Improving the Accuracy of Jarrell-Ash Microphotometers

John A. Detrio and Vincent L. Donlan

A modification to a Jarrell-Ash model 23-100 recording microphotometer used to read spectroscopic platesis described. The modification allows the motion of the plate to drive incrementally a stepper motor chartrecorder. The modification uses inexpensive off-the-shelf components and was installed in the field. Theplate reading accuracy of the instrument was improved by a factor of 2-5, and problems associated with oc-casional plate drive slippage were eliminated.

1. Introduction

We are engaged in measuring the optical spectra ofrare earth ions in crystals. For these spectra, highresolution, and therefore small aperture, spectrome-ters are needed for the most precise work. The com-bination of high resolution and small aperture oftendictates the use of photographic plates as the record-ing media for the spectra. In our work, we use a Jar-rell-Ash 3.4-m spectrograph, one of several excellentcommercial spectrometers designed for photographicwork. The use of photographic plates allows one toselect an optimum combination of slit widths and ex-posure times for most rare earth ion transitions and,more important, to superimpose on the unknownspectra the emission lines of a wavelength calibrationsource. We use an iron-neon hollow cathode lampfor this purpose; wavelengths of about a thousand ofthe lines from such a lamp have been precisely mea-sured and tabulated.'

Wavelength data from photographic plates areconveniently obtained by scanning the plates on amicrophotometer or a densitometer and measuringthe distances between lines on the resulting chart re-cordings. Using the positions of the Fe-Ne lines, wegenerate a wavelength dispersion curve in the form ofa least squares-fit fourth-order polynomial. The dif-ferences between the tabulated Fe-Ne wavelengthsand those calculated from the polynomial indicatethe quality of the fit. The polynomial is then used tocalculate the wavelengths of the unknown crystalspectral lines that fall among the set of Fe-Ne linesused in the fit.

J. A. Detrio is with the University of Dayton Research Institute,Dayton, Ohio 45469; V. L. Donlan is with Air Force Materials Lab-oratory, Wright-Patterson AFB, Ohio 45433.

Received 21 January 1974.

To yield the precision inherent in the high spectro-graphic resolution and in the Fe-Ne wavelengths, thecharts must faithfully reproduce the line positions toan accuracy equivalent to E2 Am of their positions onthe plate. In this paper we describe an inexpensiveimprovement we have made on a Jarrell-Ash model23-100 recording microphotometer that allows its usein the precision tracing of plates just described.

I. Jarrell-Ash Microphotometer

The Jarrell-Ash 23-100 microphotometer is an in-expensive (relative to the higher precision densitome-ters and microphotometers available commercially)instrument designed for scanning of plates and filmin routine spectrochemical analysis work. As such,the 23-100 has an excellent and highly reproduciblephotometric system, but only a fairly accurate drivemechanism. It has a series of three synchronous mo-tors and magnetic clutches with appropriate gears togenerate a twelve-speed plate stage drive. The out-put from the photometric system is fed to an inde-pendently driven strip chart recorder. The precisionin plate position is thus limited by the precision andreproducibility of the two independent drive systems.In addition, we have found that despite careful main-tenance, the microphotometer's clutch drive is proneto frequent slippage. These slippages, while thechart recorder continues to advance, lead to grossdistortions of the chart-to-plate ratio, necessitatingfrequent retracing and constant supervision. Evenin good runs, however, the precision in wavelengthmeasurements is limited by the basic accuracy andreproducibility of the drive.

111. Design Considerations

The design philosophy for the selection of a meth-od to improve the accuracy of the photometer chartrecordings and to eliminate the effects of driver sys-tem slippage was that it (1) require no modificationsto the twelve-speed drive, (2) be able to be ingtalled

2236 APPLIED OPTICS / Vol. 13, No. 10 / October 1974

in the field with a minimum of alteration and ma-chining of the photometer housing, (3) be reliableand inexpensive, and (4) have a maximum accuracycompatible with the spectral resolution available.

