可变强度的可见单色光设计探究
导读：这篇文章目的是设计可变强度的可见单色光，这是一种光学装置。其主要思想是使用PIC单片机多色LED来构建系统，将包括八个多色的LED，可以使用两种不同的模式。本文由英国论文网代写留学生作业频道Engineering Essay栏目整理提供。
Abstract
This project is about designing a Variable Intensity Visible Light Monochromator which is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation.

The main idea is to use a PIC microcontroller with multicolor LEDs to build the system which will consist of eight multicolor LEDs that can be used in two different modes,

1.All LEDs operate together in 100 Hz frequency.
2.All LEDs will operate in sequence with 100 Hz frequency.
Also the user will have the ability to change the intensity and the color of the LEDs using external potentiometers and there will be an LCD to display the options that the system has.

Also the user will have the ability to control the system using the computer by a simple graphical user interface program using the MatLab program and the RS232 as the communication protocol.

CHAPTER ONE: INTRODUCTION
Technologies using monochromatic light have a wide range of application, from astrophysics and astronomy to forensic science. The term monochromatic derives from the Greek words monos, meaning one or sole, and chromos, meaning color. Monochromatic light, or one-color light, is essentially electromagnetic radiation derived from photon emissions from atoms. Photons propagate, or travel, as energy wave fronts of different lengths and levels of energy. Energy levels determine the frequency of light, and the length of a wave determines its color. The bands of light wavelengths that humans can see are called visible light.

Nature of Electromagnetic Waves
Light is an electromagnetic wave. Electromagnetic waves (EM Waves) are produced by charged particles when they vibrate. As the charged particles execute SHM, a sinusoidal electric field and a sinusoidal magnetic field are simultaneously produced. These two fields are mutually perpendicular to each other and constitute an electromagnetic wave. An e.m wave is able to propagate through vacuum without the presence of any medium. The figure below shows an electromagnetic wave.

EM waves exhibit the following properties:

1.They consist of two sinusoidal fields the Electric-field and Magnetic-field, which are oscillating in phase and at right angles to each other.
2.They are transverse waves.
3.All electromagnetic waves can travel through vacuum (or free space).
4.In vacuum, they travel with the same speed c = 3.00 x 108 ms-1.#p#分页标题#e#
5.All EM waves exhibit properties such as reflection, refraction, interference, diffraction and polarization.
Although all electromagnetic waves travel through vacuum with exactly the same speed c, they have a wide range of frequencies (or wavelengths). Their properties vary distinctly with frequencies. Based on their frequencies or wavelengths, they are given different names. The following figure shows the full spectrum of electromagnetic waves. Radio waves have the longest wavelengths and gamma waves have the shortest wavelengths. Note that visible light is in the wavelength range 4.0 x 10-7 7.0 x 10-7 m or 400 nm 700 nm (violet to red region).

Nature of Electromagnetic Waves
Ultraviolet (UV) light has shorter wavelengths than visible light. Though these waves are invisible to the human eye, some insects, like bumblebees, can see them.

The near ultraviolet, abbreviated NUV is the light closest to optical or visible light. The extreme ultraviolet, abbreviated EUV, is the ultraviolet light closest to X-rays, and is the most energetic of the three types. The far ultraviolet, abbreviated FUV, lies between the near and extreme ultraviolet regions. It is the least explored of the three regions.

Though some ultraviolet waves from the Sun penetrate Earth's atmosphere, most of them are blocked from entering by various gases like Ozone. Some days, more ultraviolet waves get through our atmosphere. Scientists have developed a UV index to help people protect themselves from these harmful ultraviolet waves.

Optical Phenomena
Common optical phenomena are often due to the interaction of light from the sun or moon with the atmosphere, clouds, water, or dust and other particulates. One common example would be the rainbow, when light from the sun is reflected and refracted by water droplets. Some, such as the green ray, are so rare they are sometimes thought to be mythical. Others, such as Fata Morganas, are commonplace in favored locations.

Optical phenomena include those arising from the optical properties of the atmosphere ; of the rest of nature (Other phenonema); of objects, whether natural or human-made (Optical effects); and of our eyes (Entoptic phenomena).

