CONTEXTUALIZACIÓN

6.4.1 Directional illumination . . . 157

6.4.2 Diffuse illumination . . . 159

6.4.3 Rear illumination. . . 159

6.4.4 Light and dark field illumination. . . 160

6.4.5 Telecentric illumination . . . 160

6.4.6 Pulsed and modulated illumination. . . 161

6.5 References . . . 162

6.1 Introduction

In Chapters2and3the basics of radiation and the interaction of ra-

diation with matter were introduced. How radiation is emitted from active sources and how incident radiation interacts with passive sur- faces of objects in the scene were both demonstrated. However, we did not specify the characteristics of real radiation sources.

137

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In this chapter we turn towards the question: How is the irradi- ance of surfaces generated in practical applications? We will introduce the most important radiation/illumination sources used in computer vision. After a short treatment of natural sources (such as solar and

sky irradiance in Section6.2), we will emphasize artificial sources for

scientific applications and machine vision in Section6.3.

The most important properties of illumination sources that have to be considered for practical applications are:

• spectral characteristics

• intensity distribution

• radiant efficiency (Section2.4.3)

• luminous efficacy (Section2.4.3)

• electrical properties

• temporal characteristics

• package dimensions

We will summarize these characteristics for each illumination source, depending upon applicability.

Single illumination sources alone are not the only way to illuminate a scene. There is a wealth of possibilities to arrange various sources ge- ometrically, and eventually combine them with optical components to form an illumination setup that is suitable for different computer vision

applications. In Section6.4we will show how this can be accomplished

for some sample setups. The importance of appropriate illumination setups cannot be overemphasized. In many cases, features of interest can be made visible by a certain geometrical arrangement or spectral characteristics of the illumination, rather than by trying to use expen- sive computer vision algorithms to solve the same task, sometimes in vain. Good image quality increases the performance and reliability of any computer vision algorithm.

6.2 Natural illumination

For outdoor scenes, natural illumination sources, such as solar irradi- ance and diffuse sky irradiance, play an important role. In some ap- plications, they might be the only illumination sources available. In other cases, they are unwanted sources of errors, as other illumination sources have to compete with them. Solar irradiance, however, is hard to overcome, as it covers the entire spectrum from the ultraviolet to

the far infrared and has an enormous power in the order 103 _Wm−2_,

6.2 Natural illumination 139 spectral irradiance [Wm m] -2 -1 µ wavelength [ m]µ 0 0 0.5 1 1.5 2 200 400 600 800 1000 1200 1400 1 1 2 6 8 0 0 0 0 0 0 0 1 0 2 4

Figure 6.1: Solar irradiance: Comparison of the solar spectrum (solid lines) at

the top of the earth’s atmosphere to a blackbody at a temperature of 6000 K (dashed line). Solar irradiance at sea level measured in multiples of the vertical path through standard atmosphere, denoted asma. The figure shows the ir- radiance forma= 0, 1, 2, and 4. Withma=0, we denote the solar irradiance

right above the earth’s atmosphere, that is, without atmospheric absorption.

6.2.1 Solar radiation

Although solar radiation has the principal shape of blackbody radia-

tion (Fig.6.1), the real origin is nuclear fusion rather than incandes-

cence. Powered from internal nuclear power, the outer regions of the sun, heated up to a temperature of approximately 6000 K, emit thermal radiation. On its way through the colder parts of the solar atmosphere

the radiation is subject to absorption (Section3.4) from gases, which

shows up as narrow absorption lines, known as Fraunhofer lines. These characteristic line spectra allow remote measurements of the presence and concentration of extraterrestrial gases along the optical path.

Within the earth’s atmosphere additional absorption occurs. At sea level parts of the solar emission spectrum are extinguished while oth-

ers remain almost unchanged (Fig. 6.1b). The latter parts are called

atmospheric windows and are of major importance for long distance remote sensing. One example is the visible window, which is of major importance for terrestrial life. Strong absorption regions visible in the

solar spectrum at sea level at about 0.9µm, 1.1µm, 1.4µm, and 1.9µm

(Fig.6.1b), are caused by water vapor (H2O) and carbon dioxide (CO2).

Another major attenuation line of CO2 is located in the IR part of

0.1 0.5 scattering (clear sky) thermal emission T= 300 Ksky 1 λ µ[ m] 5 10 50 0.1 0.0 1.0 E ( ) (relative units) sky λ

Figure 6.2:Schematic illustration of the contributions from scattering and at-

mospheric emission to the diffuse background radiation.

tance for the greenhouse effect, responsible for global warming. The

increasing concentration of CO2 in the atmosphere causes an increas-

ing reabsorption of longwave IR radiation, which is emitted from the earth’s surface, and thus increased heating up of the atmosphere.

