Digikröm
DK 240 ¼ Meter
DK 242 Double ¼ Meter
DK 480 ½ Meter
Monochromator / SpectrographUser Manual
Document 1049461-g
March, 2009

Spectral Products
2659-A Pan American Frwy., NE Albuquerque, NM 87107 Tel (505) 343-9700 Fax (505) 343-9705
www.spectralproducts.com

1About Spectral Products
Spectral Products is a world leader in optical instrumentation technology and products. Spectral
Products builds on its rich heritage and extensive technical knowledge to offer a broad range ofinnovative solutions from components to systems and modules. Spectral Products is an industry
leader in the design and manufacture of optical instruments including spectrometers,monochromators, spectrographs, spectrophotometers, detection systems, light sources as well as
fiber optic cables and couplers.
Headquartered in Putnam, Connecticut, Spectral Products' focus on quality, value and service hascreated an innovative approach to manufacturing and design. With employees in Putnam,
Connecticut, Albuquerque, New Mexico and Seoul, South Korea, Spectral Products continues the
tradition of design innovation, high quality products and exceptional value.

Spectral Products
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1.1 Mission Statement........................................................................................................... 4
1.2 Warranty ......................................................................................................................... 4
1.3 Copyrights....................................................................................................................... 5
3.1 Verify Shipping Contents ............................................................................................... 7
3.2 Hardware Connections.................................................................................................... 7
3.3 Monochromator Control ................................................................................................. 7
3.4 Software .......................................................................................................................... 8
4.1 DK240/DK480................................................................................................................ 9
4.2 DK242............................................................................................................................. 9
4.3 Wavelength Accuracy................................................................................................... 10
4.4 Resolution ..................................................................................................................... 10
4.5 Grating Drive Precision ................................................................................................ 10
4.6 Slits ............................................................................................................................... 10
7.1 Sources of error............................................................................................................. 22
7.2 Acceptance criteria........................................................................................................ 23
8.1 Command Summary ..................................................................................................... 25
8.2 Remote Operation ........................................................................................................ 34
8.3 Encoding/Decoding Data Bytes................................................................................... 37
8.4 Status Bytes................................................................................................................... 39
8.5 Novram Program.......................................................................................................... 40
8.5.1 READ FROM NOVRAM.......................................................................................... 40
8.5.2 WRITE TO NOVRAM.............................................................................................. 40
8.6 RS-232 (Serial) Connection Diagram........................................................................... 41
8.7 GPIB(IEEE-488) Interface Option .............................................................................. 43
9.1 Operation....................................................................................................................... 45
10.1 Optical Diagram of DK240/480.................................................................................. 62
10.2 Optical Diagram of DK242 ........................................................................................ 63
11.1 Calibrating Zero with DK2401 ................................................................................... 64
11.2 Calibrating a Wavelength with DK2401..................................................................... 64
11.3 Slit Calibration ........................................................................................................ 65
11.4 Calibrating Zero with DKDemo VB Software ....................................................... 65
11.5 Calibrating a Wavelength with DKDemo VB software ......................................... 67
11.6 Slit Calibration Using DKDemo VB Software....................................................... 68
11.7 DK242 Calibration...................................................................................................... 69

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1 Introduction
1.1 Mission Statement
Our mission is to provide our customers with reliable products, on time, and at a fair price.
We are continually striving to maintain the highest standards, by assuring defect-free
products and by providing prompt and courteous customer service.
The staff at Spectral Products (SP)` will be happy to answer any questions about our
products and our services. For immediate assistance, please contact Spectral Products
directly at (505) 343-9700, by fax (505) 343-9705, or by e-mail atsales@spectralproducts.com
1.2 Warranty• This product is warranted to be free of defects in materials and workmanship for
one year from date of purchase.
•This manual and the software it describes are provided free of charge as a service
to the customer. The software is intended to be used as a tool for development andas an example of one possible method of code implementation. It is not intended
to be a “user application.”•

Any software associated with this product is provided “as is” with no warranty,
expressed or implied. While it is Spectral Products’ intent to provide error-freedevelopment tools, no guarantee is made regarding either the accuracy or
usefulness of this material.

•

Failures or damages resulting from lack of operator attention to proper
procedures, failure to follow operating instructions, unauthorized modifications,and natural disasters are not covered under this warranty.

•

SP reserves the right, without prior or further notice, to make changes to any of its
products described or referred to herein to improve reliability, function, or design.

•

SP accepts no liability for incidental or consequential damages arising from theuse of this software.

•

SP does not recommend the use of its components or software products in life
support applications wherein a malfunction or failure of the product may directly
threaten life or result in injury.

•

SP does not recommend the use of this product on the same power line as otherequipment with high current draw requirements.

•

The Digikröm DK240/480 does not contain any user serviceable parts. Removing
its cover, without explicit written permission from Spectral Products, will voidany written or implicit warranty.

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1.3 Copyrights
Spectral Products maintains the copyright on this material, but grants the user rights to use or
to modify the software described herein without obtaining Spectral Products’ permission andwithout the requirement to reference Spectral Products as the source of the material.
Lab VIEW® is a registered trademark of National Instruments.
Windows™, Microsoft® Visual Basic™ and Microsoft® Quick Basic™ are registeredtrademarks of Microsoft Corporation.

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2. Product Overview
The Digikröm DK240/480 are ¼ and ½ meter, Czerny-Turner type monochromator/spectrographs.
Focal lengths are 240mm and 480mm respectively. The grating(s) of your Digikröm are controlled
by a microprocessor-driven stepper motor, which is coupled to the grating table. Thus, there is nosine-bar drive mechanism in the Digikröm monochromators. This design permits a simple rugged
mechanism, which is less likely to drift out of calibration during extensive use, and/or roughhandling.
The Digikröm is controlled by a handheld controller, direct RS-232 computer control, or by using
the optional GPIB (IEEE-488) interface. All necessary protocol and command functions are givenin this manual.

Basic Czerny-Turner monochromator design

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3. Getting Started
3.1 Verify Shipping Contents
The Digikrom 240/480 monochromators do not require removal of any interior
shipping restraints. NOTE: This equipment contains static sensitive devices.
Handle equipment in a static safe environment until power can be supplied to the unit.
The following items are shipped with your order of a DK series monochromator:
1
1
1
1
1

DK240/480/242
DK24Vxx power supply
Serial communications cable
Demonstration CD software.
DK Utility CD containing test data, calibration data.

3.2 Hardware Connections
Power is supplied to the DK240/480 via the TPS65-0515DS power supply.
Attach the power cord to the three-prong outlet on the back of the powerpack.
Attach the connector from the power supply to the monochromator.
Plug the power cord into your wall or power strip outlet.
The
monochromator will reset in approx. 3 minutes and find home position.
3.3 Monochromator Control
The monochromator can be controlled by an optional handheld controller orwith a computer.
To control the monochromator from a computer, connect the supplied serialinterface (RS-232) cable, from the computer, directly to the monochromator
25 pin connector located at one end of the monochromator.
To control the monochromator using the hand held control module, see
control module instructions in chapter 3.2.

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3.4 Software
DK240/480 Demonstration Software-Windows™
DK series monochromator demonstration software is written in Microsoft® Visual
Basic™ 16 bit, Ver. 4.0 for Windows™ and will run on Windows™ 3.11, 95, 98,
2000, and NT 4.0. The demonstration software, along with instructions for operation,is found on the CD software disk. If you are interested in writing custom software that
supports the DK240/480, we will be pleased to send this source code upon request. If
you have any questions about the operation of your monochromator or if you havesuggestions, please contact us. We appreciate your comments and suggestions.

Spectral Products
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3. Product Specifications
4.1 DK240/DK480
Wavelength Drive: Worm and wheel with microprocessor control. Bidirectional.
Design: Czerny-Turner, triple-grating turret.
Focal Length: 240/480 mm.
F/#: 3.9/7.8.
Gratings: 68 x 68 mm ruled is standard. Holographic available.
Wavelength Precision: 0.01 nm with 1200 g/mm grating.
Wavelength Accuracy: + 0.3 nm with 1200 g/mm grating.
Scan Speed: 1 to 1200 nm/minute with 1200 g/mm grating.
Maximum Resolution: 0.2/0.1 nm with 1200 g/mm grating.
Slits: Computer controlled. Width – 10 to 3000 m. Height – 2 to 20 mm.
Software: Demo control program with source is included. A Lab VIEW®
Driver is available upon request.
Power: UL listed 110/220 V power pack, meets or exceeds UL1950, CSA
1402C, and IEC 950.
Interface: RS-232 standard.
Warranty: One year from delivery date.
CE marked.
4.2 DK242
Wavelength Drive: Worm and wheel with microprocessor control. Bidirectional.
Design: Czerny-Turner, triple-grating turret.
Focal Length: 240 mm.
F/#:
3.9.
Gratings: 68 x 68 mm Ruled is standard. Holographic available.
Wavelength Precision: 0.01 nm with 1200 g/mm grating.
Wavelength Accuracy: + 0.3 nm with 1200 g/mm grating.
Scan Speed: 1 to 1200 nm/minute with 1200 g/mm grating.
Maximum Resolution: 0.2 nm with 1200 g/mm grating.
Slits: Computer controlled. Width – 10 to 3000 m. Height – 2 to 20 mm.
Software: Demo control program with source is included. A Lab VIEW®
Driver is available upon request.
Power: UL listed 110/220 V power pack, meets or exceeds UL1950, CSA
1402C, and IEC 950.
Interface: RS-232 standard.
Warranty: One year from delivery date.
CE marked.

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94.3 Wavelength Accuracy
Grating (g/mm)
3600
2400
1200
600
300
150
75
50
4.4 Resolution
Grating (g/mm)
3600
2400
1200
600
300
150
75
50

DK240/242
.1nm
.2nm
.3nm
.6nm
1.2nm
2.4nm
4.8nm
7.2nm
DK240
.05nm
.1nm
.2nm
.4nm
.8nm
1.6nm
3.2nm
4.8nm

DK480
.1nm
.2nm
.3nm
.6nm
1.2nm
2.4nm
4.8nm
7.2nm
DK242

.05nm
.1nm
.15nm
.4nm
.8nm
1.6nm
3.2nm
4.8nm

DK480
.04nm
.07nm
.1nm
.2nm
.4nm
.8nm
1.6nm
2.4nm

4.5 Grating Drive Precision
Grating (g/mm)
Micro stepped
3600.01nm/step
2400.01nm/step
1200.01nm/step
600.02nm/step
300.04nm/step
150.08nm/step75.16nm/step50.24nm/step
4.6 Slits
Type
Unilateral
Bilateral