The starting point for selecting a method was therequired accuracy. According to sampling theory,some five to ten microphotometer readings per spec-trometer slit width are required. For a 20-gm spec-trometer slit width, this requires a resolution of 4-2Jim/reading. Interferometric techniques wouldtherefore have unnecessary resolution that would notjustify their cost and complexity. An interferometermodification was bread-boarded successfully, but itsprojected cost exceeded that of the method finally se-lected. Another approach was to use a precision leadscrew drive. This modification would have requiredextensive reworking of the basic instrument andprobably could not have been done in the field. Itsprojected cost also was considered excessive.

IV. Encoder Selection

The method chosen for modifying the microphoto-meter was to install a Teledyne Gurley model 8712-20 linear encoder and to replace the synchronouslydriven chart recorder with a Houston Instrument Se-ries 3000 stepper-motor driven strip chart recorder.This model linear encoder has been used in retrofit-ting milling machines and has demonstrated abilityto function in harsh environments. Its precision of10,000 pulses/in. (2.54 gm/step) closely matches ourover-all system resolution and accuracy require-ments. The off-the-shelf electronics package andencoder scale cost $800.00 and are designed to beused over a distance of 50 cm with a cumulative accu-racy of +8.5 gim. The installation was made in thelaboratory using only hand tools. Other sources oflinear encoders exist, but the limited space availableforced the selection of the TG model 8712. Higherresolution (20,000 ppi) scales are available, but theywould have restricted the working length to less than25 cm (10), the length of one photographic plate.

V. Details of the Mechanical Components

An over-all view of the installation is shown in Fig.1, and the details of the read head, scale mounting,

Fig. 1. Isometric sketch of the linear incremental encoder modifi-

cation to the Jarrell-Ash microphotometer. The overhead pho-tometer arm is cut away to show the location of the read head as-sembly. The encoder scale and its protective housing are mountedon the rear portion of the plate carriage. The box shown on the

top of the photometer arm contains the electronic components.

and the sliding dust shield (which is an essential con-sideration in ensuring long and reliable service) areshown in Fig. 2.

The read head assembly is attached to an invertedplatform that contains a frictionless hinge and ad-justing screws for setting the tilt of the read head.The tilt or rotation about an axis perpendicular tothe encoder scale is required for proper alignment.The platform is in turn attached to two bracketsmounted on the sides of the overhead photometerarm. Screws located in oversize holes in the bracketsprovide the fine adjustment of the read head to scaledistance and the alignment of the read head with re-spect to the long axis of the scale.

The precision glass scale is mounted in a protectivehousing. The base and two sides are ridged and pro-vide support for the scale from the sides. This sidemounting allows for clearance below the scale for themovement of the dust covers.

The dust covers consist of two strips of Mylar filmthat are attached to the ends of the read head plat-form. The plastic strips are restrained by grooves on

Fig. 2. Detailed sketch of themechanical components. The com-ponents identified approximately in

f3 the order in which they are de-2/¢§) ,R, scribed: (1) photometer arm; (2)

plate carriage; (3) encoder read headassembly; (4) inverted platform; (5)

_______ frictionless hinge; (6) adjustingK r, AtS screw; (7) mounting plate; (8) sup-

port brackets; (9) encoder scale; (10)housing; (11) support for encoderscale; (12) flexible dust shield; (13)clamp for dust shield; and (14)

groove for dust shield.

October 1974 / Vol. 13, No. 10 / APPLIED OPTICS 2237

the insides of the enclosure walls. These groovesguide the dust cover around the end of and under thescale as the scale and its housing move relative to theread head assembly.

The encoder scale and housing are bolted to therear or reference plate carriage of the microphoto-meter.