There are many phenomena which result from either the particle or the wave nature of light. Some are quite subtle and observable only by precise measurement using scientific instruments. One famous observation was of the bending of light from a star by the Sun during a solar eclipse. This demonstrated that space is curved. Theory of relativity

Dispersion (optics)（英国论文网http://www.ukthesis.org/）
In optics

, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency. Media having such a property are termed dispersive media. Dispersion is sometimes called chromatic dispersion to emphasize its wavelength-dependent nature, or group-velocity dispersion (GVD) to emphasize the role of the group velocity.#p#分页标题#e#

The most familiar example of dispersion is probably a rainbow , in which dispersion causes the spatial separation of a white light into components of different wavelengths(different color ColorColor or colour is the visual perceptual property corresponding in humans to the categories called red, yellow, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...s). However, dispersion also has an effect in many other circumstances: for example, GVD causes pulses to spread in optical fiber s, degrading signals over long distances; also, a cancellation between group-velocity dispersion and nonlineareffects leads to soliton waves. Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g. a waveguide ), such as sound waves.

There are generally two sources of dispersion: material dispersion and waveguide dispersion. Material dispersion comes from a frequency-dependent response of a material to waves. For example, material dispersion leads to undesired chromatic aberrationin a lens or the separation of colors in a prism . Waveguide dispersion occurs when the speed of a wave in a waveguide (such as an optical fiber) depends on its frequency for geometric reasons, independent of any frequency dependence of the materials from which it is constructed. More generally, "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure (e.g. a photonic crystal ), whether or not the waves are confined to some region. In general, both types of dispersion may be present, although they are not strictly additive. Their combination leads to signal degradation in optical fiber s for telecommunication s, because the varying delay in arrival time between different components of a signal "smears out" the signal in time.

Material Dispersion in Optics
Material dispersion can be a desirable or undesirable effect in optical applications. The dispersion of light by glass prisms is used to construct spectrometer and spectroradiometers. Holographicgratings are also used, as they allow more accurate discrimination of wavelengths. However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade images in microscopes, telescopes and photographic objectives. The phase velocity (v), of a wave in a given uniform medium is given by where (c) is the speed of light in a vacuum and n is the refractive index of the medium. In general, the refractive index is some function of the frequency f of the light, thus n = n(f), or alternately, with respect to the wave's wavelength n = n(?). The wavelength dependence of a material's refractive index is usually quantified by an empirical formula, the Cauchy or Sellmeier equations.

Optics can be the subject of the impact of dispersion desirable or undesirable in the applications of light. It is used for the dispersion of light from glass building workshop spectrometer and spectrometers. Also it used holographic gratings, as it allows more accurate discrimination of the waves. However, in lenses, dispersion causes chromatic aberration, an effect is undesirable that may degrade images in microscopes and telescopes and objectives of photography.#p#分页标题#e#

The phase speed (v), wave in the middle of a particular standardized by where (c) is the speed of light in a vacuum and (n) is the refractive index of the average. In general, the refractive index is some function of the number and frequency of light, and thus n = n (f), or alternately, with respect to n a wave of wavelength n = (?). Quantitative index is usually rely on the wavelength of the material and refraction of the trial version, or equations Kochi

Because of the Kramers Kronig relations, the wavelength dependence of the real part of the refractive index is related to the material absorption, described by the imaginary part of the refractive index (also called the extinction coefficient). In particular, for non-magnetic materials , the susceptibility that appears in the Kramers Kronig relations is the electric susceptibility. The most commonly seen consequence of dispersion in optics is the separation of white light into a color spectrumby a prism. From Snell's law Snell's lawit can be seen that the angle of refraction of light in a prism depends on the refractive index of the prism material. Since that refractive index varies with wavelength, it follows that the angle that the light is refracted by will also vary with wavelength, causing an angular separation of the colors known as angular dispersion.

