Optical Spectrometers Filters and Light Sources

Objective:

In this laboratory exercise, students will learn the basics of spectrometer design and function. The frequency separating and intensity monitoring abilities of two spectrometer systems will be studied. These spectrometers will be used to take absorption and emission spectra.

Background:

Since the optically active transitions of atoms and molecules occur at unique frequencies, we may study these species by examining the frequencies of the transitions and how the microscopic environments surrounding the ion affect these transitions. To separate the light due to different transitions, several spectrometer systems have been developed. In this exercise, we focus on one type (Czerny-Turner) of optical spectrometer pictured below.

Light entering the spectrometer through the entrance slit falls on a mirror that collimates the light. The collimated light strikes a diffraction grating mounted on a rotating table. The light reflects from the grating, satisfying the conditions for interference

nl=2d sin q

where n is an integer, called the order of the diffraction, l is the wavelength of the light, d is the distance between parallel grooves on the grating, and q is the angle the light makes with the normal to the grating surface. Thus, we see the light is separated into its wavelength, or frequency, components. The light diffracted from the grating strikes a second mirror and is refocused on an exit slit. A photomultiplier tube, sensitive to the intensity of light, is mounted on the exit slit of the spectrometer. The detector measures the intensity of light incident on it while the grating is rotated. The rotation of the grating varies the angle q, thus allowing a measurement of the intensity of light as a function of grating position, which can be converted into a measure of the frequency of light.

The Ocean Optics USB4000 series spectrometers used for the following exercises vary from the basic design pictured above in that they contain a fixed grating and a detector, which can collect data from a wide spectrum rather than the narrow exit slit. This design eliminates the need to rotate the grating in order to obtain data of intensity as a function of frequency.  However, the resolution and sensitivity to low light levels is reduced.

Apparatus:

The USB4000 is a newly developed spectrometer based on fiber optic coupling and integrated computer design. The detector in the USB4000 is a CCD (charge-coupled device) array capable of measuring the intensity of light over a wide range of grating angles. Thus the USB4000 takes a "snapshot" of the spectrum without the need to rotate the grating. Since there is no exit slit in this type of spectrometer, the width of the pixels composing the diode array determines the minimum value for this parameter.

Prior to measurement on your sample it is still necessary to take a reading of the Dark current and Reference spectrum.

Begin by running the control software. An icon labeled SpectraSuite can be found on the desktop or in the Program Menu. Double click this icon to bring up the control software. The control software is a user-friendly interface with push buttons that enable the experimenter to set the acquisition parameters and acquire spectra in a variety of ways. Of importance to this experiment, there are buttons for acquiring dark current, reference, and data spectra. The dark current should be acquired prior to switching on the white light source or opening the light shutter.

After switching on the white light source, the reference spectrum should be acquired with the appropriate parameters. Be careful not to saturate the detector, as this will make your reference measurement useless.

Experiment I: Emission of Incandescent and Fluorescent Bulbs

Collect an emission spectrum from a tungsten filament lamp.

• Switch on the tungsten lamp and the USB4000 control computer, and run the software.
• Turn off the room lights and collect a dark spectrum (press the grey light bulb button).With the same background conditions, collect the emission spectrum of the tungsten lamp.
• Now switch off the tungsten lamp and measure the emission spectrum of the fluorescent room lights by reflecting light into the fiber optic probe.
1. Estimate the temperature of the tungsten filament. Give an idea of how well (how sig figs) you can determine this number.
2. Present your two spectra and comment on the differences between the two. How can objects appear so similar to our eyes when being illuminated by such different sources?

Experiment II: Emission of Gas Discharge Lamps

Collect emission spectra from the hydrogen gas-discharge lamp provided and examine the resolution of the spectrometer.

• Load the gas bulb into the lamp base and switch the power on.
• Place the collection end of the fiber optic near the lamp so that the spectrum of the emitted light is seen on the computer screen.

The spectrum observed is produced by the discrete transitions of the atoms in the gas as they "hop" from one energy level to another after being excited by colliding with electrons accelerated between the electrodes of the electrical discharge. The lifetimes of the excited states are on the order of 1ns.

1. Compare the values for the hydrogen lines to the accepted values.
2. Zoom in (Use Set Scale in View Menu) on a few of the spectral lines (one at a time) and measure the width of each line from the gas discharge lamp.
3. When the resolution of the spectrometer (a function of the size of the spectrometer, the coarseness of the grating, the detector, and the entrance slits) distorts the width of a spectral line, the data is said to be "resolution-limited" by the spectrometer. Based on the manufacturer's specifications for the spectrometer (~1.5nm), what is the best resolution we can expect?
4. Does the width of the line you measured accurately portray the width or is your measurement resolution-limited?

Emission spectra are valuable measurements because they show researchers how energy leaves a system. Once a collection of ions is excited (in our case by the collisions with fast electrons) the energy they receive must somehow escape. If we detect light from the excited ions we know that the energy is carried away by photons. The energy, or frequency, of the emitted photons tells specifically which atomic transitions are responsible for the creation of the photons.

In the following experiment, we will examine absorption spectra. Absorption spectra tell researchers which energy levels of materials are capable of being coupled by photons. By measuring the frequency, and thus energy, of the photons which couple the levels, we can precisely place the levels in energy.

Experiment III: Colored Glass, Neutral Density, and Interference Filters

Examine the light transmitted through a variety of glass filters.

• Close the lamp shutter, turn off the room lights, and store a dark spectrum.  You should subtract it from any signal you collect.
• Open the lamp shutter.  Adjust the amount the shutter is opened and the time of collection so that the a good spectrum is observed. View and store the reference spectrum. The reference spectrum tells you how strong the lamp is and how sensitive the detector is at each frequency separated by the grating. Click on the "T" button to view the transmission spectrum.

When a filter is placed in the optical path, the filter will attenuate the reference spectrum according to the filter's profile.  The filter profile can be found by dividing the output spectrum by the reference spectrum.  This gives the fraction of the spectrum that is transmitted as a function of wavelength.

1. Place additive and subtractive interference filters in the sample space and record their spectrum.
2. Summarize your findings and write an explanation about additive and subtractive colors.  Information on the color filters can be found at Edmund Industrial Optics site. Give an example of a technology that uses each kind of filter.
3. Collect transmission spectra of individual and combinations of ND filters (0.15, 0.3, 0.6).
4. Determine how well the equation %T = 10-D describes the filters. T is the transmission and D is the optical density. A graph of what you measure vs. what you expected would be very helpful.  Also, comment on the "neutral" nature of these filters.
5. Take a transmission spectrum of the green interference filter. Hold the filter up to your eye and observe transmitted room light.  Tilt the filter with respect to your eye.  Describe what  you see. Look up "optical intereference filter" and explain what you see. What is the center wavelength and bandwidth of this filter?