Increment
1µ1µMinimum
10µ
10µ

Maximum
3000µ
5000µ

Slit aperture height is adjusted by removing the plate covering the slit with a 3/32” hex head
wrench. Then loosen the aperture securing screw with a 3/64” hex head wrench, slide theaperture to the desired height, and retighten.
DO NOT FULLY REMOVE THE APERTURE RETAINING SCREW, OR THE APERTURE WILL
FALL INTO THE MONOCHROMATOR BODY.
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5. Theory of Operation
The optics of monochromators are designed so that, for monochromatic light, an image of theentrance slit is formed at the exit slit. Scanning the monochromator rotates the grating and moves
this image across the exit slit. If one were to measure the intensity of the light exiting themonochromator as this scanning occurs, one would see that a triangular intensity profile results.
This is shown in Fig 2.1. Diffraction and other aberrations cause deviations from this ideal situation.
Because of the physics of diffraction gratings, entrance slit images are formed at different angles fordifferent monochromatic wavelengths. Therefore, rotating the grating also selects a changing
wavelength region. This is described by the grating equation.
2 * d * COS(Æ) * SIN(q)l = —————————————nImagine a source that sends two monochromatic lines into a monochromator. If the wavelengths are
sufficiently different, the two monochromatic slit images will not overlap at the exit slit. However,the finite width of the slits allows the possibility of overlap for some wavelength difference. That is,
the slit width limits the ability to resolve two closely spaced wavelengths.
Wider monochromator entrance slits allow more light to enter into the instrument. Narrower slits
allow for better resolution between wavelengths. This is one of the basic trade-offs in the use of
monochromators.
The wavelength that is passed by the monochromator, lambda, is described by the grating equationthat was presented earlier.
2 * d * COS(Æ) * SIN(q)l [nm] = —————————————
nor in wavenumberss [cm-1] = n * (0.5 / COS (Æ) ) * N * CSC(q)whered — is the grating groove spacing in meters
N — is the number of grooves per centimeterÆ — is the Ebert angle. This is a fixed angle determined by the positions of the grating,the collimating mirror, the camera mirror and is approximately 18 degrees for the
DK240.
q — is the angle of grating rotation measured from the point at which white light is
specularly reflected through the instrument.
(Note that » 70º is the maximum grating angle.)
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n — is the order of diffraction. Typically, for light incident normally to a grating, someof the light will be reflected (zero order), some will be diffracted to the right (+1 order),and some will be diffracted to the left (-1 order). Diffraction at greater angles also
occurs, but is not significant (orders +2, -2, +3...). The DK240 grating drive provides a
Dq of 7.5 x 10-3 degrees.
Because entrance slit images are formed at different angles for different monochromatic
wavelengths, different wavelengths will be exiting the monochromator at different angles. The
grating causes an angular dispersion as a function of wavelength and this angular dispersion ispreserved at the exit slit.
In a single monochromator the angles at which light strikes the grating is independent of wavelength.
In the second half of a double monochromator, the angle at which the light strikes the grating
depends on the wavelength. (The first grating has introduced angular dispersion as a function of
wavelength.)
If the second grating rotates in the same direction as the first grating, then the angular dispersion of
the second grating will add to that of the first grating. The dispersion is doubled. If the entrance,
center and exit slits are approximately the same width, then it is the entrance and exit slits that limit
the band pass. Because the dispersion at the center slit is half of that at the exit slit, the band pass ofthe center slit is twice that of the exit slit.
If the second grating rotates opposite to the first grating, then the angular dispersion of the second
grating will subtract from that of the first grating. The net dispersion is zero. Now the entrance andcenter slits determine the band pass. Because the dispersion at the exit slit is zero, its width has noeffect on the band pass.
Subtractive dispersion is useful in imaging applications and in pulse studies.
In trying to relay an image through a single monochromator, the image is distorted by the angulardispersion that exists at the exit slit. This angular dispersion is cancelled in the subtractive double.
In pulse analysis, a single monochromator will cause temporal broadening because of the unequal
path lengths for light at the grating. In a subtractive double, these unequal path lengths arecancelled.
For users who wish further information we recommend the review article by Murty or Hutley's bookon diffraction gratings. Specific questions about the DK series can be answered by the staff at SP.

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6. Basics of Optical Spectrometers
NOTE TO READERS: The basics of optical spectrometers are familiar to mostreaders of this manual. However, terminology and interpretations of instrument
characteristics vary somewhat and these basics are repeated here as they apply toSpectral Products dispersive grating instruments come in two forms; monochromators that select one
wavelength and spectrographs that output a range of wavelengths, generally for use with an array
detector. Both share the same optical concept; they are one to one imaging systems in which one
image of the entrance slit appears at the exit for each wavelength passed through the instrument.
Figure 1 shows the optical elements of a typical monochromator. Light enters through an entrance
slit and is made into a nearly collimated beam by the collimating mirror. The light strikes adiffraction grating, which then disperses different wavelengths at different angles in the plane ofincidence. The focusing mirror collects the light from the grating over a range of angles (andtherefore from a range of wavelengths) and images the light to distinct positions near the exit slit.
The physical position of the image depends on its angle on the camera mirror, and the angle depends
upon its wavelength.

Entrance slit

Figure 1: Typical monochromator.

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13During Monochromator scanning, the intensity of light that passes the exit slit waxes and wanes as
the images of the entrance slit move across (see figure 2). The intensity at any time is the
convolution of the intensity profile of the entrance slit image with the transmission profile of the exit
slit.

Figure 2
Formation of a spectral line

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Figure 3 illustrates collection of a spectrum with a spectrograph and an array detector. In this case,the array detector elements see a signal that is proportional to the amount of the entrance slit image
that falls on the element.

Intensity

Pixel number
Pixel 0Array detector
Pixel 120

Entrance slit

Figure 3: Array detection in a
spectrograph and the resulting spectrum

Real sources are polychromatic, not monochromatic. For each wavelength present in the source animage of the entrance slit appears at slightly different position near the exit. The relationshipbetween wavelength and position is referred to as the reciprocal linear dispersion (RLD) of thespectrometer – often referred to as just dispersion and expressed in units of nanometer per
millimeter. The magnitude of the dispersion depends upon the wavelength, the focal length of the
focusing mirror, and the grating. The intensity of polychromatic light passed by the monochromatoris a complicated convolution of the intensity profile of the entrance slit image, the dispersion of thespectrometer, the spherical profile of the source, and the transmission profile of the exit slit.
Most optical spectrometers use diffraction gratings as the wavelength-dispersing element. The
diffraction grating has a series of parallel grooves spaced at about the wavelength of light. Eachgroove diffracts the incident light, and interference between the diffracted light from each groove
allows only one wavelength at each angle. Amazingly, gratings with as few as five grooves show
the properties of diffraction gratings.

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With the grooves of the grating perpendicular to the plane of incidence, light incident at angle i, isdiffracted at an angle, r, for wavelength lambda
n*lambda=d*(sin(i)+sin(r))
Where d is the grating groove spacing and n is the order of the diffraction (see Figure 4).

Incident light

Grating
normalBisector
Diffracted light

Figure 4: Diffraction by a grating. Light is incident at angle i.
Light is diffracted at angle r. The sum of l and r is a constant (2phi) in a monochromator. Theta = r-I is the angle of rotation from
specular reflection.
The preceding discussion is a simplification, neglecting the optical aberration resulting from a lessthan perfect image of the entrance slit. Nevertheless, the model provides a good foundation forunderstanding the properties of monochromators and spectrographs as discussed below. For a moredetailed treatment, see M.V.R.K.Murty, Theory and Principles of Monochromators, Spectrometers,
and Spectrographs, Optical Engineering, Vol.13, No.1, Jan 1974.
Properties of Spectrometers
The four important specifications in selecting a spectrometer are:
1. Wavelength resolution: the ability of the instrument to differentiate between different
wavelength of light.
2. Through-put: the percentage of light that can be sent from a light source through thespectrometer.
3. Spectral purity: the ratio of the inband light passed by the spectrometer to the transmitted
light that falls outside the selected spectral band.
4. Price: wavelength resolution, transmission effeciency, and spectral purity can vary
dramatically between instruments as the size, type, quality, and design of optical systems
differ.

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The relative importance of these specifications depends upon the application. A tradeoff can befound between wavelength resolution and price by selecting an instrument with a focal length
between 1/8 meter and ½ meter. Or, a double monochromator offers unbeatable spectral purity.
Wavelength resolution and its relatives
The resolution of a spectrometer is classically defined as the wavelength separation (∆λ) between
two ideal monochromatic spectral lines of equal intensity when their half maximum intensities
overlap (since the spectral lines are monochromatic, their line shape comes from the instrument).
This is approximately equivalent to saying that the resolution is the full width at half maximum(FWHM) measured for a single monochromatic line. Ideally, the resolution is limited by the numberof illuminated grooves on a grating (N) providing the grating is uniformly illuminated:(λ/∆λ) <= NIn practice however, the resolution is typically limited by slit widths, dispersion, and opticalaberrations in the system.
Other physical factors that may be external to the system can also impact resolution. Since three
data points are needed to define a peak, a monochromator made to take large scanning steps willshow a triangular peak (if it doesn’t miss the peak altogether). The FWHM of this triangle will be
wider than could be seen by taking finer steps. A similar effect limits the resolution of array-coupledspectrographs, where the minimum image size per data point is equivalent to a single pixel. At least
three pixels are needed to define the monochromatic peak, so the FWHM becomes limited to valueof 2-3 pixels widths multiplied by the RLD. Since there is no benefit to send multiple entrance slit
images to the same pixel, the entrance slit should be set to a width no narrower than one pixel width.
Band pass (Monochromator)
The FWHM is also called the band pass as it defines what band of wavelengths is passing throughthe spectrometer. Most often the term band pass is reserved for the FWHM at moderate to wide
equal width slits. Optical aberrations, diffraction, scanning method, pixel width, slit height,
uniformity of illumination and the like are negligible in this regime. The bandwidth then becomes
the dispersion times the slit width.
Bandwidth (Spectrograph)
Spectral Products uses the term Bandwidth to distinguish between the narrow band selected by the
monochromator and the wide spectrum output by a spectrograph. When coupled to an array
detector, the bandwidth designates the wavelength range thrown across the entire array.

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17Dispersion
The reciprocal linear dispersion of a spectrometer can be found in the following (Leon Radziemski,
Calculation of dispersion for a plane grating in a Czerny-Turner mount: a comment, Applied Optics.
Vol 20, No. 11, 1 June 1981):∆λ d ⋅ cos(r )
=
∆x
n⋅ f
Where r is the diffraction angle, x is the lateral distance along the focal plane, n is the order, and f the
focal length of the focusing mirror. Reciprocal linear dispersion is not a constant; it varies withwavelength and can exceed a factor of two over the useful spectral range.
Wavelength Precision, Reproducibility, and Accuracy
Wavelength precision is the gradation on the scale that the spectrometer uses in determining
wavelength. DK series monochromators and spectrographs employ a micro stepping grating drivethat gives a wavelength precision of .01nm per step (averaged) with a 1200 l/mm grating.

Wavelength reproducibility is the ability of a spectrometer that has been set to a wavelength given to
return to the original wavelength after the wavelength setting has been changed. This is a measureof the mechanics of the wavelength drive and the over all stability of the instrument.
Wavelength accuracy is the difference between the spectrometer’s set wavelength and the true
wavelength. It is not meaningful to apply a wavelength accuracy to a spectrograph because a wideband of wavelengths exists onto the detector array in a spectrograph. In monochromators,
wavelength accuracy must be checked against known spectral line wavelengths. SP checks its
monochromators at ten wavelengths across the 30% transmission spectral region of each grating.
Etendue, Spectral Energy Density, and Throughput
The percentage of light that can be sent from a light source through a spectrometer would be adesirable measure of its throughput. Unfortunately, the properties of sources vary so much that thismeasure would not provide a useful standard. Instead, two separate specifications are useful;etendue, a measure of the degree of coupling that can be achieved, and transmission efficiency, ameasure of how much of the input light exits the monochromator.