For visible light, most transparent materials (e.g. glasses) have: or alternatively: that is, refractive index n decreases with increasing wavelength ?. In this case, the medium is said to have normal dispersion. Whereas if the index increases with increasing wavelength the medium has anomalous dispersion. At the interface of such a material with air or vacuum (index of ~1), Snell's law predicts that light incident at an angle ? to the normal will be refracted at an angle arcsin(sin(?)/n). Thus, blue light, with a higher refractive index, will be bent more strongly than red light, resulting in the well-known rainbow pattern.

Optical Spectrometer
The optical spectrometer is a precision instrument capable of an accuracy of measurement far greater than that found in most other areas of physical measurement. The function of the spectrometer is to disperse light into its various component wavelengths (or frequencies) and to determine the wavelength (or frequency) of each resolved component. The dispersive element is usually a diffraction grating but it could also be a glass prism.

The collimator renders a parallel beam of light through the two coaxial cylindrical tubes. One end of the collimator has a slit through which light enters the tube and falls on lens L situated at the other end. Prism table is a circular plate fixed over a vertical stand of adjustable height. The free end of stand consists of a circular scale graduated in degrees from 0o to 360o along with verniers to enable to read the position of the prism. Telescope is meant for observing the spectrum and is mounted horizontally on a vertical stand attached to the circular scale. The telescope can be rotated about the prism table.#p#分页标题#e#

The telescope is turned towards a distant object and is focused to see a clear image of object. It is then brought in line with the collimator. A clear image of the slit is obtained by adjusting the screws in the collimator. The prism is kept over the prism table.

Prism
A prism is a portion of a transparent medium bounded by two plane faces inclined to each other at a suitable angle.

Angle A between the two refracting surfaces ABQP and APRC is called the angle of prism. A ray of light suffers two refractions on passing through a prism.If KL be a monochromatic light falling on the side AB, it gets refracted and travels along LM. It once again suffers refraction at M and emerges out along MN. The angle through which the emergent ray deviates from the direction of incident ray is called angle of deviationd

The prism is placed over the table such that parallel rays from collimator falls on the sides AB and AC. Move the telescope in the position T1 to catch the brightest image of the slit formed by reflection of light at faces AB and AC. The cross wire is made to coincide with image and reading on the circular scale is noted. The telescope is turned to position T2 and the same procedure is repeated. If q is the difference between the two readings through which the telescope is turned then

Fluor meter
An instrument used to measure the intensity and the wavelength distribution of the light emitted as fluorescence from a molecule excited at a specific wavelength or wavelengths within the absorption band of a particular compound. Characteristic fluorescence bands may be used to identify specific pollutants such as the poly nuclear aromatic hydrocarbons. Excitation spectra of impurities can be observed by scanning the wavelength of the excitation light which is incident on the sample over a range of wavelengths and observing the relative intensity of the fluorescence emitted at a given wavelength. These spectra are also characteristic of the impurity.

Czerny-Turner Configuration
The Czerny-Turner (CZ) monochromator consists of two concave mirrors and one plano diffraction grating (see Figure 9). Although the two mirrors function in the same separate capacities as the single spherical mirror of the Fastie-Ebert configuration, i.e., first collimating the light source (mirror 1), and second, focusing the dispersed light from the grating (mirror 2), the geometry of the mirrors in the Czerny-Turner configuration is flexible. By using an asymmetrical geometry, a Czerny-Turner configuration may be designed to produce a flattened spectral field and good coma correction at one wavelength. Spherical aberration and astigmatism will remain at all wavelengths. It is also possible to design a system that may accommodate very large optics.

Monochromators
Monochromators are optical subassemblies used to isolate narrow portions of a light spectrum. They accept polychromatic input from a lamp or laser, and outputs monochromatic light. With monochromators, polychromatic light enters via a fixed port, such as a slit or optical fiber.Inside the monochromator, a dispersive element, grating, crystal prism, or mirror diffracts the light into its spectrum.If the monochromator can be rotated, the dispersive element is manual or motor-driven.The angle at which the element is rotated determines the wavelength of the output monochromatic light output, as well as the specific color of the light.#p#分页标题#e#

When selecting monochromators for an application, it is important to consider certain parameters and how they will affect the desired output. These parameters include bandpass, dispersion, resolution, acceptance angle, and blaze wavelength. Bandpass is the wavelength range in which the monochromator transmits. Dispersion, resolution, acceptance angle, and blaze wavelength are important parameters to consider when choosing monochromators. Dispersion in monochromators is the wavelength dispersing power, which is usually expressed as spectral range or slit width (nm/mm). Dispersion depends on the focal length, grating resolving power, and the grating order. Resolution is the minimum bandpass of the spectrometer, which is usually determined by aberrations in the optical system. With monochromators, the acceptance angle (f/#) is a measurement of the light-collecting ability and focal length / mirror diameter. Blaze wavelength is the wavelength of maximum intensity in the first order of monochromators.