The radiation luminous efficacy of solar irradiation can be deter-

mined to be approximately 90-120 lm W−1 _{for the lowest angle of inci-}

dence (midday).

6.2.2 Diffuse sky irradiation

In addition to direct solar irradiation, natural illumination consists of diffuse sky irradiation, commonly referred to as sky-background radia- tion. It is caused by two major contributions: scattering of the sun’s ra-

diation for wavelengths shorter than 3µm; and thermal emission from

the atmosphere for wavelengths beyond 4µm (Fig.6.2).

Depending on the cloud coverage of the sky, different scattering

mechanisms dominate. As already outlined in Section3.4.1, the two

basic mechanisms are Rayleigh scatter, for particles smaller than the wavelength, such as atmospheric molecules, and Mie scatter, for par- ticles with sizes about the wavelength of the radiation, such as micro- scopic water droplets. The solar scattering region dominates for wave-

lengths shorter than 3µm because it is restricted to the region of solar

irradiance. The spectral distribution changes depending on the scat- tering mechanism. For clear sky, Rayleigh scattering dominates, which

has aλ−4 _{wavelength dependence. Thus short wavelengths are more}

efficiently scattered, which is the reason for the blue appearance of the clear sky. For cloud-covered parts of the sky, Mie scatter dominates the solar region. As this type of scattering shows a weaker wavelength de- pendency (which is responsible for the greyish appearance of clouds),

6.3 Artificial illumination sources 141

100 200 400 600

(nm) ( m)µ

800 1000 1200 1400 1600 1800 2000 2.2 2.4 2.6 2.8 36 38 40 42 LASERS

ARC LAMPS (DC AND PULSED) QUARTZ TUNGSTEN HALOGEN LAMPS

INFRARED ELEMENTS

D LAMPS2

Figure 6.3:Usable wavelength regions for commercially available illumination

sources (Courtesy Oriel Corporation, 1994).

the scatter spectrum is more closely approximating the solar spectrum, attenuated by the transmittance of the clouds. Additionally, the solar region of the scatter spectrum is modified by a number of atmospheric absorption bands. These are mainly the bands of water vapor at 0.94,

1.1, 1.4, 1.9, and 2.7µm, and of carbon dioxide at 2.7µm. The effect of

these bands is schematically shown in Fig.6.2.

The thermal region of the sky-background beyond 4µm is repre-

sented by a 300 K blackbody irradiance. Figure6.2 shows the corre-

sponding blackbody curve. In this region, the absorption bands of the atmosphere have an inverted effect. Bands with strong absorption have a strong emission and will approach the blackbody curve appropriate to the temperature of the atmosphere. Conversely, bands with high trans- missivity have correspondingly low emissivity and thus contribute only a small fraction of the blackbody irradiance. This effect is schematically shown in Fig.6.2.

It is important to note, that the exact shape of the sky-background irradiance strongly depends on the elevation angle of the sun, as well as on meteorological parameters, such air humidity, air temperature, and cloud distribution.

6.3 Artificial illumination sources

Although being the basic physical process used in a large variety of illumination and radiation sources, thermal emission of radiation (Sec-

tion2.5) is only one among other possible mechanisms generating radi-

ation. In this section, the most important commercial radiation and illu- mination sources are introduced, together with the underlying physical processes of radiation emission, practical implementation, and specifi- cations.

Commercially available illumination sources cover the entire spec- tral range from the ultraviolet to the mid-infrared region. They are man- ufactured in a variety of package sizes and geometrical arrangements,

spectral irradiance [mWm nm ] -2 -1 0.1 200 400 600 800 wavelength [nm] 6332 50W QTH 6282 50W Hg 6316 STD D2 6251 Std 75W Xe 1 10 6263 75W Xe ozone free

Figure 6.4: Overview of spectral irradiance curves for arc, quartz tungsten

halogen, and deuterium (D2) lamps at a distance of 0.5 m (Courtesy Oriel Cor- poration, 1994).

optimized for specified applications. Figure6.3shows an overview of

available illumination sources for different spectral regions. In the fol- lowing sections we will focus on the following illumination sources:

• incandescent lamps

• (arc) discharge lamps

• fluorescent lamps

• infrared emitters

• light emitting diodes (LED)

• laser

A more detailed overview can be found in [1], [2], and in catalogs of

manufacturers, such as the one from the Oriel Corporation [3].