The etendue of an instrument is the product of an instrument’s physical aperture (cm2) and its
angular aperture (steridians). For a source of a given brightness [watts/(cm2*steridian)], themaximum power (watts) that can be coupled into an instrument is the product of the brightness andthe etendue. This is true because the brightness of a source cannot be changed; changing theapparent emission angle changes the apparent size in inverse proportion. The brightness (a LaGrange
Invariant) is unchanged. For a monochromator the etendue is:
E = Sw * Sh * Wg2 / f2

Where Sw = slit width
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Sh = slit height
Wg = grating width
F = instrumental focal length
In a chain of optics or optical instruments, the component with the smallest etendue will determine
the etendue of the system. For spectrometers it is useful to find the spectral energy density(watts/nanometer) that can be coupled. This can be found by dividing the etendue by the spectral
bandwidth.
D = E / (Sw / (f * A))
D = (Sh / f) * Wg2 * A
Where A is the angular dispersion of the grating.
The ratio of usable slit height to focal length is approximately constant across all monochromators. It
is limited by aberrations. Therefore, the spectral energy density depends primarily on the grating
width, and secondarily on the dispersion. To get the maximum throughput, use the widest, highest
dispersion grating available!
Etendue defines the coupling between a light source and a spectrometer. Transmission efficiency
describes the light loss within the spectrometer. The transmission efficiency becomes:
T = (Rm)n * Rg

Where Rm is the reflectance of a single mirror; n is the number of mirrors; Rg is the diffraction
efficiency of the grating.
Mirror reflectance is typically 0.92 for a protected aluminum mirror. In a four mirror system, about
70% is transmitted by the mirrors. SP offers custom broadband high reflectance coatings that canboost this efficiency to almost 95% in a four mirror system over about a wavelength octave.
Grating diffraction is quite complicated; it is both wavelength and polarization dependent. Grating
diffraction efficiency for a ruled grating typically reaches 90% at the blaze wavelength, falling off to
20% at 0.6 * lambda blaze, and 1.5 * lambda blaze. Holographic gratings typically have a flatter
30% efficiency. Careful selection of gratings to match the spectral regions of interest will allow goodtransmission efficiency at any wavelength.

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We can get a measure of total spectrometer through put per nanometer by multiplying the spectralenergy density by the transmission efficiency. The result is:
H = (Sh / f) * Wg2 * A * (Rm)n * Rg
The F/# Misconception
F/# is the measure of the acceptance angle of an optical instrument, and is generally defined at theratio of diameter to focal length. For years, F/# has been promoted as the measure of
monochromator throughput. However, as previously discussed, grating size is the dominant factor inthroughput. For example, a F/4 monochromator with a 30 mm2 grating will have 44% morethroughput than a F/2.5 monochromator with only a 25mm2 grating. Similarly, a F/4 monochromatorwith a 68mm2 grating will have 85% more through put than a F/3 monochromator with a 50mm2
grating. However, F/# is a useful concept in judging optimum coupling between spectrometers andsources or detectors. When F/#s are matched, the full aperture of the spectrometer will be utilized.
Stray Light
Stray light is all out of band light transmitted by a spectrometer. Because the spectral profile of thesource and the spectral sensitivity of the detector may enhance or under estimate the measured
spectral purity, two distinct methods of stray light measurement have evolved.
The ASTM has published a filter method for measuring stray light in spectrometers. This method
uses an incandescent lamp together with long and short pass blocking filters. This is useful formeasuring the contribution of stray light originating far from the band pass region when using acontinuum source.
Instruments SA introduced another method in the 1960s that is particularly relevant for laser
spectroscopy. Their measure of stray light is the inverse ratio of light at the peak of a 632.8nm laser
source to the light measured at 5 band passes from the peak. This method measures the contributionof stray light originating near the band pass region when using a line source. The following
discussion reviews most of the sources of stray light in spectrometers.
Rediffracted Light
Rediffracted light originates when a secondary order of diffracted light goes from the grating back to
the collimating mirror. The light may then be reflected back to the grating where it is rediffracted.
Because of the double bounce on the collimating mirror, the rediffracted light arrives at the exit slit
unfocused. Typically, it will be 0.1% of the ordinary signal. (A discussion of rediffracted light isgiven in Mittledorf and Landon). Rediffracted light usually appears as a long wavelength spectral
impurity in short wavelength light. Eliminating rediffracted light is a matter of design geometry.

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Secondary Sources
Secondary sources are sources of reflection within the spectrometer that direct stray light back into
the main beam path. A typical example is a well intentioned but troublesome baffle placed nearlyparallel to the beam path. Grazing incidence reflection off such a baffle will send light close enoughto the correct beam path to end up as stray light at the exit. Stray light from secondary sources
generally appears as a broad, flat-topped band in a spectral scan. Its intensity will be very sensitive
to the illumination conditions. Blackening the baffle does not reduce the effect because of the strong
reflectance of most materials at grazing incidence. In Spectral Products’ spectrometers, baffles aretreated as if they were mirrors. They are used to direct stray light out of the beam path.
Higher Order Diffraction
Higher order diffraction is an inescapable source of stray light in grating based spectrometers. The
grating equation N * lambda = d * (sin(i) + sin(r)) allows not only first order diffraction of
wavelength lambda, but coincident diffraction of wavelengths lambda/2, lambda/3, lambda/4…
Depending upon the blaze wavelength of the grating, the efficiency for diffraction of these higher
orders may actually be greater than the first order diffraction efficiency. The only way to eliminate
these shorter wavelengths is with a filter. Fortunately, long pass filters are easily obtained. Spectral
Products offers a series of filters that have been especially selected to suppress higher orderdiffraction.
GhostsGhosts are spurious spectral lines that originate in periodic irregularities in the diffraction grating.
For a 1200 l/mm grating, for example, a .01% spurious modulation of the groove profile at 1000
l/mm would produce ghost lines. Those ghost lines would have a spacing of about lambda * (1 –1/1.2) from each spectral line at wavelength lambda. Ghosts were originally observed inspectrometers because they used ruled gratings manufactured on mechanical ruling machines that
had intrinsic periodic errors in their mechanisms. Modern interferometrically controlled rulingmachines produce gratings that are free of such errors. Holographic gratings can also exhibit ghosts,
some with intensities exceeding conventionally ruled gratings. These holographic ghosts originate
in extraneous reflections during the hologram exposure.
Scatter
Scatter in a spectrometer is the primary source of diffuse background. Scatter does not originate inreflections from walls or other non-optical objects within the spectrometer. The probability of suchlight exiting the spectrometer is low. Scatter is diffuse reflection at the optical surfaces; the result of
surface roughness, scratches, and digs. Scatter from the optical surfaces is important because it ismost intense at low angles. This low angle scatter has a high probability of reaching the detector.
Spectral Products has extensive experience in producing low scatter laser optics, with scratch anddig of 10/5 or better. All of this experience has been applied to the optics of Spectral Products’
spectrum.

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7. Sources of Errors in Spectrometers
Spectral Products’ monochromators use a two-point calibration method, that is, the zero-order point
and one wavelength. The zero-order point can be determined using virtually any light source,
broadband or monochromatic, diffuse or coherent, since the grating is acting essentially as a mirror
at this point. The slits are taken down to their minimum aperture (typically 10 m) and then thegrating position is adjusted to produce maximum throughput. The “zero” command then stores thislocation into non-volatile RAM; the number stored is the number of motor steps from the device’s
physical home position (determined by location sensors on the grating turret and motor shaft) to the
optimized optical zero-order point.
The second point can be calibrated at almost any arbitrary wavelength, usually chosen to be
somewhere in the middle of the particular grating’s spectral response. The monochromator compares
its actual physical location with the ideal location for that wavelength (in terms of motor steps fromzero) to produce the calibration number. This calibration number is not a count of motor steps or
physical location, but a scaling factor used as a multiplier throughout the range of grating motion.
Therefore, the monochromator takes the ideal number of motor steps (if the unit were optically andgeometrically perfect) and scales it by the calibration factor. Each grating in a multiple grating
monochromator has it’s own zero and calibration numbers, compensating for mechanical or opticalvariations as the gratings are changed.

7.1 Sources of error
The wavelength appearing at the exit slit of a Czerny-Turner monochromator (the design used in allSpectral Products monochromators) is given by the following equation.
λ =2cos(φ/2)sin(θ)/(Ν∗Κ)

where φ = Ebert angle
18.7° for a DK240
9.2° for a DK480
25.4° for a CM110
θ = Grating rotation from 0nm (deg)Ν = Groove density (g/mm)
Κ = Diffraction order

Any of the above terms (with the exception of Κ an integer) may be in error. The Ebert angle, that is
the angle subtended at the grating surface by the central rays from the collimating and focusing
mirrors, will vary from unit to unit. The mirrors may not be ground to precisely the same focal
length or may be mounted slightly off center, either of which will shift Κ slightly. Similarly, thegroove density of the grating may not be ideal. Gratings cut from the same master will be very close
to one another, but may differ by some percentage from the stated value.
Both of the above values will affect the calibration of a given instrument, but once fixed, theyremain constant for that particular unit and are accounted for by the calibration factor. By far themost critical source of error is the value of θ. Spectral Products monochromators use a worm/wheel
grating drive driven by a step motor. The sources of error in such a system are multiple: nonSpectral Products
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22

linearity of the worm wheel, non-linearity of the worm, step-angle errors in the motor, eccentricity of
any of the shafts or assemblies, and any play in any part of the assembly. We attempt to ameliorate
these errors through such means as:
1.
2.
3.
4.
5.

Specifying ABEC 7-tolerance level in the bearings.
Specifying AGMA Q14 tolerance worms and worm wheels.
Specifying bores to 0.00025” tolerance, shaft run outs to .001”.
Utilizing the highest quality step motors and driver electronics available.
Testing and run-in of all assemblies prior to integrating them into a monochromator,
rejecting and/or rebuilding them as necessary.
6. Testing in the final unit, rejecting and replacing drives that do not meet criteria for
accuracy and repeatability.
The above factors are all an attempt to achieve accuracy on the order of step-size resolution of each
instrument: 0.00025° for the DK series. But such accuracy is not theoretically possible, even withthe tightest of tolerances.
As an example, the DK series use a 64-pitch worm wheel, 180 tooth, and 2.8125” pitch diameter.
AGMA Q14 tolerances give a tooth-to-tooth error tolerance of 0.00014”, with a total composite
tolerance of 0.00032”. Therefore, the tooth-to-tooth angular error is given by:∆θ =sin -1(0.00014/(2.8125/2))=0.0057°
Total composite error (i.e. from one random tooth to another) would be 0.013 so; the worm wheelalone can contribute an absolute error of 50 micro steps in a standard DK. For a 1200g/mm gratingaround 600nm, that error would be about 0.35nm, and we are only considering errors introduced by
the worm wheel itself. Experiments have demonstrated that the motor/shaft/worm assembly
contributes errors much more important in determining the usability of a given grating drive. Theseerrors tend to be pseudo-sinusoidal, cycling every 2°of grating motion, and at least as great in
amplitude as the maximum wheel error.

