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Preliminary Project 2 Proposal

By Rebecca Hunt,2014-01-20 02:57
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Preliminary Project 2 Proposal

    Preliminary Project 2 Proposal

    Spectrophotometer

    December 6, 2002

    Deryck Hong

    Suryadi Gunawan

? Project Description:

    The objective of this project is to design a spectrophotometer, which will measure

    the color spectrum from a sample object at any wavelength. The object will be a piece of

    single colored paper. The spectrophotometer will determine the color of the paper, and

    display the color on a computer screen.

    The block diagram 1 shows the basic setup of the system. The halogen light will be shined on the colored paper. The colored paper will reflect the light. By using the

    lens, the reflected light can be aligned into a diffraction grating. Diffraction grating is

    typically small about 8 to 10 cm wide. It consists of a reflecting surface on which many

    thousands of narrow parallel groves have been made close together. A beam of light

    directed at such a surface is diffracted in all directions at each groove. The light waves

    reinforce each other in certain directions and cancel out in other directions. The direction

    of diffraction is different for each wavelength. By letting the reflected light shine on the

    diffraction grating, the colors of the light are separated (color spectrum).

    After the colors of the light are separated, the next step is to measure how many components exist in each wavelength, which is associated with color. In order to do that,

    the color spectrum must be captured and converted into binary data. A Charged Coupled

    Device (CCD) is used to capture the color spectrum. CCD will convert the light source

    into voltage which in turn will be converted into binary data by an analog-to-digital

    converter. The digital data will then be converted to RGB values. By pressing the “Get

    Color” button on the computer screen (Fig 4), the color and its RGB values will be

    displayed on the screen.

    Diagrams?

    CCD Sensor

    Source Lens to focus the Diffraction grating light reflection

    Analog to digital converter

    to convert voltage to binary sequences

    Core processing

    Result shown in the screen (RGB value and color sample)

    Block diagram 1: basic setup of the system

Block diagram 1 shows the basic setup of the system. The CCD sensor is mounted onto

    the circuit board (Fig 1). The core processing will be carried out on any window-based

    platform, and C programming language will be used. The results can be seen on the

    screen.

Fig 1: 2048 pixels CCD line scan camera circuit board with EPP interface

Fig 2: Block diagram for the CCD line scan camera circuit board.

Fig 3: Example of diffraction gratings

     User Interface:

    RGB values display

    Output Color displayscreenGetColor

    Workstation

     Fig 4: The screen will display the outputs

    After the colored paper is placed in front of the first lens, when the “Get Color”

    button is pushed on the screen, the color of the source object will be displayed on the

    screen.

? Analytical Component:

     As shown in Block diagram 1, the source object, two lenses, diffraction

    grating, and CCD will be mounted on a piece of metal bar. After shining the halogen

    light on the colored paper, the light travels through the lens (Fig 5) and shines on the

    diffraction grating.

    Fig 5: Example of the refraction of the lens

With the lens equation to be:

     1/ object distance + 1/ image distance = 1 / focal length

     The lenses that will be used for this project will have focal length of 4 cm. In order to

    produce a real image, the object distance has to be greater than the focal length. The

    following table shows a list of object distance with its corresponding image distance with

    the focal length of 4 cm.

    Object distance Image distance

    (cm) (cm)

    4.5 36

    5 20

    5.5 14.7

    6 12

    6.5 10.4

    7 9.3

    7.5 8.57

    8 8

    8.5 7.56

    9 7.2

    9.5 6.9

    10 6.67

    Table 1: Object distances with their corresponding image distances

After the diffraction grating, the source light will be separated into different orders (Fig

    6). The colors of the light are separated for each order except the zero order group (m =

    0). The zero order contains the original light.