Additional features to consider when choosing monochromators are nitrogen-purging abilities, vacuum capabilities, and fiber optic ready connection.Monochromators with a nitrogen purge feature have a port for nitrogen purging. Nitrogen purging is important because it allows monochromators to operate more deeply into ultraviolet (UV) light.Similarly, monochromators that operate with internal vacuum conditions are also able to extend their wavelength further into the ultraviolet range.Monochromators that have a fiber optic ready connection can be coupled with waveguides for the easier delivery of light output and data retrieval.

A range of accessories are available for use with monochromators, including cooled photo multiplier detectors, silicon detectors, light guides, arc lamp sources, and integrating sphere. Motorized drive monochromators are also available which can include electronic drivers to interface with software for automation.

Monochromator System Optics（英国论文网http://www.ukthesis.org/）
To understand how a complete monochromator system is characterized, it is necessary to start at the transfer optics that brings light from the source to illuminate the entrance slit (see Figure 10). Here we have "unrolled" the system and drawn it in a linear fashion.

Aperture Ratio (f/value, f/Number), and Numerical Aperture (NA)
The light gathering power of an optic is rigorously characterized by Numerical Aperture (NA). Numerical Aperture is expressed by:

Aperture Ratio (f/value, f/Number), and Numerical Aperture (NA)
In any spectrometer system a light source should be imaged onto an entrance slit (aperture) which is then imaged onto the exit slit and so on to the detector, sample, etc. This process inevitably results in the magnification or demagnification of one or more of the images of the light source. Magnification may be determined by the following expansions, taking as an example the source imaged by lens L1 in Figure 10 onto the entrance slit:#p#分页标题#e#

Similarly, flux density is determined by the area that the photons in an image occupy, so changes in magnification are important if a flux density sensitive detector or sample are present. Changes in the flux density in an image may be characterized by the ratio of the area of the object, S, to the area of the image, S', from which the following expressions may be derived:

These relationships show that the area occupied by an image is determined by the ratio of the square of the f/values. Consequently, it is the EXIT f/value that determines the flux density in the image of an object. Those using photographic film as a detector will recognize these relationships in determining the exposure time necessary to obtain a certain signal-to-noise ratio

Slit Height Magnification
Slit height magnification is directly proportional to the ratio of the entrance and exit arm lengths and remains constant with wavelength (exclusive of the effects of aberrations that may be present).

CHAPTER TO: ELECTRONIC PARTS
This chapter contains the main electronic parts that will be used in design of monochromator and the features of each one, the design procedures are illustrated in details in chapter three.

PIC microcontroller:
This powerful (200 nanosecond instruction execution), (only 35 single word instructions) CMOS FLASH-based 8-bit microcontroller packs Microchip's powerful PIC architecture into a 40-pin package.

The PIC16F877A features 256 bytes of EEPROM data memory, self-programming, an ICD, 2 Comparators, 8 channels of 10-bit Analog-to-Digital (A/D) converter, 2 capture/compare/PWM functions, the synchronous serial port can be configured as either 3-wire Serial Peripheral Interface (SPI) or the 2-wire Inter-Integrated Circuit (IC) bus and a Universal Asynchronous Receiver Transmitter (USART). All of these features make it ideal for more advanced level A/D applications in automotive, industrial, appliances and consumer applications.

PIC16F877A features:
1.PWM 10-bit
2.256 Bytes EEPROM data memory
3.ICD
4.25mA sink/source per I/O
5.Self Programming
6.Parallel Slave Port
I have used the PIC 16F877A microcontroller from microchip because it has all the modules that I wanted in my project.