7.2 Acceptance criteria
Monochromators are aligned and calibrated at Spectral Products by using a HeNe laser to level andalign the optics, and to give an approximate calibration (assuming the laser frequency is within thegrating’s response range). A spectral line source such as an Ar or Hg lamp is then used to fine-tune
the calibration, checking for repeatability and accuracy. Typically 10 known spectral lines fallingwithin the 30% transmission range are examined for each grating. The calibration factor isdetermined by calibrating to the particular spectral line, which gives the best fit to the line set being
examined. Automated scans then check for repeatability throughout the line set, recalibrating theunit as necessary. A technician, who writes the calibration into non-volatile RAM, then checks final
calibration.
Acceptable errors for various grating groove densities are listed below.

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Density (g/mm)
3600
2400
1200
600
300
150
75
50

Accuracy (nm)
±0.10
±0.15
±0.30
±0.60
±1.20
±2.40
±4.80
±7.20

Repeatability (nm)
±0.03
±0.06
±0.10
±0.20
±0.40
±0.80
±1.60
±2.40

Note that the acceptable error varies inversely with the groove density. This is because it is actually
the same angular error of grating position. Further, these numbers are generalized to the middle of
the grating range (about 30° from the zero order point). As can be seen from the grating equation, theoutput wavelength is not a linear function of the grating angle, therefore the same absolute error in
grating position will produce a varying amount of wavelength error across the range of the grating.

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8. Software Operation
8.1 Command Summary
The subscript D indicates the decimal value of the byte is listed.
CLEAR
This command restores factory calibration values for the grating and slits.
This command also executes a reset, which returns the grating to home position.

To DK240/480: <25> D
From DK240/480: <25> D
From DK240/480: <Status Byte>
DK240/480 Action:
Reset monochromator
From DK240/480: <24> D
CSRThis command sets monochromator to Constant Spectral Resolution mode. The slit width willvary throughout a scan. This is useful, for example, where measurement of a constant interval of
frequency is desired (spectral power distribution measurements).
To DK240/480: <28> D
From DK240/480: <28> D*To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status Byte>
DK240/480 Action:
Set mono to CSR mode
From DK240/480: <24> D*Band pass value = HighByte*256 + Low Byte (in hundredth’s of nanometers)
See Appendix F, page 36, Constant Spectral Resolution
ECHO
The ECHO command is used to verify communications with the DK240/480.
To DK240/480: <27> D
From DK240/480: <27> D
DK240/480 Action:
No action
GCAL
This command allows recalibration of the monochromator positioning scale factor and should be
used immediately after using the ZERO command (see page 15). The monochromator should be
set to the peak of a known spectral line, then the position of that line is input using the
CALIBRATE command.
CAUTION: Use of this command will erase factory settings.

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To DK240/480: <18> D
From DK240/480: <18> D
To DK240/480:<High Byte> <Mid Byte> <Low Byte>
DK240/480 Action:
If 65536 * <High Byte> + 256 * <Mid Byte>+ <Low Byte> (inhundredths of nm) is a valid position; then the scale factor used in determining
position will be recalibrated to make the current position agree with the input
position. The grating will return to home after completion.
From DK240/480: <Status Byte>
From DK240/480: <24> D
GOTOThis command moves the monochromator to a selected position. Valid values of position are
grating dependent and are described in Appendix C.
To DK240/480: <16> D
From DK240/480: <16> D
To DK240/480:<High Byte> <Mid Byte> <Low Byte>
DK240/480 Action:
If valid, grating will move to 65536 * <High Byte> + 256 * <Mid
Byte>+ <Low Byte> (in hundredths of nm)
From DK240/480: <Status Byte>
From DK240/480: <24> D
For example, the command to instruct the monochromator to GOTO the wavelength 250 nm
could be sent as the 4 bytes <16> D <0> D <97> D <168> D (units are in hundredths of nm). Here,<16> D specifies the GOTO command while <0> D <97> D <168> D specifies the destination of
25000(in hundredths of nm).
GRTID?
Returns the 6 byte current grating ruling identifier.
To DK240/480: <19> D
From DK240/480: <19> D
From DK240/480: <Byte 1><Byte 2><Byte 3><Byte 4><Byte 5><Byte 6>
1 = number of gratings installed in the monochromator (1-3)
2 = number of grating currently in use (1-3)
3 = high byte of current grating ruling (g/mm)
4 = low byte of current grating ruling (g/mm)
5 = high byte of current grating blaze wavelength (nm)
6 = low byte of current grating blaze wavelength (nm)From DK240/480: <Status Byte>
DK240/480 Action:
No action
From DK240/480: <24> D

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GRTSEL
This command changes gratings , if additional gratings installed..
To DK240/480: <26> D
From DK240/480: <26> D
To DK240/480: <One Byte> (1, 2, or 3 depending on gratings installed)
From DK240/480: <Status Byte>
DK240/480 Action:
If valid, slews to new grating and automatically resets..
From DK240/480: <24> D
RESETThis command returns the grating to home position.
To DK240/480: <255> D <255> D <255> D
DK240/480 Action:
Grating will return to home position
SLIT RESETThis command resets one or all gratings to home position.
To DK240/480: <43> D
From DK240/480: <43> D
To DK240/480: <Slit Byte>
0: All slits
1: Entrance slit
2: Exit slit
3: Middle slit (DK242 only)
From DK240/480: <Status Byte>
DK240/480 Action:
One or all slits will return to home position.
From DK240/480: <24> D
SCANThis command scans the monochromator between the present position and a alternate specified
wavelength, at a rate determined by the SPEED command. Valid values of position are grating
dependent.
To DK240/480: <12> D
From DK240/480: <12> D
To DK240/480:<High Byte> <Mid Byte> <Low Byte>
From DK240/480: <Status Byte>
DK240/480 Action:
Scans the grating to the desired wavelength
From DK240/480: <24> D

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SCAN UP
Scans (faster than slewing) the grating towards the longer wavelength until the DK receives a 24
or reaches the limit of the grating operation.
To DK240/480: <9> D
From DK240/480: <9> D
DK240/480 Action:
Slews until 24 received or limit is reached.
To DK240/480: <24> D
From DK240/480: <Status Byte>
From DK240/480: <24> D
SCAN DOWN
Scans (faster than slewing) the grating towards the shorter wavelength until the DK receives a 24
or reaches the limit of the grating operation.
To DK240/480: <3> D
From DK240/480: <3> D
DK240/480 Action:
Slews until 24 received or limit is reached.
To DK240/480: <24> D
From DK240/480: <Status Byte>
From DK240/480: <24> D
SERIAL?
Returns the 5 digit serial number of the monochromator.
To DK240/480: <33> D
From DK240/480: <33> D<1st Digit><2nd Digit><3rd Digit><4th Digit><5th Digit>
From DK240/480: <Status Byte>
DK240/480 Action:
No action.
From DK240/480: <24>
SLEW UP
Slews the grating towards the longer wavelength until the DK receives a 24 or reaches the limitof the grating operation.
To DK240/480: <8> D
From DK240/480: <8> D
DK240/480 Action:
Slews until 24 received or limit is reached.
To DK240/480: <24> D
From DK240/480: <Status Byte>
From DK240/480: <24> D

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SLEW DOWN
Slews the grating towards the shorter wavelength until the DK receives a 24 or reaches the limitof the grating operation.
To DK240/480: <2> D
From DK240/480: <2> D
DK240/480 Action:
Slews until 24 received or limit is reached.
To DK240/480: <24> D
From DK240/480: <Status Byte>
From DK240/480: <24> D
SLIT?
Returns the current four byte (six byte for DK242) slit width. First two bytes are high and low
byte of the entrance slit width in microns. Second two bytes are the high and low byte of the exit
slit width. For DK242, the last two bytes are for middle slit width.
To DK240/480: <30> D
From DK240/480: <30> D
From DK240/480: <4 or 6 Bytes><Status Byte>
DK240/480 Action:
No action.
From DK240/480: <24> D
SLTADJ
Adjusts all slits to a given width.
To DK240/480: <14> D
From DK240/480: <14> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
If valid, adjusts all slits to the new width.
From DK240/480: <24> D
S1ADJ
Adjusts entrance slit to a given width.
To DK240/480: <31> D
From DK240/480: <31> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
If valid, adjusts the entrance slit.
From DK240/480: <24> D

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S2ADJ
Adjusts exit slit to a given width.
To DK240/480: <32> D
From DK240/480: <32> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
If valid, adjusts the exit slit.
From DK240/480: <24> D
S3ADJ (DK242 0nly)
Adjusts middle slit to a given width.
To DK240/480: <34> D
From DK240/480: <34> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
If valid, adjusts the middle slit.
From DK240/480: <24> D
S1CAL
Allows for entrance slit calibration. Uses the same procedure as GCAL but with a two byte slit
width specifier.
To DK240/480: <22> D
From DK240/480: <22> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
No immediate action.
From DK240/480: <24> D
S2CAL
Allows for exit slit calibration. Uses the same procedure as GCAL but with a two byte slit width
specifier.
To DK240/480: <23> D
From DK240/480: <23> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
No immediate action.
From DK240/480: <24> D

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S3CAL (DK242 only)
Allows for middle slit calibration. Uses the same procedure as GCAL but with a two byte slit
width specifier.
To DK240/480: <35> D
From DK240/480: <35> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
No immediate action.
From DK240/480: <24> D
SPEED
Selects the speed at which the monochromator may scan.
To DK240/480: <13> D
From DK240/480: <13> D
To DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:No immediate action. If a valid value (in nm/min) is selected, the SCAN
command will thereafter cause the monochromator to move at approximately
a speed value of (256 * <High Byte> + <Low Byte>)
From DK240/480: <24> D
Relevant scan speeds (nm/minute):
If grating grooves are greater than or equal to 1200 g/mm, then the valid values for scan speeds
will be from 1 to 600
If grating grooves are less than 1200 g/mm, then the valid values for scan speeds will be: (integer
numbers from 1 to 600) x 1200/current grating groove. For example, with a 600-groove grating,valid values are: 2, 4, 8, …1200.
Speed will be an integer number and is truncated after the calculation.
SSPEED?
Returns the current scan speed.
To DK240/480: <21> D
From DK240/480: <21> D
From DK240/480: <High Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
No immediate action.
From DK240/480: <24> D

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STEP DOWN
Moves the grating one step toward UV.
To DK240/480: <1> D
From DK240/480: <1> D
From DK240/480: <Status byte>
DK240/480 Action:
If valid, moves the grating to a shorter wavelength in 1 motor step
From DK240/480: <24> D
STEP UP
Moves the grating one step toward IR.
To DK240/480: <7> D
From DK240/480: <7> D
From DK240/480: <Status byte>
DK240/480 Action:
If valid, moves the grating to a longer wavelength in 1 motor step
From DK240/480: <24> D
TESTPerforms automatic self diagnosis.
To DK240/480: <17> D
From DK240/480: <17> D
From DK240/480: <Status byte>
DK240/480 Action:
Mono initiates self diagnostic routine and will reset after sending <24>
From DK240/480: <24> D
WAVE?
Returns the 3 byte current wavelength setting.
To DK240/480: <29> D
From DK240/480: <29> D
From DK240/480: <High Byte> <Mid Byte> <Low Byte>
From DK240/480: <Status byte>
DK240/480 Action:
The current wavelength is: 65536 * <High Byte> + 256 * <Mid
Byte>+ <Low Byte> (in hundredths of nm)
From DK240/480: <24> D

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ZERO
This command sets the current wavelength to 0.00nm.
CAUTION: Use of this command will erase factory settings.
To DK240/480: <52> D
From DK240/480: <52> D
To DK240/480: *<One byte> D
DK240/480 Action:
The current zero offset values of the gratings are saved as the zeroorder position and sets the current position to 0.00 nm.
From DK240/480: <Status Byte>
From DK240/480: <24> D*<One Byte>
can be 1 or 2.

