Physics 418
Instructor: Tao Zhou
Lab 1 : Spectra of various light sources
I. Objective
The purpose of this experiment is the following:
1. To introduce to the student what is a spectrometer, how to calibrate it, and how to use it to
measure spectra.
2. To illustrate that the most complete characterization of light source is its spectrum, from which
the color and intensity can be derived. Different light sources with the same color can have very
different spectra.
II. Introduction
Light source is the first component of any imaging system. It is used either as an image itself, such as the
LED array in a display, or more likely, used to illuminate the object. For any incoherent light source, i.e.,
any source that is not a laser, its spectrum I() contains the most complete information of this source.
The total intensity is decided by I = ∫ I()d while the color can be derived from the CIE color diagram
method using I(). Thus, it is of great importance to measure the spectra of light sources experimentally.
The experimental instrument used for spectrum measurement is called spectrometer, the schematics is
shown in Fig. 1. Its main component is either a prism or gratings, which is used to disperse light spatially
according to their wavelength. The dispersed rainbow of light will then impinge on a CCD detector array,
and each element of the CCD array will read out the intensity of the light at one particular wavelength,
i.e., I() can be obtained. The size of the entrance slit of the spectrometer is the main parameter to
determine the resolution of the spectrum. The spatial size of the CCD array and the gratings dispersive
power determine the spectral range of each measurement.
Fig. 1. Schematics of a spectrometer
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Before the measurement, it is essential to calibrate the spectrometer, i.e. to know the absolute position of
the spectrometer and the spectral range of each acquisition. In this experiment, student will use Hg lamp
to calibrate the spectrometer. The spectral lines of Hg lamp most commonly used are: 404.656 nm,
435.833 nm, 546.073 nm, 576.960 nm, and 579.066 nm. Students are then to measure the spectra of a
variety of light sources, including incandescent lamp, fluorescent lamp, red, green and white LEDs.
III. Required equipment:
Gratings spectrometer
Computer with KestrelSpec software installed
ND filter holder with mount
ND filter set
Incandescent lamp, fluorescent lamp, green, red and white LEDs
IV. Procedure
A. Calibrating the spectrometer
1. Turn on the Hg lamp, with light impinging on the entrance slit of the spectrometer.
2. Turn the spectrometer gratings’ reading to about 560 nm. Acquire the spectrum so that the mercury
lamp’s 3 lines of 546.073 nm, 576.960 nm and 579.066 nm appearing together he same graph.
3. Select SETUP: X Axis calibration menu option. Entering pixel numbers and their corresponding
wavelengths. Save the Cal Curve. Record the exact reading of the gratings.
4. Calculate the spectral range of the CCD detector, i.e. how many nanometers of wavelength interval
between pixel 1 to pixel 750? This range will be assumed to remain uniform across the whole 400 – 760
nm range.
5. Move the gratings to a position so that pixel 1 will correspond to 400 nm. This position on the gratings
needs to be calculated, based on the information obtained above.
B. Acquiring spectra of light sources
1. Put the incandescent lamp at the source position, as illustrated in the schematics, then acquire its
spectrum, starting from 400 nm, and end at 760 nm. The whole range requires up to 10
acquisitions, each one covering about 40 nm. Record the starting position of each acquisition. If
during acquisition the CCD detector is saturated, put neutral density filter in, and repeat the
whole procedure with the same neutral density filter.
2. Repeat the same procedure with fluorescent lamp.
3. Repeat the same procedure for green, red and white LEDs.
4. Save all the graphs using ASCII format.
V. Summary of Results
1. Sort out the spectral data using Excel spread sheet. The data sheet should look like the following
table:
Wavelength (nm)
Intensity (CCD count)
400
1000
400.5
1200
401
1300
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………
758.5
759
759.5
760
3500
3000
2900
2700
Of course the exact wavelength interval is not 0.5 nm. Instead it should be whatever you recorded with
your data, and the same thing for the intensity number. Yet the format should be like the table above,
with all the acquisition appended to each other to form a continuous spectral range.
2. Repeat the process for every light source, so you should have 5 spreadsheet data. You don’t have to
print them.
3. Plot them out using Excel, and you should have plots similar to the following graphs, with wavelength
in nm as the x axis, and intensity in CCD count as the y axis.
3.
Make some comments of these different spectra, based on what we learnt in the lectures.
4. EXTRA CREDIT: what will be the CIE xyz value for these different light sources, and what is the corresponding
color according to the CIE color diagram? It would be very nice if you can calculate at least one source, but it is not
required.
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