Multicolor LED:
The multicolor LED is a LED that contains three LEDs with RGB colors and with these LEDs you can have all the colors you need by Applying different voltages on these LEDs.

The Features:

1.256 color capability with red, green and blue chips
2.High intensity
3.Water clear lense
4.Reliable, rugged, long life
5.Low power requirement
6.VF (forward voltage): Red = 2V, Green = 3.5V, Blue = 3.5V
7.Luminous Intensity: 800 to 4000 mcd at 20mA
7805 Regulator:
The regulator is a devise that is used to keep the voltage at certain level (in this case 5 V) so that the other devices may operate in the best way they can.#p#分页标题#e#

The Features:
1.Complete specifications at 1A load
2.Output voltage tolerances of 2% at Tj= 25C and 4% over the temperature range (LM340A)
3.Line regulation of 0.01% of VOUT/V of ?VIN at 1A load (LM340A)
4.Load regulation of 0.3% of VOUT/A (LM340A)
5.Internal thermal overload protection
6.Internal short-circuit current limit
7.Output transistor safe area protection
8.P+ Product Enhancement tested
2N1711 Transistor
The 2N1711 BJT transistor is used as a switch to handle the high currents that the multicolor LED require and which the PIC can't supply.

The Features:
1.Emitter
2.Bases
3.Collector, connected to case
74HC08 AND Gate:
The 74HC/HCT08 are high-speed Si-gate CMOS devices and are pin compatible with low power Schottky TTL (LSTTL). They are specified in compliance with JEDEC standard no. 7A. The 74HC/HCT08 provide the 2-input AND function.

The Features:
1.Complies with JEDEC standard no. 8-1A
2.ESD protection:
3.HBM EIA/JESD22-A114-A exceeds 2000 V
4.MM EIA/JESD22-A115-A exceeds 200 V.
5.Specified from -40 to +85 C and -40 to +125 C.
Potentiometer:
Potentiometer is a variable resistor device that I used to change the voltage on the multicolor LED so I can have different color

IC MAX 232
The MAX232 is an IC that is used to communicate through the RS232 protocol between PC and PIC microcontroller.

CHAPTER THREE: PROPLEM ANALYSIS
The aim of the project is to develop a monochromator for use within an optical process tomography system. The monochromator will be an electronic device that will be use super bright multicolor visible light LEDs to produce differing single wave length of visible light on its output that will be fed into a light array and subsequently fired through transparent sections of pipeline. The single wavelength visible light that is produced must be able to be applied constantly or in a pulsed fashion and the intensity of the light is user adjustable. Also the instrument is stand alone and can be computer controlled.

The design are built with a visible light emission unit, pulsation mechanism, electronic switching, electronic switching mechanisms, electronic circuit to control light intensity, and both electromechanical and computer controlled user interface

PIC and LCD Programming
The PIC and LCD were programmed using C++ code then using CCS Compiler because it is a professional program and it gives the user a lot of possibilities, capabilities and libraries to use.

REFERENCES
1.Chi, Chang H., Ed., 1980, "Periodic Structures, Gratings, Moire Patterns and Diffraction Phenomena".
2.Goldstein, S. A. and Walters, J. P., 1976, "A Review of Considerations for High Fidelity Imaging of Laboratory Spectroscopic Sources Parts 1 and 2", Spectrochimica ACTA, 31B, 201316.
3.James, J. F. and R. S. Sternburg,1969, The Design of Optical Spectrometers Chapman & Hall Ltd., London, England.#p#分页标题#e#
4.Godfrey Onwubolu,2005 Mechatronics: Principles and Applications", Butterworth-Heinemann .
5.Clarence W. de Silva, (October 17, 2007), " Mechatronic Systems: Devices, Design, Control, Operation and Monitoring Series) " ,CRC
6.Robert H. Bishop, (November 19, 2007) , " Mechatronic System Control, Logic, and Data Acquisition (The Mechatronics Handbook, Second Edition) ",CRC
7.Nikolay Avgoustiv,2007 "Modelling in Mechanical Engineering and Mechatronics: Towards Automous Intelligent Software Models, Springer
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