For the DK240/480, the <one byte> is always 1, for the DK242 the <one byte>

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8.2 Remote Operation

The Digikröm 240/480 can be controlled by any remote computer that has an RS-232 serial
communications port. Total control over the grating table and continuously variable slits can beachieved through a simple protocol. The RS232 connection requires a cable with a DB25-M
subminiature connector (to connect to the monochromator) and a computer communications port
connector at the other end, as appropriate for the user. Spectral Products offers a DK24AT,
DK24PS, DK24MA and DK24IC cable for connecting to AT, PS2, MAC style computers and GPIB,
respectively.
Pin Assignments for the Female DB-25 Connector at Ends of DK240/480
Pin
1
2
3
4
5

NAME
GND
TxD
TxD
RTS
CTS

6

DTR

7

GND

8-24
25

+5v

FUNCTION
Chassis Ground
Data in (from computer to DK)
Data out (from DK to computer)
Clear to Send (output from host to DK)
Request to Send (output from DK to
host)
Data Terminal Ready(output from DK
to host)Signal Ground (common with chassis
ground)Not used
Receive current loop return

The pin assignments above are mapped one-to-one between the cable connection of
a Digikröm and an IBM-AT style serial communications port.
The Digikröm emulates data communication equipment (DCE) when communicating with a remotecomputer. No crossing of data or handshake lines are necessary. The request to send/clear lines areused for handshake protocol of control communications. The Digikröm DK240/480 is factory
configured and the character length; number of stop bits and parity cannot be changed. Its signal
levels and format are the same as those that are specified for the RS-232.
The computer must be set to the Digikröm DK240/480 data type and baud rate
Character length:
8 bitsBaud rate:
9600 bits/sec
Stop bits:
1
Parity:None

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Clear

Byte
Description(Decimal)<25>
Returns grating and slits to original factory calibration.

CSR

<28>

Adjusts the entrance and exit slits to the CSR value and sets the mono
to the CSR mode.

Echo

<27>

Remote handshake byte, <27> = yes.

Gcal

<18>

This command prompts the user to “Enter the Calibration value” in thecurrent units. Changes made using this command will erase the values
preset at the factory.

Goto

<16>

This command allows the user to enter a new wavelength. Press the“Enter” key to complete this command.

Grtid?

<19>

Returns the current grating ruling identifier.

Grtsel<26>

Changes grating if additional gratings are installed.

Reset

<255>

This command returns the grating to the home position.

Scan

<12>

This command allows the user to enter the wavelength.

Scan Up<9>

Scans (faster than slewing) the grating towards the longer wavelength
until the DK receives a 24 or reaches the limit of the grating operation.

Scan Down<3>

Scans (faster than slewing) the grating towards the shorter wavelength
until the DK receives a 24 or reaches the limit of the grating operation.

Serial?

<33>

Returns the serial number of the monochromator.

Slew Up<8>

Slews the grating towards the longer wavelength until the DK receives
a 24 or reaches the limit of the grating operation.

Slew Down<2>

Slews the grating towards the shorter wavelength until the DK receives
a 24 or reaches the limit of the grating operation.

Slit Reset<43>

This command resets one or all slits.

Slit?

<30>

Returns the current slit width.

Sltadj<14>

Adjusts slits to a given width.

Speed

<13>

Sets the scan rate at which the grating rotates during Scan operation.

Sspeed?

<21>

Returns the current scan speed.

Step down<1>

Steps the grating motor one step towards UV.

Step up<7>

Steps the grating one step toward IR.

S1adj

<31>

Adjusts entrance slit only.

S2adj

<32>

Adjusts the exit slit only.

S3adj

<34>

Adjusts the middle slit on DK242 only.

S1cal

<22>

Allows for entrance slit calibration. Changes made using thiscommand will erase the values preset at the factory.

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S2cal<23>

Allows for exit slit calibration. Changes made using this command will
erase the values preset at the factory.

S3cal

<35>

Allows for middle slit calibration on DK242 only. Changes madeusing this command will erase the values preset at the factory.

Wave?

<29>

Returns the current wavelength setting.

Zero

<52>

Sets the current wavelength to 0.00nm. Changes made using thiscommand will erase the values preset at the factory.

The Cancel byte, <24> (sometimes preceded by a status byte) terminates operation of the
Digikröm. This does not apply to the Echo and Reset commands. A status byte is used toindicate errors or status information.

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8.3 Encoding/Decoding Data Bytes
Many computer-based commands (RS-232) both send and receive information in the form of multi-byte specifiers. For anumber given in decimal form, such as base 10, to be sent to the monochromator, the number must first be broken down
into hexadecimal bytes (8 bits). Then, each byte is converted into a decimal value. This decimal value is transmitted as a
ASCII character to the communication device. Then, the monochromator translates the characters into the form
necessary to perform the operation. Conversely, the monochromator sends the data back in decimal characters. Each isa byte long, and the computer application must convert these separate bytes back to a useful decimal value.

•

ENCODING DATA BYTES

The desired command is GOTO 100 nm.
The GOTO command in RS-232 is specified as:<16><HIBYTE>><MIDBYTE><LOWBYTE>

Where the current UNITS selected determine the units for the two-byte specifier. For this example,
the units are in Angstroms.
Step 1:NOTE:

Convert the desired specifier to proper units.
100 nm = 10000 hundredths of nm

The following steps will be shown two ways: (A) with conversions performed by aunspecified algorithm, for example, using a calculator with decimal-hex conversion
capability, and (B) using a numeric algorithm that is more suitable for computers.

• Method A:
Step 2:
Convert to Hexadecimal
1000(base 10) = 2710(base 16)
Step 3:

Break the hex value into three bytes
2710(base 16) => 00 | 27 | 10
Hi Mid Lo

Step 4:

Convert each byte to its decimal equivalent
Hibyte:00(base 16) => 00(base 10)
Midbyte: 27(base 16) => 39(base 10)
Lowbyte: 10(base 16) => 16(base 10)

Step 5:

Send the command. The specifiers are 0, 39 and 16.

• Method B:
Note: All the following numbers are given in decimals.
Step 2:

Divide by 65536 and round down to the nearest whole number.
EX: 1000 / 65536 = 0.01526 rounds to 0 = Hibyte

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Step 3:Calculate middle byte
EX: 10000 - (65536 x Hibyte) = 10000 - 0 = 10000
10000/256=39.0625. Truncates to 39 = Middle byte

Step 4:

Adjust the remainder.
EX: 0.0625 x 256 = 16 = Lowbyte

Step 5:

Send the command. The specifiers are 0, 39 and 16.

•

DECODING DATA BYTES

The desired command is QUERY POSITION.
The QUERY POSITION command returns two bytes indicating the current wavelength, in the form
<HIBYTE>><MIDBYTE><LOWBYTE>
To be useful to the user, the two bytes must be converted back to a single decimal number. As
before, we can do this by either method A or method B, by essentially reversing the above
procedures.
For this example, the QUERY POSITION command returns the ordered pair (5, 4, 106), Hibyte,
Lowbyte respectively, as the current wavelength. For this example, the units are in Angstroms.
Method A:
Step 1:

Convert each byte to its hex equivalent
Hibyte:05(base 10) = 5(base 16 )Midbyte: 04(base 10) = 4(base 16 )Lowbyte: 106(base 10) = 6A(base 16)

Step 2:

Concatenate the 3 bytes to form one hex number
05 | 04 | 6A = 05046A(base 16)

Step 3:

Convert the hex number to a decimal05046A(base 16) = 328810 = 3288.10 nm.

Method B:
Note: All of the following numbers are in hundredths of nm.
Step 1:
Use the formula:
Wavelength (λ) =(Hibyte x 65536) + (Midbyte x 256) + Lowbyte(5 x 65536) + (4 x 256) + 106 = 328810 hundredths of nm = 3288.10nm

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8.4 Status Bytes
Whenever the DK240/480 is given a command, it will respond with a status byte that indicates whether thecommand was accepted. Each bit in the status byte has a meaning, which is given below. When acommand is not accepted, some of the bits of the status byte will indicate the reason. In general, if <Status
Byte>D is smaller than 128, then the command was accepted.

Bit 7: 0 if specifier value acceptable (bit 4 active, bits 5,6 inactive).
1 if specifier value not acceptable (bits 5,6 active, bit 4 inactive).
Bit 6: 0 if specifier value not equal to present value (bit 5 active)
1 if specifier value equal to present value (bit 5 inactive)
Bit 5: 0 if the specifier was too small
1 if the specifier was too large
Bit 4: 0 if scan is negative going (GOTO and SCAN only)
1 if scan is positive going (GOTO and SCAN only)
Bit 3: 0 not used
Bit 2: 0 if monochromator is not in CSR mode.
1 if monochromator is currently in CSR modeBit 1: 0 not used
Bit 0: 1 if motor movement in negative orders (for ZERO operation only)
0 if motor movement in positive order (for ZERO operation only)

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8.5 Novram Program

These commands are Read from Novram and Write to Novram. There are 128
memory locations in the Novram, and their addresses are from 0 to 127. Table on page
40 gives the address and the meaning in the Novram memory.

8.5.1 READ FROM NOVRAM
These commands read a word (0 to 65535) to a Novram address (1 to 64) indicated byaddress byte.
To DK240/480: <56> D
From DK240/480: <56> D
To DK240/480: <Address Byte>
From DK240/480: <Data High Byte><Data Low Byte>
DK240/480 Action:
No immediate action. The word read from the Novram
address is (256*<High Byte><Low Byte>)
From DK240/480: <Status Byte>
From DK240/480: <24> D
Data Byte contains a returned value, and Address Byte is 0 through 127.

