This manual contains basic operating instructions for the Perkin-ELmer Lambda-3B spectrophotometer located in the Mass, Optical, and Infrared Spectroscopy Lab (V14A) at Niagara College. A complete system, consisting of the spectrophotometer and an R100A pen recorder, this spectrophotometer is an excellent piece of research laboratory equipment. Its double beam optics provide a stable flat baseline (the 100% reference) and mean that the sample does not need to be removed for every wavelength measured. It has a 1 nm spectral bandwidth and can scan the entire 190-900 nm wavelength range. Our unit has a special thin-film filter holder installed since it is primarily used in PHTN1432: Thin-film and High Vacuum Technology as an analytical tool for thin-film coatings.
Remove all samples from the sample chamber and turn on both the spectrophotometer (the switch is on the RIGHT REAR of the machine beside the power cord) as well as the recorder. Press the PEN LIFT button on the recorder and install a pen. You must now wait for one minute while the machine initializes - the display will show a countdown while this occurs. Turn on the lamp for the range being investigated using the switches on the lower-right (front) of the unit: UV for the UV range below 380nm, VIS for the range 400-900nm. Since the UV lamp is a deuterium lamp which is valued at $746.05 (!) use this lamp sparingly only if investigating the UV region (i.e. using quartz NOT glass substrates).
Setting the Wavelength:
When the unit is ready, select the start wavelength (the TOP of the scan: if you are scanning from 400nm to 600nm, 600nm is the starting point) by entering the wavelength on the keypad, in nanometers, and pressing the GOTO (LAMBDA) button. The display will show the current wavelength (likely in the IR when the unit first starts) and will slowly decrease until the target is reached. In the photo on the left, the wavelength is seen as 500.6nm.
Press the MODE button until the "%T" indicator is lit (usually this is the default). When the wavelength indicator shows it has reached the target (the start of the scan), press the AUTO ZERO button with NO SAMPLE INSTALLED and the SAMPLE DOOR CLOSED. This will set the 100%T reference and the rightmost display will now read 100.0 as it does here.
Inserting The Sample:
Open the sample door by lifting it and insert the sample into the front beam path (the thin-film filter holder) - a filter usually fits between the aluminum block and the spring clips. Insert any desired reference, such as a blank substrate, into the reference (rear) holder. This is an optional step which allows the spectrophotometer to subtract the effects of a substrate from a coating.
Starting the Scan:
We are ready to begin a scan but first, select the SCAN SPD on the spectrophotometer using the appropriate button. Speeds are in terms of nm/minute. Normally a scan speed of 60nm/minute is sufficient. Now select the recorder paper speed as well - a speed of 60mm/minute is normal - using the SPD button on the recorder as shown. When selecting recorder paper speed ensure the indicator on the right side of the speed indicator is lit as shown in the photo. This combination (60nm/min, 60mm/min) means that wavelength scale of the final graph produced will be scaled so that one mm is one nm: a 300nm scan from 400nm through 700nm will consume 30cm of paper.
Press the PEN LIFT button on the recorder to lower the pen. Mark this as the beginning of the scan (high wavelength). Now press RUN on both the recorder and the spectrophotometer simultaneously. The wavelength display on the Lambda-3B will begin decreasing and the pen will move accordingly on the recorder. When the scan is complete (watch the wavelength display) press STOP on both machines. Press PEN LIFT then PAPER ADV on the recorder to advance the paper and tear it off.
An example scan of a commercial dielectric filter. A pencil can be used to mark directly on the paper the start and stop wavelengths. Note the line at 600nm created when the pen was left in the DOWN position when the filter was initially installed (with no filter in place, the pen jumps to the 100% position drawing a line).
Installation instructions for the Specular Reflectance Accessory
Raise the pen on the recorder. Turn the LAMP switches (lower-right of the unit) off. Remove all samples from the sample chamber. Wait one minute for the unit to cool gradually then turn off the power switches on the spectrophotometer as well as the recorder. Remove the pen from the recorder.
The unit uses two lamps as sources: a 30W 'DZA' halogen lamp for this visible range (extending to 900nm) and a cold-cathode deuterium lamp for the UV range below 380nm. Light from the source passes through a monochromator consisting of a diffraction grating and slits to select a single wavelength. Monochromatic light then passes through a chopper which interrupts the sample beam alternatingly passing the light between the reference and sample beams. These beams pass through their respective cells and onto a photomultiplier tube where the miniscule amount of light passing through the samples is detected.
As a backgrounder, see the Scientific American Amateur Scientist article in January 1975 which describes a dual-beam machine.Copyright (C) Niagara College, Canada, 2004