The amount of energy falling on the Earth's surface from the sun is approximately 5.6 billion billion (quintillion) megajoules per year. Averaged over the entire Earth's surface, this translates into about 5 kilowatt-hours per square meter every day. The energy input from the sun in a single day could supply the needs for all of the Earth's inhabitants for a period of about 3 decades. Obviously, there is no means conceivable (nor is it necessary) to harness all of the energy that is available; equally obvious is that capturing even a small fraction of the available energy in a useable form would be of enormous value.

Introduction - Only in the last few decades has mankind begun to search in earnest for mechanisms to harness the tremendous potential of solar energy. This intense concern has resulted from a continuing increase in energy consumption, growing environmental problems from the fuels that are now consumed, and an ever-present awareness about the inevitable depletion of fossil fuel resources upon which we have become so heavily dependent. Among the topics discussed in this section are photosynthesis, the photoelectric effect, solar cells, charge-coupled devices, fuel cells, and nuclear fusion.

Electronic Imaging Detectors - The range of light detection methods and the wide variety of imaging devices currently available to the microscopist make the selection process difficult and often confusing. This discussion is intended to aid in understanding the basics of light detection and to provide a guide for selecting a suitable electronic detector (CCD or video camera system) for specific applications in optical microscopy.

The MOS Capacitor - At the heart of all charge-coupled devices (CCDs) is a light-sensitive metal oxide semiconductor (MOS) capacitor, which has three components consisting of a metal electrode (or gate), an insulating film of silicon dioxide, and a silicon substrate.

Introduction to CMOS Image Sensors - CMOS image sensors are designed with the ability to integrate a number of processing and control functions, which lie beyond the primary task of photon collection, directly onto the sensor integrated circuit. These features generally include timing logic, exposure control, analog-to-digital conversion, shuttering, white balance, gain adjustment, and initial image processing algorithms. Inexpensive CMOS image sensors are entering the field of optical microscopy in educational instruments that combine acceptable optical quality with user-friendly control and imaging software packages.

Photomultiplier Tubes - A photomultiplier tube, useful for light detection of very weak signals, is a photoemissive device in which the absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux.

Photosynthesis - Green plants absorb water and carbon dioxide from the environment, and utilizing energy from the sun, turn these simple substances into glucose and oxygen. With glucose as a basic building block, plants synthesize a number of complex carbon-based biochemicals used to grow and sustain life. This process is termed photosynthesis, and is the cornerstone of life on Earth. The tutorial demonstrates the basic molecular steps in the photosynthetic process.

Solar Cell Operation - Solar cells convert light energy into electrical energy either indirectly by first converting it into heat, or through a direct process known as the photovoltaic effect. The most common types of solar cells are based on the photovoltaic effect, which occurs when light falling on a two-layer semiconductor material produces a potential difference, or voltage, between the two layers. The voltage produced in the cell is capable of driving a current through an external electrical circuit that can be utilized to power electrical devices. This tutorial explores the basic concepts behind solar cell operation.

Incandescent Lamp Filaments - Nearly every source of light depends, at the fundamental level, on the release of energy from atoms that have been excited in some manner. Standard incandescent lamps, derived directly from the early models of the 1800s, now commonly utilize a tungsten filament in an inert gas atmosphere, and produce light through the resistive effect that occurs when the filament temperature increases as electrical current is passed through. This interactive tutorial demonstrates the sub-atomic activity within a conducting incandescent lamp filament that results in resistance to current flow, and ultimately leads to the emission of infrared and visible light photons (with a corresponding rise in color temperature).

Hydrogen Fuel Cell Basics - Fuel cells are designed to utilize a catalyst, such as platinum, to convert a mixture of hydrogen and oxygen into water. An important byproduct of this chemical reaction is the electricity generated when hydrogen molecules interact (through oxidation) with the anode to produce protons and electrons. This interactive tutorial explores the major steps in fuel cell operation.

Interaction of Photons with Silicon - In a charge-coupled device (CCD) incident light must first pass through a silicon nitride passivation coating as well as several thin films of silicon dioxide and polysilicon gate structures before being absorbed into the silicon substrate. This interactive tutorial explores the interaction of photons with silicon as a function of wavelength.

Building A Charge-Coupled Device - Explore the steps utilized in the construction of a charge-coupled device (CCD) as a portion of an individual pixel gate is fabricated on a silicon wafer simultaneously with thousands or even millions of neighboring elements. The interactive tutorial examines and illustrates each individual stage in the fabrication of the CCD photodiode sensor element.

Full-Frame CCD Operation - Full-frame charge-coupled devices (CCDs) feature high-density pixel arrays capable of producing digital images with the highest resolution currently available. This popular CCD architecture has been widely adopted due to the simple design, reliability, and ease of fabrication.

Photomultiplier Tube Operation - In the end-on photomultiplier tube design, photons impact an internal photocathode and transfer their energy to electrons, which then proceed through a chain of electron multipliers termed dynodes ending in the anode. This tutorial explores how electrons are generated by the photocathode and amplified by passing through a dynode chain.

Reference Listing - Gathered from our vast library of literature on optical microscopy, the listed reference materials are an excellent source of additional information on the topic of light and energy and their interrelationships. Included in this section are references to books, select book chapters, and review articles that discuss various aspects of the theory and applications regarding how light is converted into energy and vice versa.

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