8.5.2 WRITE TO NOVRAM
These commands write a word (0 to 65535) to a Novram address (1 to 64) indicated byaddress byte.
WARNING !!!
Improper use of this command may corrupt the configuration andcalibration information of the monochromator. See ‘Restore Disk’supplied to restore values
To DK240/480: <59> D
From DK240/480:<59> DTo DK240/480: <Address Byte><Data High Byte><Data Low Byte><Checksum Byte>
DK240/480 Action:
No immediate action. Writes a word (2 bytes) to themonochromator’s non-volatile memory. CheckSum Byte = Address Byte + Data
High Byte + Data Low Byte. The addition is operated in one byte method. The Carybit will truncate if it exists. Therefore, it is always Checksum <=255.
From DK240/480: <Status Byte>
From DK240/480: <24> D

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8.6 RS-232 (Serial) Connection Diagram

To DK

To
ComputerConnector, DB25 Female

Connector, DB9, Male

Fig. I-2 DK To PC 25-Pin Serial Port

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NOVRAM ADDRESS
Address
1
2
3
4
5
6
7
8
9
10
11 / 42
12 / 43
13 / 44
14 / 45
15 / 46
16 / 47
17 / 48
18 / 49
19 / 50
20 / 51
21 / 52
22 / 53
23 / 54
24 / 55
25 / 56
26 / 57
27 / 58
28 / 59
29 / 60

The meaning of the content
AAAAH if programmed, else random
Serial Number
Source 1 (not used)
Source 2 (not used)
Source 3 (not used)
Source 4 (not used)High byte: Current source (not used)
Low byte: IEEE address
Number of motor steps of Slot 1
Number of motor steps of Slot 2
Number of motor steps of Slot 3
Grating 1 Blaze
Grating 2 Blaze
Grating 3 Blaze
Zero Offset of Machine 1, Grating 1
Zero Offset of Machine 1, Grating 2
Zero Offset of Machine 1, Grating 3
Zero Offset of Machine 2, Grating 1
Zero Offset of Machine 2, Grating 2
Zero Offset of Machine 2, Grating 3
Grating 1 Calibration, High byte
Grating 1 Calibration, Low byte
Grating 2 Calibration, High byte
Grating 2 Calibration, Low byte
Grating 3 Calibration, High byte
Grating 3 Calibration, Low byte
Entrance Slit Offset
Exit Slit Offset
Middle Slit Offset, for DK242 only
High byte: # of gratings
Low byte: Bit 0 – 0 = Full step, 1 = Micro step(1)
Bit 1 – 0 = 1 machine, 1 = 2 machine(2)
Bit 2 – 0 = no OMA, 1 = OMA(4)
Bit 3 – 0 = no CSR, 1 = CSR(8)
Bit 4 – 0 = no GPIB, 1 = BPIB(16)
Bit 5 – 0 = Unilateral, 1 = Bilateral(32)

30 / 61
31 / 62
32 / 63
33 / 64
34
35
36
37
38
39
40
41

Model Number
Grating 1 groove/mm
Grating 2 groove/mm
Grating 3 groove/mm
Machine 2 Slot 1
Machine 2 Slot 2
Machine 2 Slot 3
Hi byte not used
Low byte subtractive
Reserved
Reserved
Reserved
Reserved

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8.7 GPIB(IEEE-488) Interface Option
The IEEE-488 Bus(General Purpose Interface Bus) provides an electrical and mechanical system forinterconnecting electronic measurement devices. With the GPIB Interface Option installed, themonochromator can be controlled by any GPIB controller. All GPIB commands are echoed back to thecontroller. The echoed commands have an apostrophe (') appended at the beginning and end of the echo
word.
Setting the Digikröm GPIB address.
The GPIB address of the monochromator can be set to any address between 1 and 31. To set or reset the
GPIB address, press the "OPTIONS" key, while the READY screen is displayed on the Digikröm Control
Module. The control module will display the following:

OPTIONS: 1 =CALIBRATE
Press the "2" key to display the REMOTE screen. The display will then show the following:

1 = REMOTE MODE
2=SET GPIB ADDRESS
Press the "2" key again to set or view the GPIB address. The display will show:

GPIB ADDRESS = x
ENTER 1 - 31
X is the current GPIB address. Input the desired GPIB address, between 1 and 31, and press the enter key.
To exit without changing the GPIB address, press the back arrow key, ←, or the "ENTER" key if noaddress has been input.
Press any key, except RESET, to return the monochromator to manual control operation.
Timeouts: After initializing the GPIB interface card, set the timeout to longer than 180 seconds. If only
one grating is installed in the monochromator the timeout can be set to 90 seconds or longer. The longer
timeout allows the monochromator to perform a Grating select routine before a timeout error can occur.
For more information on sending and receiving GPIB commands, see GPIB demonstration programprovided with your monochromator.
GPIB status bytes are returned in ASCII form as defined below. The status bytes are enclosed in the backand forward arrow keys (<>) and separated by commas .eg. <OK,SC+,CSR>.
<OK> <SV> <SV0> <SV1> <SV2> <SC+> <SC-> <CSR> -Command executed and specifier value acceptable
Specifier value sent is the same as the current value
Specifier value is too small
Specifier value is too big
Invalid CSR bandwidth value sent
Positive going SCAN or GOTO(towards longer wavelengths)
Negative going SCAN or GOTO(towards shorter wavelengths)
Monochromator is currently in the CSR mode

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GPIB Commands/Query

Specifier value definition'GOTO XXXXXXX'XXXXXXX = wavelength unit, enter as tenths of angstroms'SCAN XXXXXXX''GCAL XXXXXXX''SSPEED yyyy'

yyyy = scan speed in nm/min'S1CAL ZZZZ'ZZZZ = slit width in microns'S2CAL ZZZZ''SLTADJ ZZZZ''S1ADJ ZZZZ''S2ADJ ZZZZ''CSR xxxx'

xxxx = Bandpass value (chart on page 37)'MANUAL s's = step direction, 7 = positive, 1 = negative'GRTSEL n'n = Grating number 1, 2 or 3

'ZERO g'

g = machine number 1 or 2

'CLEAR''TEST''WAVE?''SLIT?''*IDN?''ECHO'
Except for the *IDN? command, all commands operate as defined in Command Summary, page 10. Whenthe *IDN? query is sent, the monochromator will return the firmware identifier string:<SP INSTRUMENTS, DIGIKROM YYYY,XXXXX>YYY is the model number of the monochromator, 240/242/480, XXXXX is the five digit serial number ofthe monochromator
After sending the Echo byte of the TEST command, the monochromator will output the TEST status byte.
The monochromator will perform a reset operation after any of the following commands are sent:*GRTSEL, *GCAL, *S1CAL, *S2CAL, TEST, CLEAR or RESET

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9. Hand Held Control Module Operation
The DK2401 Handheld Controller sends command instructions to the DK240/480.

9.1 Operation
The DK2401 receives power from the monochromator. Connect the controller RS232
cable to the RS232 connector at one end of the DK240/480. Apply power to themonochromator. Press the ON/OFF button of the DK2401 to turn it on. The controller
display will read
Spectral, LLC.
Digikröm, DK-Series

Once the DK240/480 finishes initialization routines, the controller display will read
Gr/mm: 01200
SS:0100nm/m
eSW:0050µ
READY…
••••••

Grtg: 1/3
SN:54321
xSW:0050µ
λ = 0100.00nm

Gr/mm : 01200 - denotes 1200 grating groove per millimeter.
Grtg : 1/3 - denotes current grating is 1 of 3 total gratings.
SS : 0100 nm/m - denotes the current scan speed 100 nanometers per minutes.
SN : 54321 - denotes the monochromator serial number.
eSW : 0050 u - denotes the entrance slit width, 50 microns.
xSW : 0050 u – denotes the exit slit width, 50 microns.
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•
•

READY: - denotes the monochromator is ready to accept commands.
λ= 0100.00 nm - denotes the current wavelength at 100 nm.

The keyboard consists of 23 keys, 13 control keys, and 10 number keys, including adecimal point. The numeric keys are used to enter parameter values in response toprompts from the LCD display. The LCD display will prompt the user to enter a value.
The user responds by pressing the appropriate numerical keys, and the decimal point key
for wavelength operations, correcting erroneous entries with the backspace key. Once thedisplayed value is correct, the user then presses the ENTER key, and the entry is acceptedby the controller. Please note that the numerical entries must agree with the units
indicated by the LCD prompts.
The command keys are used to initiate (or halt) the many functions of the Digikröm
240/480. The keyboard layout is shown above. Described below are the various
command key functions of the Digikröm monochromator.
•

MANUAL/. This command allows the user to change the rotation of the gratingby one motor step. It also allows the user to set a new zero position for either λequal to zero or a nonzero value.

The MANUAL/. key shifts control of the grating table back and forth between manualand automatic. When the user shifts from AUTO to MANUAL mode by pressing the
MANUAL/. key, the display responds as the image below.
MANUAL:
1: Stp-7: Stp+
CURRENT

2:Slw-8:Slw+

CNCL=quit
3:Run-9:Run+λ = 0100.00 nm

In MANUAL mode, the wavelength of the monochromator is controlled by pressing
selected keys on the numerical pad. The numbers on the bottom row 7, 8, and 9, control
the scan speed in the increasing wavelength direction, and the numbers on the top row 1,
2, and 3, control the decreasing wavelength scan speed. The numbers on the middle row
have no effect on the scan. The 7 or 1 number key in each row produces a single step
each time it is pressed. The 8 or 2 number key in each row produces a continuous scan at
one half the scan speed, as long as the key is pressed. The 9 or 3 number key in each row
will scan at the speed shown in the display.
The user can return control to the AUTO mode by pressing the MANUAL/. key again orpressing CANCEL key.
All other keys, with the exception of the 1, 2, 3, 7, 8, 9, CANCEL and MANUAL/. keys,
will have no effect on the status of the DK240/480. If the user attempts to use inactive
keys while in the MANUAL mode, a high pitched inactive key tone will result.
Note: All of the following commands must be executed from the AUTO mode.
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•

GOTO
The user can change the λ wavelength by entering a value for a
new wavelength and then pressing enter. This command changes the grating
angle, which in turn changes the wavelength at the exit slit.

Use the GOTO command to instruct the DK240/480 to find a discrete wavelength. The
values of wavelength are grating dependent. Once the key is pressed, the display reads:
ENTER: goto
CURRENT

CNCL=quit
λ = 0100.00 nm

GOTO

λ = xxxx.xx nm

ENTER. During this part of the operation, the display reads:

ENTER: goto
CURENT

CNCL=quit
λ = 0100.00 nm

RUNNING……
Once the DK240/480 finds the specified wavelength, the GOTO operation stops and the
READY screen appears.
•

SCAN
The user can scan the intensity of light leaving the exit slit over a
wavelength range defined as λ1-λ2.
The user can scan different ranges of wavelength by entering the values of λ1 and λ2 withthis command, then pressing enter.

The SCAN key allows the user to scan between a start and an end position specified by
the user. The START position(λ1) may be greater or smaller than the END position(λ2).
Valid values of position are grating dependent. The scanning speed for the SCAN
command is a constant and is determined by the user. Once the SCAN key is pressed,
the display reads:
ENTER: scan
CURRENT

CNCL=quit
λ = 0100.00 nm

ENTER

λ1 = XXXX.XX nm

Type the starting wavelength and press ENTER. The display will then read:
ENTER: scan
CURRENT

CNCL=quit
λ = 0100.00 nm

ENTER

λ2 = YYYY.YY nm

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Type the ending wavelength and press ENTER. The display will then read:
SCAN:# of Repetitions :CNCL=quit

Enter Numbers :rrrType a number from 1 to 999. Press Enter. If no number is typed and you press Enter,scan will start and complete one repetition. If you enter a number, the display will then
read:
SCAN:
# of Repetitions :CNCL=quit
rrr

Secs to Pause :ppp

Type a number from 1 to 999. Press Enter. If no number is typed and you press Enter,the scan will start, completing one repetition. If you enter a number, the display will then
read:
SCAN:
CNCL=quitλ’ = xxxx.xx λ” = yyyy.yy
Rep. : rrr
Sec. : ppp
Scanning……… Secs to Pause : ppp
After the number is entered, the scan will begin.
READY screen will be displayed.