6.3.1 Incandescent lamps

Incandescent lamps are among the most popular all-purpose illumina- tion sources. The most prominent examples are standard light bulbs used in almost every household. The classic light bulb uses a carbon filament, which is placed in an evacuated glass enclosure in order to avoid oxidation (burning) of the carbon filament.

More modern versions of incandescent lamps use tungsten filaments instead of carbon fibers. The practical setup of tungsten incandescent lamps are tungsten filaments of various shapes (rectangular dense and

coiled filaments) in quartz glass envelopes (Fig. 6.5). The coiled fil-

6.3 Artificial illumination sources 143 a filament b normal to filament highest irradiance

Figure 6.5: Quartz tungsten halogen incandescent lamps: asetup of a coiled

filament lamp;bsetup of a rectangular filament lamp (Courtesy Oriel Corpo- ration, 1994).

long axis of symmetry of the housing. For the rectangular filaments, the

light output strongly depends on the direction (Fig.6.5b). The quartz

glass housing is transparent only for wavelengths up to 3µm. It does,

however, heat up by absorption of long-wave radiation and thermally emits infrared radiation corresponding to the glass temperature ex- tending the spectrum into the mid-infrared region.

Incandescent lamps have a high visible and near infrared output.

With an emissivity of tungsten of about ˜ε = 0.4 (in the visible), the

spectral exitance of tungsten incandescent lamps is close to the exi- tance of a graybody. It does, however, deviate for wavelengths of about

the peak wavelength and above. Figure6.6shows the spectral exitance

of an incandescent tungsten surface, compared to a graybody with an

emissivity of ˜ε=0.425 at a color temperature of 3100 K.

The radiant efficiency of incandescent lamps is in the order of 80 %, as incandescence very efficiently converts electrical input power into radiant output. The output within the visible region, however, is much lower. Operated at a color temperature of approximately 3000 K, tung- sten incandescent lamps have a relatively low radiation luminous effi- cacy ofKr =21.2 lm W−1_{, as the main part of the spectrum lies in the}

infrared (Section2.5.4). The lighting system luminous efficacy is only

Ks =17.4 lm W−1_{. The values are taken for an individual tungsten in-}

candescent light bulb [4] and are subject to fluctuations for individual

realizations.

Two important modifications allow both radiant efficiency and the lamp life to be increased:

1. In all tungsten filament lamps, the tungsten evaporates from the fil- ament and is deposited on the inside of the envelope. This blackens

0 0.5 1 1.5 2 2.5 3 0 1 2 3 0.5 1.0× × 10 10 6 6 1.5×106 2.0×106 2.5×106 3.0×106 3.5×106 sp ectral exit a nce [Wm m] -2 -1 µ wavelength [ m]µ

Figure 6.6:Spectral exitance of (1) a blackbody; (2) a graybody with emissivity

of= 0.425; and (3) a tungsten surface, all at a temperature of 3100 K (Courtesy Oriel Corporation, 1994).

the bulb wall and thins the tungsten filament, gradually reducing the light output. With tungsten halogen lamps, a halogen gas is filled into the envelope. The halogen gas efficiently removes the de- posited tungsten and returns it to the filament, leaving the inside of the envelope clean, and providing long-term stability. This thermo-

chemical process is called the halogen cycle [3].

2. Some manufacturers produce new-generation halogen lamps with infrared coatings on the envelope. These coatings are made such that infrared radiation is reflected back onto the tungsten filament. Thus, the temperature of the envelope and the infrared output of the lamp are reduced, which increases luminous efficacy. At the same time, the filament is heated by the emitted infrared radiation, which yields a higher radiant efficiency, as less current is needed to maintain the operating temperature. Both effects increase the lighting system luminous efficacy.

As the exitance of an incandescent lamp is given by the tempera- ture, which does not immediately follow changes in the voltage, the light output does not follow rapid (kHz) voltage changes. It does, how- ever, follow slow voltage changes, such as the net frequency under ac operation, with an amplitude in the order of 10 % of the absolute exi-

tance [3]. This effect might cause beating effects, if the frame rate of

the video camera is at a similar frequency. For demanding radiomet- ric applications it is recommended to use regulated dc power supplies. The smaller the filament, the lower the thermal mass and the faster the response of the lamp.

