of temperature, surface charge is produced in a particular direction as a result of the change in its spontaneous polarization with temperature. Pyroelectricity has been known for the last twenty-four centuries, but a while ago in 1938 a chemists at the Sorbonne in Paris, proposed that tourmaline crystals could be used as IR sensors in spectroscopy. Some research on pyroelectric detectors was conducted in the next decade in the United Kingdom, United States, and Germany, but the results appeared only in classified documents. In 1962, Cooper presented the first theory of the pyroelectric detector and conducted experiments using barium titanate. The pyroelectric detector is an AC device, unlike other thermal detectors that detect temperature levels rather than temperature changes. This generally limits the low frequency operation, and for a maximum output signal the rate of charge of the input radiation should be comparable to the electrical time constant of the element. The pyroelectric detector can be considered as a small capacitor with two conducting electrodes mounted perpendicularly to the direction of spontaneous polarization, as shown in Figure with its equivalent electrical circuit. To orient the sensitive element before use, the material is heated and an electrical field applied. When the detector is operated, the change in polarization will appear as a charge on the capacitor and a current will be generated, the magnitude of which depends on the temperature rise and the pyroelectrical coefficient of the material. The incident radiation flux hits the radiation-sensitive element with the area AS and absorption coefficient α. The absorption of radiation flux results in a temperature change ΔT(t) within the pyroelectric material. The pyroelectric effect generates charges ΔQ(t) on the electrodes. These charges are transformed into a signal voltage uS´(t). There are three major noise sources in a pyroelectric detector with a shunt resistor: Thermal fluctuation noise, Johnson noise and Amplifier noise. It has been assumed that both the thermal and electrical time constants are longer than one second. In nearly all practical detectors the thermal noise is insignificant and is often ignored in calculations. It can be seen that the loss-controlled Johnson noise dominates above 20 Hz, while below this frequency the resistor-controlled Johnson noise and the amplifier current noise (Va) contribute almost equally significantly to the total noise. At very high frequencies the amplifier voltage noise (Vav) dominates. Relative magnitudes of noise voltages in a typical pyroelectric detector Photon Detectors Spectral detectivity curves for a number of commercially available IR detectors are shown in Figure. Interest has centered mainly on the wavelengths of the two atmospheric windows 3–5 μm [middle wavelength IR (MWIR)] and 8–14 μm (LWIR region; atmospheric transmission is the highest in these bands and the emissivity maximum of the objects at T ≈ 300 K is at the wavelength λ ≈ 10 micron), though in recent years there has been increasing interest in longer wavelengths stimulated by space applications. The spectral character of the background is influenced by the transmission of the atmosphere, which controls the spectral ranges of the infrared for which the detector may be used when operating in the atmosphere. Progress in IR detector technology is connected with semiconductor IR detectors, which are included in the class of photon detectors. In this class of detectors the radiation is absorbed within the material by interaction with electrons either bound to lattice atoms or to impurity atoms or with free electrons. The observed electrical output signal results from the changed electronic energy distribution. The photon detectors show a selective wavelength dependence of response per unit incident radiation power. They exhibit both perfect signal-to-noise performance and a very fast response. But to achieve this, the photon detectors require cryogenic cooling. Photon detectors having long wavelength limits above about 3 μm are generally cooled. This is necessary to prevent the thermal generation of charge carriers. The thermal transitions compete with the optical ones, making noncooled devices very noisy. Photon Detection Process Photon detectors are based on photon absorption in semiconductor materials. A signal whose photon energy is sufficient to generate photocarriers will continuously lose energy as the optical field propagates through the semiconductor. Inside the semiconductor, the field decays exponentially as energy is transferred to the photocarriers. The material can be characterized by an absorption length, α, and a penetration depth, 1/α. Penetration depth is the point at which 1/e of the optical signal power remains. The power absorbed in the semiconductor as a function of position within the material is then The number of photons absorbed is the power (in watts) divided by the photon energy (E = hν). If each absorbed photon generates a photocarrier, the number of photocarriers generated per number of incident photons for a specific semiconductor with reflectivity r is given by where 1 ≤ η ≤ 1 is a definition for the detector’s quantum efficiency. Absorption coefficient for various photodetector materials in spectral range of 1–14 μm. Comparison of the D*(spectral detectivity) of various commercially available infrared detectors when operated at the indicated temperature. Chopping frequency is 1000 Hz for all detectors except the thermopile (10 Hz), thermocouple (10 Hz), thermistor bolometer (10 Hz), Golay cell (10 Hz), and pyroelectric detector (10 Hz). Each detector is assumed to view a hemispherical surrounding at a temperature of 300 K. Theoretical curves for the background-limited D* (dashed lines) for ideal photovoltaic and photoconductive detectors and thermal detectors are also shown. PC, photoconductive detector; PV, photovoltaic detector; PE, photoemissive; and PEM, photoelectromagnetic detector.