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SCAN SPEED
This command selects the speed at which the DK240/480
will scan the intensity of light at the exit slit for a given wavelength range. The
user should refer to Appendix B, for a list of scan speeds that are appropriate forvarious gratings.

The SCAN SPEED key allows the user to control the rate at which the wavelengthchanges. Values of speed are grating and units dependent and are given in Appendix E.
Once the key is pressed, the display reads:#=next1: Scan Speed
2: Rd. Novr.
Enter Selection : _CNCL=quit
3: Wr. Novr.

Caution: do not press number 3. This will change factory settings.

Press 1 and display will read
• SCAN SPEED
ENTER=new
CNCL=quit
CURRENT SCAN SPEED: xxxx nm/m
NEW SPEED :yyyy nm/m‘xxxx’ is the current scan speed. Enter new scan speed and press Enter. The display
will return to Scan Speed screen. Press Cancel, the Ready screen will be displayed,
and the new speed will be displayed within the screen (SS).
• Rd Novr. (Read Novram)
READ NOVR.
Address [1-64] :Novram Value :Address + Enter :CNCL=quit
xx
yyyy
zz

Enter the new ‘zz’ address and press Enter. The ‘xx’ will be replaced by ‘zz’, and thecontroller will query the Novram Value and will display the value in ‘yyyy’ location.
User can increase the xx address by pressing Options key or decrease the xx addressby pressing Scan Speed key.

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•

Wr. Novr. (Write Novram)

OPT’N: Addr
Address [1-64] :Value [0-65535] :Value + Enter :CNCL=quit
xx
yyyy
zzzz

Enter the ‘zzzz’ value and press Enter. The ‘yyyy’ will be replaced by ‘zzzz’, and thecontroller will send a command to write the ‘yyyy’ value to the ‘xx’ Novram address.
User can increase the ‘xx’ address by pressing OPTIONS key or decrease the ‘xx’
address by pressing SCAN SPEED key.
•

SLIT ADJThis command allows the user to change slit widths.

This SLIT ADJ allows the user to adjust the entrance and exits slits to desired widths.
The power up default slit width is 50 microns. This width may be changed to any value
between 10 and 3000 microns in the SLIT ADJ routine. After the SLIT ADJ ispressed, the display will read:
ENTER: new
0: AllEnter

1: Ent.
4: Reset
Selection : _CNCL=quit
2: Exit
5: Cal.

DK242 only

ENTER: new
0: All
1: Ent.
3: Mid.
4: ResetEnter Selection : _CNCL=quit
2: Exit
5: Cal.

If ‘0’ is pressed, the screen will display the box that allows a user to enter a new width for
all slits. If ‘1’ is pressed, a user can enter a new width for entrance slit. And, the sameway for number ‘2’ that is used for exit slit. Pressing CANCEL will cancel any SLITADJ routine and return to the READY screen.

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If ‘4’ is pressed, the Reset Slit screen will be shown. Then, a user can select ‘0’ for all,‘1’ for entrance slit, and ‘2’ for exit slit.
If ‘5’ is pressed, the Slit Calibration screen will be shown. A user can calibrate theentrance or exit slit.
CAUTION: Use of the slit calibration will erase factory settings.
•

SLIT ADJENTER: new
CNCL=quit
CURRENT WIDTH:
0050µ
ALL SLITS
NEW WIDTH? :xxxx µm

ENTER: new
CNCL=quit
CURRENT WIDTH:
0050µ
Entrance SLITNEW WIDTH? :xxxx µm

ENTER: new
CNCL=quit
CURRENT WIDTH:
0050µExit SLITNEW WIDTH? :xxxx µm•

SLIT RESETENTER: reset
0: All

1: Ent.

CNCL=quit
2: Exit

NEW WIDTH? :xxxx µm

ENTER: reset
0: All

CNCL=quit
2: Exit

1: Ent.

Running………………………….

If ‘0’ is pressed, all slits will be reset. ‘1’ is for Entrance slit, and ‘2’ is for Exit slit.

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•

SLIT CALIBRATIONThis command will allow a user to calibrate entrance and exit slits.

ENTER: new
CNCL=quit
SLITS CALIBRATION
1: Entrance
2: ExitEnter Selection :ENTER: new
CNCL=quit----SLITS CALIBRATION---Current Width :0050 µDesired Width :xxxx µ _CAUTION: Use of this command will erase factory settings.

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•

QUERY
This key contains three different functions: Test, Slot Width, andGPIB address.

1: Test2: Slot WidthEnter•

Selection :CNCL=quit
3: GPIB_

Test Initiates an internal diagnostic routine and resets the monochromator.
Similar to RESET.

1: Test2: Slot WidthEnter

CNCL=quit
3: GPIBDK is OK
Selection : _If ‘1’ is pressed, the below images is shown on LCD. After the test, the LCD will
display either “DK is OK” or “DK is Not OK”.
•

Slot Width Displays the current slot width of the grating turret table.

1: Test2: Slot Width
Slot Width is :Enter Selection :CNCL=quit
3: GPIB
xxxxx_

After the ‘2’ is pressed, the xxxxx will shown on LCD. The xxxxx is the current
slot width of the turret table.
•

GPIB Sets the monochromator GPIB address if available.

ENTER: set
OPT’N : up
GPIB Address :New Addr. (1-31) :CNCL=quit
SCNSPEED:dn
09
xx

If ENTER is pressed, ‘xx’ is the new address that will replace ‘09’. ‘xx’
can be increased by pressing OPTIONS or be decreased if SCAN SPEED
pressed.

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•

OPTIONS This command offers 3 menu commands to change all calibrations,grating selection, and constant spectral resolution (CSR) function.

In the event that you feel the instrument needs recalibration, this can be initiated from thecontrol module or external computer via the RS-232 cable. At this point, themonochromator must be set to a known wavelength that you have determined by means
external to the DK240/480 (laser, atomic line, etc).
NOTE: Grating calibration should only be performed after grating zero is checked, andif necessary, re-zeroed using the OPTIONS command.

OPTIONS:
1: Calibration
2: GratingEnter Selection :•CNCL=quit
3: CSR_

Calibration

The Calibration command offers four functions: Calibrate Grating, Zero,
Calibrate Slits, or Clear. Once selected, each option has submenus that willprompt the user for information. Pressing the ‘Cancel’ will exit the OPTIONS
mode and return to the READY screen.
CALIBRATE:
1: Cal. Grating
3: Cal. Slits
Enter Selection :•CNCL=quit
2. Zero4: Clear_Cal. GratingThis command allows recalibration of the monochromator positioning
scale factor and should ALWAYS be used immediately after the ZERO
command. The monochromator should be set to the peak of a known
spectral line, then the position of that is input using the CALIBRATE
command.
CAUTION: Use of this command will erase factory settings.
GRATING CALIBRATIONCNCL=quit
Current λ :xxxx.xx nm
yyyy.yy nm
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54You then enter the externally established wavelength and the DK240/480
adjusts its calibration point then performs a hardware reset. At the end of
the reset cycle the new calibration will be in effect. If problems areencountered during this procedure, consult SP for assistance.
Limits have been set on the extent to which you can alter the factory
calibration of the DK240/480. If the display indicates the desired
calibration parameter is too large or too small then the method for
determining the calibration point should be reviewed. Grating wavelength
limits are shown below.
Groove/mm
3600
2400
1200

Limit (nm)
500
750
1500

Groove/mm
600
300
150

Limit (nm)
3000
6000
12000

Groove/mm
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Limit (nm)
24000
36000
80000

55

•

Zero The Zero command will make the current grating position
0.00nm. User can use 1, 2, 3, 7, 8, 9 keys to set to the peak of the zeroorder transmission for each grating. If the peak is on the negative sideof zero order, a user must use ‘1’ to step down. If the peak is found,
ENTER key will be pressed to set the current grating to 0.00nm.
‘xxxxx’ is the previous zero offset value, and ‘yyyyy’ is the new zero
offset value after ENTER had been entered.

CAUTION: Use of this command will erase factory settings.
ZERO:
Zero Offset 1 :CNCL=quit
xxxxx

DK is moving to 0 nm
ENTER: Zero
1: Stp-7: Stp+
CURRENT

2:Slw-8:Slw+

CNCL=quit
3:Run-9:Run+λ = 0000.00 nm

ENTER: Zero
1: Stp-2:Slw-7: Stp+
8:Slw+Is below 0.00 nm

CNCL=quit
3:Run-9:Run+

ENTER:
Zero Offset 1 :CNCL=quit
xxxxx

Zero Offset 1 :yyyyy

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56•

Cal. Slits

This command allows a user to calibrate entrance and exit slits.
CAUTION: Use of this command will erase factory settings.
ENTER: new
CNCL=quit
SLITS CALIBRATION
1: Entrance
2: ExitEnter Selection : _ENTER: new
CNCL=quit----SLITS CALIBRATION---Current Width :0050 µDesired Width :xxxx µ•Clear

This command restores factory calibration values for the grating and slits.
It also executes a reset, which returns the grating to home position.
CALIBRATE:CNCL=quit
1: Cal. Grating
2. Zero3: Cal. Slits
4: Clear
RUNNING …………………….
•Grating

When the ‘Grating’ is selected, the LCD shows the current grating groove per
millimeter, the current grating number per total gratings, the current grating blazein nanometers, and the selection for the grating if the total grating is more than 1.
If the new grating is selected, it takes more than two minutes to change to a new
grating.
GRATING:
Gr/mm : ggggg
Blaze (nm) : bbbb
Enter New Grating : x

CNCL=quit
Grtg : 1/3

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57

GRATING:
CNCL=quit
Gr/mm : 00600
Grtg : 1/3
Blaze (nm) : 1000
RUNNING ……………………..

•

CSR

This command sets the monochromator to Constant Spectral Resolution(CSR) mode. The slit width will vary throughout a scan.
o xx.xx nm - is the maximum allowed for current grating.
o yy.yy nm - is the current bandwidth.
o zz.zz nm - is the new width that will be set.
oUser can use OPTIONS or SCAN SPEED key to adjust the new width.
OPT’N: next
Maximum BW :Bandwidth:
New Width:

CNCL=quit
xx.xx nmyy.yy nmzz.zz nm

The CSR function is described in further detail below.
•

ENTER/ STOP

Press the ENTER/STOP key after every command to carry out that action.
•

RESET

This command resets the grating turret to the home position. The READY screen
will be displayed after the reset has been completed.
SPECTRAL PRODUCTS
Monochromator Is
ResettingWaiting …………………………

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•

FILTER(Optional)

This option is enabled when an AB300-T filter wheel is attached. The Filter option is
described in detail in the Filter operations chapter. The Filter option allows you torotate the 6 position filter wheel assembly so that the selected filter will be placed in
the monochromator beam path. You may select any one of up to six filter positions.
FILTER:
CNCL=quit
Current Filter : x***************************New Filter ? [ 1 – 6 ] : y
FILTER:
CNCL=quit
Current Filter : x***************************
Running ………………………...

•

INVALID VALUE ENTRY

Whenever a value is entered that is out of the grating range, the following screen appears
VALUE INVALIDHIT ANY KEY TO CONT.

When an invalid value is entered for grating SCAN SPEED, the following screen appears

Invalid key ……………………...
HIT ANY KEY TO CONT.