6.3 Artificial illumination sources 145

6.3.2 Discharge lamps

Discharge lamps operate on the physical principle of gas discharge. At low temperatures, such as ambient temperature and below, gases are nonconducting. The gas molecules are neutral and can not carry elec- trical current. In a statistical average, a small number of molecules is ionized due to natural radioactivity. These ions, however, have very short lifetimes and immediately recombine. In gas discharge lamps, a strong electric field is generated in between two electrodes, separated

by distanced. Within this field, randomly generated gas ions are ac-

celerated towards the electrodes of opposite charge. Upon impact on the cathode, the positively charged gas ions release electrons, which in turn are accelerated towards the anode. These electrons eventually hit other atoms, which can be excited and recombine under emission of light, corresponding to the difference between two energy levels.

Spectral lamps. Spectral lamps are plain gas discharge lamps without additional fluorescence coatings, as opposed to fluorescence lamps. As the energy levels of the light emission in gas discharge are character- istic for the gas molecules, gas discharge lamps emit the characteris- tic line spectra of the corresponding fill gas. A prominent example is the low-pressure sodium vapor lamp used for street illuminations. The bright yellow light corresponds to the Na-D line at a wavelength of 590 nm. Because the spectral exitance consists of a single line in the visible spectrum, the sodium vapor lamp has an extremely high radi- ant luminous efficacy of 524.6 lm W−1 _{(Osram GmbH). Accounting for}

the electrical power consumption yields a net lighting system luminous efficacy of 197 lm W−1_.

In order to increase the luminous efficacy, the gas pressure within the lamp can be increased by allowing the bulb to heat up. As a conse- quence, the spectral lines of the exitance are widened. In extreme cases the spectral distribution shows a continuum without spectral lines.

Other examples of fill gases of discharge lamps are xenon (Xe), mer- cury (Hg), and mixtures of Xe and Hg. The spectral exitance of these gas discharge lamps is similar to that of arc lamps with the same fill gases (shown in Fig.6.4).

Fluorescent lamps. The spectral output of gas discharge lamps, such as Xe or Hg lamps, shows a high contribution from the ultraviolet region well below 400 nm. Radiation at these wavelengths is invisible, causes severe sunburn, and damages the tissue of the eye’s retina.

Fluorescent lamps are discharge lamps (usually filled with Hg) that are additionally coated with special fluorescent materials. These lay- ers absorb ultraviolet radiation and convert it into longer wavelength radiation in the visible region, which is finally emitted. The exact spec-

tral content of the emitted radiation can be varied depending upon the compounds of the fluorescence layer. Examples are lamps with a high content of red light at 670 nm, which is photosynthetically active and can be used as an illumination source for greenhouses.

As the wavelength of light is shifted from about 250 nm towards 500 nm, the energy of the re-emitted radiation is only half the energy of the incident radiation. The remaining energy is absorbed within the fluorescence material. This energy constitutes the main energy loss in fluorescence lamps. Thus, the lighting system luminous effi- cacy is relatively high, compared to incandescent lamps. Typical val-

ues of the luminous efficacies are in the order ofKs=71 lmW−1_{, and}

Kr =120 lmW−1_{. The radiant efficiency lies in the order of ˜η=50 %.}

These high values are due to the fact that almost no heat is generated and the major part of the spectrum is emitted in the visible region. Fluorescent lamps are the perfect choice for low-energy room illumina- tion.

For many years tube-shaped fluorescent lamps have been used in both homes and public buildings. Modern developments in lamp man- ufacturing have led to a huge variety of shapes and color temperatures of fluorescent lamps. They have most recently been advertised as low- energy substitutes for incandescent light bulbs. In order to reduce the size of the lamp and to overcome the elongated shape, narrow tubes are coiled to light bulb-sized compact illumination sources.

All gas discharge lamps are subject to fast brightness fluctuations when operated with ac power supplies. If stable illumination over time is required, these lamps have to be operated with special high frequency power supplies.

6.3.3 Arc lamps

For high currents, the electrodes of discharge lamps get extremely hot. At a certain temperature, the emission of electrons from the cathode is due mainly to incandescence of the electrode material, and the gas discharge is turned into an arc discharge. This effect can be facilitated by a cone shaped cathode, which focuses the electric field.

Xenon and mercury arc lamps. Figure6.7a shows a diagram and the technical setup of commercial arc lamps. The anode and cathode are made of tungsten and sealed in clear quartz glass. The tungsten is doped with materials, such as thoria, to enhance electron emission. When the lamps run, the internal pressure increases to 15-75 bar, de- pending on the lamp type.

Arc lamps constitute the brightest manufactured broadband sources. The major light output is restricted to the arc, which can be made small depending on electrode geometry. The small radiating area makes

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