Lab VI Light Emitting Diodes ECE 476

Transcription

1 Lab VI Light Emitting Diodes ECE 476 I. Purpose This experiment examines the properties of light emitting diodes. The use of the monochromator for studying the spectrum of light sources is also examined. II. Background The Oriel Spectrometer System The spectrum of the LEDs will be examined using a grating monochromator. Diffraction grating were studied in an earlier experiment. The monochromator we will be using has a 1200 line/mm grating with a narrow slit at the input of the monochromator. The narrower the slit is for the input of the light, the higher the resulting spectral resolution. In order to automate the spectrum data collection, the output spectrum from the monochromator is measured using an array of 512 diodes. This diode array is interfaced to a computer that displays and prints the spectrum. Instructions for the Oriel Spectrometer System: A-1. Log into the computer using your engineering domain username and password. A-2. Launch the program: Start Menu->All Programs->SpectraArray SL->SpectraArray SL A-3. Three graphs are displayed when the program starts. The graph to the right is the sample graph; select this graph by clicking on the upper right corner. The square in the upper right corner should be blue to indicate that this is the selected graph. A-4. Select Mode->Spectrum from the menu. Make sure that Sample (raw data) and Nanometers are selected. Press OK. A-5. Set the monochromator center wavelength by setting the micrometer dial. For a center wavelength of 400 nm -- set the micrometer to 4.0 mm. For a center wavelength of 500 nm -- set the micrometer to 5.0 mm. For a center wavelength of 600 nm -- set the micrometer to 6.0 mm. etc. For a center wavelength of 900 nm -- set the micrometer to 9.0 mm. Note: The micrometer needs to be set to 4.0, 5.0, 6.0, 7.0, 8.0 or 9.0 mm. The monochromator typically measures center wavelength plus/minus nm. A-6. Select "Setup->Wavelength Calibration" from the menu. Enter the C0, C1 and C2 coefficients appropriate for the micrometer setting. The table below gives the appropriate

2 coefficients. Micrometer Setting C0 C1 C2 4.0 mm e mm e mm e mm e mm e mm e-6 A-7. Select "realtime mode" icon. Align spectrometer with the light source to be measured. When a spectrum signal is obtained and it looks good, use the mouse to select the "single spectrum" icon to record the spectrum of interest. If the size of the spectrum signal exceeds 3500 or the peaks are flat at the top, the detector is saturating and the light source either needs to be attenuated (or moved further away), or the integration time needs to be reduced (use the clock symbol icon). If the signal is too weak-- either the alignment of the light source to the monochromator needs to be improved, the light source needs to be placed closer to the monochromator, or the integration time needs to be increased (use the clock symbol icon). The appearance of the spectrum graph can be adjusted using the zoom and move icon symbols. A view of just the sample window can be obtained by clicking window-> open sample window-> channel 1. Watts to lumens conversion for a monochromatic source Figure 1: Graph for Watts to lumens conversion.

3 For the desired wavelength, find the "relative sensitivity" from the graph and divide it by 100. Multiply this by 673 lm/w. Then multiply that by the optical power of the source (in Watts). The result should be luminous flux, given in units of lumens. If the wavelength of the light is beyond the range of the graph, assume a measurement of 0 lumens. Luminous_Flux = Relative_Sensitivity/100 * Power * 673 lm/w Remember that this conversion only works for monochromatic light sources (i.e. intensity highly concentrated around one wavelength). This is an OK approximation to make for LEDs. III. Procedure Part A: Light Output versus Input Current Set-up 1. In this experiment the optical output power of two LEDs will be measured as a function of the input current. There is a vector board with all of the LEDs on it in your cabinet. The board can be seen below in figure 2. Figure 2: LED board.

4 1. Connect the variable DC power supply to the high-efficiency red LED. Using the specifications sheet provided in the lab, look up the wavelength and maximum allowable input current of the LED. Calibrate the optical power meter to the wavelength of the LED. Measure the LED output light as the current is increased from 5 to 25mA. Record the light output at 5mA current increments. Plot the results. Note: It is impossible to measure current to an accuracy of 5 ma using the DC power supply. 2. Repeat this measurement with the infrared LED provided. Also use the IR sensor card to examine the shape of the light output, and describe it in your lab report. Part B: LED Efficiency 1. LED efficiency is a percent: (light power out)/(electrical power in) * 100. Different LEDs have different efficiencies. Measure the efficiency of several LEDs at a current of 25 ma. To get the electrical power to the LED you will need to measure the voltage across the LED. To get the most accurate light power measurements, change the wavelength setting of the optical power meter to match that of the LED that you are measuring. Measure the efficiency of the following LEDs: Infrared High-efficiency red Standard red Yellow Green Blue 2. Construct a table showing the following data for each of the six LEDs: LED color Wavelength Light power in watts at 25 ma Electrical power in watts at 25 ma Efficiency Light output in lumens at 25 ma (the curve to convert from watts to lumens was handed out earlier in class) Part C: LED Spectrum Set-up 1. You will be measuring light spectrums using the Oriel Spectrometer System in the lab. Follow the instructions given in the Background section to set up and operate the system. 1. Set the center wavelength on the dial to 600 nm (6.0mm on the dial). Direct light from the room s lights into the monochromator by adjusting its position. Take a spectrum

5 measurement. You should observe a strong signal at nm and two small signals at nm and nm (to see the small signals you may have to increase the integration time considerably). These are spectral lines of mercury, which is one of the elements in the light bulb. The line is a strong green emission line of mercury and the two other lines are called the yellow doublet lines of mercury. The other gas in the light bulb is argon. Note: If the intensity of the light is 3500 on the computer display, the diodes have saturated because of too much light. Adjust the monochromator position and repeat your measurement. If the peaks are very small (<100) and noisy, you need to increase the light level. Often the alignment of the light entering the monochromator needs to be improved. Either print out the spectrum you measured, or capture the screenshot and save it to your network drive to include in your lab report. 2. Measure the spectrum of the high efficiency red LED. Obtain printouts. From your printout determine the width of the LED emission (in nm) at half the peak output power. IV. Conclusion Draw conclusions on the comparison of different LEDs as well as the usefulness of the spectroscopy system.