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59•

Constant Spectral Resolution (CSR)

TheoryThe CSR mode allows the DK240/480 to vary the slit widths throughout a scan.
This is useful, for example, where measurement of a constant interval of frequency isdesired (spectral power distribution measurements). With the exception of slit setting,operation in the CSR mode leaves the other functions unchanged. When the
monochromator is positioned using GOTO, or scanned using SCAN, the slit widths will
be continuously and automatically adjusted to provide constant bandwidth.
The spectral dispersion of a monochromator is frequently quoted in units ofnanometers per millimeter of exit width. However, the spectral dispersion is a functionof wavelength. The spectral dispersion will change by about a factor of two as a single
grating is scanned over its full range. When a monochromator with fixed slits is scannedover a wide wavelength range, the change in spectral dispersion will change thebandwidth of light that is passed by the fixed slits.
In the Constant Spectral Resolution mode (CSR), a changing slit width compensates
for the changing dispersion to maintain fixed spectral resolution. Instead of selecting
fixed slit widths, the user selects a fixed band pass. At each wavelength, the Digikröm
then sets the slit width to allow the pre-selected bandwidth to be maintained.
Some important limitations apply to this method:•

The effective entrance and exit slit widths may be limited by the source anddetector. If the source is imaged on the entrance slit, and if that image width isless than the slit width, then change of the physical slit width will have no effect.
Similarly, if the exit slit is imaged on a detector, and if that detector is smaller
than the slit image, changing the physical slit width will again have no effect.

•

The resolution of a monochromator depends in part on uniform illumination of thegrating. If uniform illumination is not maintained as the slits are widened, thenconstant spectral resolution may not be obtained.

•

The slit settings are not continuously variable; the changes in the slit widths thatare used to maintain constant spectral resolution may result in observable steps in
the output intensity. To restrict this effect, the smallest slit width that is used in
the CSR mode is 106 microns or 1% of the slit width increment.

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60

The CSR mode is selected by pressing the SLIT ADJ key four times. The display will respond as shown
below.

BANDWIDTH
xx.xxnm=If the monochromator is currently in the CSR mode, then XX.XX will be the current bandwidth. If themonochromator is not in the CSR mode then XX.XX will be the maximum allowed bandwidth for thecurrent grating. The CSR bandwidth is entered at the prompt, using the numeric keys, then pressing the
ENTER/STOP key. The bandwidth should be selected from the allowed band pass values listed below.
If an invalid bandwidth has been entered, a beep will sound and the display will not change. If a valid
bandwidth has been selected, the slit widths will be automatically adjusted and the display will return to the
READY screen. When the CSR mode has been selected, the character for current grating number, at the
top right corner of the READY screen, will be displayed in reverse video (white on black).
To escape from the CSR display without entering a value, press the cancel key.

•

Specifications
The allowed band pass (resolution) values in the CSR mode are given below. With
uniform grating illumination and effective slit widths determined only by the physicalslits, the actual band pass values will be within ± 9% of the nominal value.
Grating (lines/mm)
2400
1200
600
300
150
75

1.50
3.00
6.00
12.0
24.0
48.0

Allowed Band pass Values (nanometers)
0.75
0.37
0.18
1.50
0.75
0.37
3.00
1.50
0.75
6.00
3.00
1.50
12.0
6.00
3.00
24.0
12.0
6.00

The instrument can be removed from the CSR mode in the following five ways:
•
•
•
•
•

By selecting a specific slit setting
By selecting a new grating
By resetting, recalibrating, or clearing the instrument
By selecting the TEST function
By removing power from the instrument

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6110. Reference Drawings
10.1 Optical Diagram of DK240/480

As indicated above, light from a source is focused on the entrance slit and directed by the turning mirror,
M1, to the collimating mirror, C. The focused beam is collimated and directed to the grating, G, which
diffracts and reflects the radiation. A particular wavelength of the light, determined by rotation of thegrating, is directed to the focusing mirror, F, which focuses it onto the exit slit via the second turning
mirror, M2.

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62

10.2 Optical Diagram of DK242

As indicated above, light from a source is focused on the entrance slit and directed by theturning mirror, M1, to the collimating mirror, C. The focused beam is collimated anddirected to the grating, G, which diffracts and reflects the radiation. A particular
wavelength of the light, determined by rotation of the grating, is directed to the focusing
mirror, F, which focuses it onto the exit slit via the second turning mirror, M2.
Optical baffles are utilized to minimize stray light due to scattering from slit edges anddust or aberrations in the optical elements.

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6311. Calibration
11.1 Calibrating Zero with DK2401

Recalibration will erase the values previously programmed in your NOVRAM’s memory.
•
•
•
•
•
•
•

Using a discrete light source, such as a HeNe laser, illuminate the entrance slit. Make sure thelight source is aligned perpendicular to the entrance slit.
Using SLIT ADJ, set both slits to 50µ
Press GOTO and set λ = 0 nm
Using a white card, look for the laser beam exiting the monochromator.
Pressing 1 will single step the grating below zero. Pressing 7 will single step grating above zero.
Step up and down until the brightest light is seen at the exit slit.
Press OPTIONS, then 1 (calibration), then 2 (zero).
Then, press ENTER to re-zero the monochromator.

11.2 Calibrating a Wavelength with DK2401

Recalibration will erase the values previously programmed in your NOVRAM’s memory.
•
•
•
•
•
•
•
•

Zero MUST be set before wavelength calibration,
Using a discrete light source, such as a HeNe laser, illuminate the entrance slit. Make sure thelight source is aligned perpendicular to the entrance slit.
Press GOTO and set λ = 0 nm. Press Enter.
Using a white card, look for the laser beam exiting the monochromator. If beam is not seen, seeabove section.
Press GOTO and set λ = (laser wavelength, i.e. 632.8nm).
Using a white card, look for the laser beam exiting the monochromator.
If the new wavelength is not correct, enter into MANUAL mode
In MANUAL mode:1 – single step below current λ2 – scan below current λ3 – slew below current λ7 – single step above current λ
8 - scan step above current λ
9 - slew step above current λ•It is best to single step, while observing detector for maximum peak intensity. Press MANUAL
again to get back to the Ready screen.

•
•
•
•
1.

To calibrate this new wavelength, Press OPTIONS.
Press 1, Calibration
Press 1, Cal Grating
Type new wavelength
Press Enter.

Note: The CANCEL key will allow you to exit this procedure at any time.

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64

11.3 Slit Calibration
Recalibration will erase the values previously programmed in your NOVRAM’s memory.
Pressing the keys “OPTIONS” then “1” then"3" will initiate slit calibration and will produce
similar displays and invoke the slit calibration routines. Please consult SP before proceeding. As
with the grating calibration, the slits must be set to a known width by a method external to themonochromator before recalibration. The most practical method is diffraction patternmeasurement, whereby laser light is passed through relatively narrow slits and the main beam ofthe diffraction pattern is measured.
Place a white card in front of the spherical mirror nearest the slit being calibrated and observe thelaser diffraction. Properly calibrated slits will have a main diffracted beam of 12.7mm for a
DK240 or DK242, and 25.4mm for a DK480 when the slits are set to 25 microns.

11.4 Calibrating Zero with DKDemo VB Software
Recalibration will erase the values previously programmed in your NOVRAM’s memory.
•
•

•
•

Using a discrete light source, such as a HeNe laser, illuminate the entrance slit. Make surethe light source is aligned perpendicular to the entrance slit.
Using the DKDemo-F program, click Commands, Control, Goto drop down tabs and enter 0
for the wavelength, and then press OK.

Click Commands, Control, Slits to set entrance and exit slits to 50µ.
If the beam exits the monochromator, no further action is needed.

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•

If the beam does not exit the monochromator, click Commands, Setup, Zero.

A new window will pop up. Click on the up/down arrow buttons until the beam exits.
Press Zero to issue a new zero offset for the grating.

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66

11.5 Calibrating a Wavelength with DKDemo VB software
Recalibration will erase the values previously programmed in your NOVRAM’s memory.
•
•
•
•

•
•

Zero MUST be set before wavelength calibration,
Using a discrete light source, such as a HeNe laser, illuminate the entrance slit. Make
sure the light source is aligned perpendicular to the entrance slit.
Click Commands, Control, Goto drop down tabs and enter 0 for the wavelength. Check to
see that the zero offset is correct for the grating in use. If not refer to the zero calibration
above.
Click Commands, Control, Goto drop down tabs and enter the wavelength you wish to
calibrate the grating.

If the current calibration offset is unsatisfactory, click the step up/step down buttons until
the laser line exits.
Click Commands, Setup, Calibrate. A second box will pop up showing the current
wavelength (Goto wavelength). Enter the desired wavelength, i.e. the laser wavelength.

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67

The monochromator will perform a Reset.

11.6 Slit Calibration Using DKDemo VB Software
Recalibration will erase the values previously programmed in your NOVRAM’s memory.
As in slit calibration procedures above, slit calibration must be done very carefully. Issuinga wrong number can result in damage to the slit blades.
•

Click the Commands, Setup, Slit Cal drop down tabs. A second box will pop up, asking
which slit you are calibrating, and what the correct slit width should be.

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6811.7 DK242 Calibration
The DK242 establishes a zero order offset value for each half of the monochromator. In other words, eachgrating has its own zero offset. Calibration offset is different in that one value is established for each pair ofgratings. It is very important to have the DK242 zeroed properly to receive a satisfactory output from themonochromator.
The DK242 MUST be properly zeroed before it can be calibrated.
Zero and calibration is best performed using the DKDEMO-F software included on the Software
Installation Disk received with your monochromator. The following instructions reference the DKDEMO-F
software.
You will need two small hex wrenches (1/16” and 3/32”) and a visible light source for this procedure. A
laser source such as a HeNe works best, but it can be done with a white light or atomic line pen lamp.
••

Remove power from the unit. Remove the slit cover plates and remove the cover.
Apply power and set your light source to the entrance slit. Once the DK242 has finished it’s reset
routine, open the DKDEMO-F software. Click the Commands, Setup, and Zero tabs. A new
window will pop up as shown below:

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•

•
•

•

The grating turrets will rotate to the current Zero order position. While observing the light source
through the middle slit, press the radio buttons labeled 1 to optimize the light passing through theslit. The left button steps the turret below the current zero offset, the right button steps the turret
upward.
Once the light sufficiently exits the first half of the monochromator, repeat the above procedureusing the radio buttons labeled 2, to get the light to exit the entire monochromator.
Once the light exits the entire monochromator correctly, click the zero radio button. A warning
screen will appear as shown below. Press the Continue button to zero the monochromator turrets.
The monochromator will perform a reset.

Press the Exit button to leave the zero routine.

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70•

Again using the DKDEMO-F program, press Commands, Control, GoTo and enter a known
spectral line of the source you are using. In this example we have entered 546.07nm.

•••

If the wavelength light exits the monochromator, no further action is required. If the light does
not exit, use the radio buttons on the main view to step the turrets until the light exits
properly.
Click the Commands, Setup, and Calibrate tabs to calibrate the monochromator. In theexample below, we stepped the monochromator to 546.52nm to get the 546.07nm line to exit.
Ensure that the Desired Wavelength matches the known wavelength you have chosen to
calibrate the instrument.
Press the OK button to calibrate the DK242. Again a warning window will appear.

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71Spectral Products
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73