VI. Electronic Details

The linear encoder system consists of a ruled glassscale that travels between light source and detectorassemblies placed on opposite sides of the read head.The incremental code pattern is generated by themotion of the 500 line pairs/2.54 cm scale interrupt-ing two transmitted light beams. The outputs of thetwo pairs of detectors (per beam) are summed to im-prove the performance and cancel certain scale er-rors. The signal detection circuitry is fully differen-tial to compensate for all common mode sources oferror, such as lamp intensity variations, supply volt-age variations, and temperature changes. The readhead output signals are sin and cos waveform at a fre-quency of 500 cycles/2.54 cm. This phase informa-tion is used to interpolate up to a total of 2500 cycles/2.54 cm in each channel. Further logic circuit ma-nipulation provides forward and reverse output puls-es and their complement in TTL format.

The stepper motor driven chart recorder requirespulses at least 5 V positive and of at least 10-,4sec du-ration. These pulses are obtained from the TG out-put by the use of a one-shot multivibrator configuredas shown in the electrical schematic (Fig. 3). Somerecorders can be driven directly by the TTL pulsefrom the TG electronics package; however, as a pre-cautionary measure, the use of a multivibrator bufferstage is recommended.

Those electronic components required in additionto the TG black box are shown in Fig. 3. If eitherforward or reverse pulses are to drive the recorder,the two outputs can be OR'd in a suitable logic ele-ment. The availability of either F (forward) and R

P.S. AND BUFFER AMPLIFIER MODULE

…i --

TO T-G1 I h. ~ LINEAR ENCODER | °pei l

' i -;NOe _ I

GROUND|

- - - - - - - - - - - - - --_-__ _ Z

ZE

GI

OUTPUTTO

RECORDERDRIVE

Fig. 3. Interconnecting wiring diagram and one-shot buffer fordriving the stepper motor chart recorder.

LIMIT SWITCH (INTERNAL TO J*Al WIRING

SINGLE LIMIT SWITCH ( DUAL CAMS)

NC

L__ LOWVOLTAGE RELAY

I IO 110 V ACPOWER TO

FUSE MOTOR DRIVE

SUB CHASSIS

- ILAMENT(PNIOL TRANSFORMER J A CONNECTOR J-10)

TB-5 TB-6PIN 8 PIN 18

TO IIOV-MAIN J-APOWER STRIP

Fig. 4. Limit switch wiring diagram showing the modification tothe Jarrell-Ash model 23-100 drive circuit. The limit switch is at-tached to the mounting plate under the photometer arm. The act-

uating cams are mounted on the outside of the scale enclosure.

(reverse) pulses or their complement F and R lends agood deal of flexibility to the circuit design. The re-corder input, however, must be +5-V pulses into 10-KQ impedance at a maximum rate of 200 pps.

The only modification made internally to the J-Amicrophotometer is the limit switch assembly shownin Fig. 4. The circuit shown in Fig. 4 is only one ofmany methods possible for interrupting the driver atthe mechanical limit of the scale. This method is notnecessarily the best since all power is removed at thelimit position, and the instrument drive must beturned off and then reset by hand. It was the sim-plest to implement, however, and probably the leastexpensive failsafe technique available.

VII. Installation and Alignment

The mounting requirements for the TG linear en-coder are a scale to read head clearance on the scaleside of 0.025 0.005-cm total run out over the 50-cm(20") working length of the scale. The variation inthe vertical direction is 0.0025 cm/30 cm of travel.In order to align properly the encoder, it is necessaryto adjust the tilt of the read head assembly about anaxis perpendicular to the scale face (along the detec-tor-light source axis). Provisions for these adjust-ments were described in Sec. V.

The installation procedure for the linear encoder isclearly detailed in the instructions provided by TG.The only additional item found to be useful is a gooddial ,indicator for initially setting the scale accuratelyon the carriage relative to the direction of motion.Another requirement is patience.