    Fig 6: Example of the separation of colors by using diffraction grating

    The diffraction grating that will be used for this project will have 1000 lines per mm and an angular separation of the maxima of 36 degrees. If there is no lens between the diffraction grating and the CCD sensor (width of the CCD is 2.8 cm), the minimum distance between CCD and the diffraction grating can be calculated:

     tan ( 36 degrees ) = height of the CCD / the distance of the separation (D)

     0.73 = 2.8 cm / D

    ? D = 3.8 cm

    The distance between CCD sensor and the diffraction grating has to be greater than 3.8 cm. If the distance is less than 3.8 cm, then the separation of the maxima will be less than 2.8 cm, which will allow optical occlusion to occur. With blue, green, and red as the primary colors for this project, then the wavelength that the CCD needs to “see” ranges from 435 nm to 740 nm (Fig 7).

    Fig 7: the spectrum of the colors

    In order to project the light that contains wavelength from 435 nm to 740 nm to the CCD sensor with width of 2.8 cm, the exact distance between the diffraction grating and CCD sensor can be calculated by using the equations in Fig 8.

    Fig 8: Multiple slits

     With d = 1/ (1000 lines/mm) = 1um, and choosing m to be 1:

     The angle for the wavelength () at 435 nm can be found: ?

    ? d*sin( ) = m * ?1

    435nm? sin() = 1d

    ? = 25.78 degrees 1

     The angle for the wavelength at 740 nm can be found by using the same equation:

    ? d*sin( ) = m * ?2

    740nm? sin() = 2d

    ? = 47.73 degrees 2

     Also,

    y1? tan() = (Equ: 1) 1D

    y2?tan() = (Equ: 2) 2D

     and are the displacements from the central maxima to the maxima of blue yy21

    and red lights. The difference of those two displacements has to be the height of the

    CCD sensor.

     Then:

     y - y = 2.8 cm (Equ: 3) 21

    Substitute ? and ?into equations 1 and 2: 12

    y1 (25.78) = (Equ: 4) tanD

    y2 (47.73) = (Equ: 5) tanD

    Subtract equation 5 from equation 4:

    yy12 (25.78) - (47.73) = - tantanDD

    y?y?2.812 -0.6172 = = DD

     4.547 cm ?D?

     In order to project light with wavelength from 435 nm to 740 nm to the CCD

    sensor, the distance between the CCD and the Diffraction grating has to be 4.547 cm.

    Without a lens between the diffraction grating and the CCD sensor, the distance

    between them has to be very precise when mounting them on the metal bar. By adding a

    lens between the CCD and the diffraction grating, the lens can be used to calibrate the

    amount of the light with wavelength between 435 nm to 740 nm to be projected on the

    CCD sensor.

    One factor that has to be considered is the CCD sensor; it might get hot under a

    long period of operation. When the CCD sensor gets hot, it will generate more current,

    and will cause some of the pixels to contain invalid data, also known as the dark current.

    With some of pixels containing invalid data, the color that will show up on screen will be

    different from the color of the source object.

     The sampling rate of the CCD’s pixels has to precise in order to generate the

    correct output color. Undersampling and oversampling occur when sampling the data is

    either too slow or too fast. Both undersampling and oversampling will cause the output

    color to be different from the color of the source object. Without all the information, the specific sampling rate cannot be determined at this point.

     The location where the color reading will be performed needs to be analyzed. The location is very small (around 1 mm in circle). Therefore, a sequence of experiments must be done to give the user a precise location.

     After RGB component values are found and displayed on the screen to the user, there may be the need to do color correction in order to compensate for the inability of the CRT monitor to display the same color as the object. This will allow the user to see the same color for both on the object and the color displayed on screen. Yet, the RGB

     value will remain unchanged.

? Calibration of the device

    The purpose of this project is to build a measurement tool measuring a RGB value

    from an object. Hence, there must be a way to calibrate the tool and compare the result with standardized measurement tool. In the first step, the spectrophotometer will be calibrated using light emission from two different elements. We know that each element from the periodic table produces light in different wavelengths (figure below). Therefore, the spectrophotometer which ranges between 400 nm to 700 nm can be calibrated by using two elements, which will then produce wavelength below 500 nm and above 600 nm. In order to get this wavelength, additional tool is required and can be obtained from the RIT Physics Department.

     The second step will compare the spectrophotometer result to other standardized spectrophotometer result which will be given the same input.

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