Vil. AccuracyThe circuitry of the Houston Instrument Series

3000 chart drive allows the frequency of the inputstepping pulses to be divided down into nine ratiosranging from 1:1 to 1:400. Each division is selectableby push button. The recorder advances by 0.070mm/step, so that at the highest stepping rate (one

2238 APPLIED OPTICS / Vol. 13, No. 10 / October 1974

step per input pulse), the chart-to-plate ratio is27.78:1. At the lowest stepping rate, the ratio is0.07:1. For the most accurate wavelength work, thelargest chart-to-plate ratio is used in order to mini-mize errors due to the finite pen width.

In most of our work, we use a spectrograph gratinghaving 600 grooves/mm. This gives a first order re-ciprocal linear dispersion of about 5 A/mm. Thegrating has a resolving power of about 65,000 so thatthe diffraction-limited slit width is on the order of20 Atm in the visible. A good rule of thumb is thatthe centers of spectral lines can be located to an accu-racy of about %0 the spectral slit width. With theabove parameters, this corresponds to a limiting ac-curacy of about 0.01 A in our measurements, provid-ed no mechanical sources of error are present. Wecan, in fact, routinely obtain rms errors of 0.01 A inthe fourth-order polynomial fit to the wavelengthdispersion curve by directly measuring Fe-Ne linepositions on photographic plates using a Gaertnertraveling stage microscope that has a micrometer ac-curacy of ±0.5 gtm. However, the microscope is notsuitable for precision measurements of the broaderlines of the rare earth ion spectra with their reducedcontrast. For these lines, the use of chart recordingsto locate the line centers is mandatory. The limitingaccuracy of +0.01 A is therefore a criterion againstwhich we can judge the accuracy of our mechanicalplate-to-chart recording system.

When working with the Jarrell-Ash microphoto-meter before the modification, our wavelength accu-racy ranged from ±0.05 A to ±0.10 A, as indicated bythe rms errors of the fourth-order polynomial fit.This range of accuracy could be obtained only bycarefully monitoring the stage drive for any clutchslippage and often selecting the best of several goodtraces of the same plate region.

After making the modification described here, wehave obtained in every run rms errors in the 0.02-0.03-A range. Further, the difference between theseaccuracies and the 0.01-A ideal can be attributed tohuman errors, i.e., uncertainties in line positions due

to the finite pen width and to interpolation betweenmarks on the precision meter stick used to measurechart distances. In fact, several retracings of thesame region of a plate can be superimposed with nodifferences in line positions apparent to the eye, evenif during one or more of these runs the microphoto-meter drive was observed to be slipping or hangingup. Automatic data recording and processing couldbe added to the modification described here to elimi-nate the human element.

IX. Conclusions

We have described an inexpensive in-the-fieldmodification that can be used to transform a micro-photometer or a densitometer of medium accuracy toone compatible with the precision inherent in highresolution photographic spectroscopy. Furthermore,the modification allows for a later adaptation of au-tomatic digital data processing equipment to elimi-nate all human sources of error. Specifically, wehave, with about $2000.00 worth of off-the-shelf com-ponents (including the cost of the Houston Instru-ment chart recorder) plus about $700.00 of additionallabor and material, upgraded a Jarrell-Ash 23-100microphotometer to the equal of the most preciseplate-reading instruments. In doing so, we have notsacrificed any of the outstanding photometric perfor-mance of the instrument or its convenient platemounting and alignment features.

We acknowledge the assistance of F. Timko and D.Holthaus with the mechanical details of the installa-tion and R. Petty for contributions to the electronics.All are with the University of Dayton Research Insti-tute. We also thank Hector Ramos, Air Force Insti-tute of Technology, for making many of the wave-length measurements.

References1. H. M. Crosswhite and G. H. Dieke, in American Institute of

Physics Handbook, D. Gray, Ed. (McGraw-Hill, New York,

1963), pp 7-43 ff.

Kenneth A. Geiger

Cryophysics

October 1974 / Vol. 13, No. 10 / APPLIED OPTICS 2239