S pe ct ra l S ensor s by Ca r l Zei s s

M M S Ð M o n o lit h ic
M in ia t u r e -S p e c t r o m e t e r

株式会社 スペクトラ・コープ〒164‑0011 東京都中野区中央4‑4‑5第一小林ビル
Tel:03‑5328‑2858 Fax:03‑5328‑2859
URL http://www.spectra.co.jp

Origins

Origins 2

A classic spectrometer or a classic monochromator typically comprises a dispersive medi-

The quasi-monolithic principle 4

um, entrance and exit slits, and imaging compo-

Concepts 4nents which produce a parallel beam path. To

Optical Components 4

record a spectrum, a detector located behind the

Total package 7

exit slit must sequentially record the incident

Back to the basics: The specifications 8

slit is moved. This mechanical movement requi-Spectral resolution 8

res time and is prone to defects. In many appli-

light, while the dispersive component or the exit

Special features ofdiode array spectrometers (DAS) 9

cations – in industry in particular – short measuring times and insensitivity to external influences

lntensity resolution 11

are a major advantage. Since the end of the 70's,

Scattered light 13

when several developments converged, Carl Zeiss

Optical interface 14

has been working on diode array spectrometers.

Optimization of adiode array spectrometer 15■ Diode arrays which – used in place of theexit slit – simultaneously recorded a complete

Control 16

spectrum within a fraction of a second (andDiode array 16

made moving components superfluous)

Preamplifier 17

became attractively priced, making their

Control electronics 17

wider use possible.

Applications 18■ Imaging gratings developed at Carl Zeiss

Emission 18

made lens elements or concave mirrors

Diffuse reflection 19
Reflection 19

superfluous. This greatly reduced the numberof components needed for instrumentation.

Transmission 20
I n t e rf e rence spectrum 20■ Quartz light fibres were launched on the

Applications/fields of use 21

market. These allowed a modular design ofSpecifications 22

spectrometers, as complex coupling devices
were no longer required.
To rigidly connect the remaining components (fibre connectors for the light supply, theentrance slit, the concave imaging grating anddiode array) a special spectrometer body madeof ceramic material was developed. All compo-

2

nents were permanently glued to each other,tings, are available for most of

resulting in a quasi-monolithic module. Its bene-

the interesting spectral ranges andfits: ruggedness, permanent alignment, highhave revolutionised measuring

speed. The potential read-out speed was consi-

technology. The days whenderably faster than the available computers wereevery manufacturer ofcapable of processing. This led to the creation ofspectral analysis built his

specific processing electronics on the basis of aown spectrometer frommicroprocessor.

discrete components area thing of the past in

These developments merged in the MCS

the same way as themodule which had its first public showing in

IC has replaced the1982 and met with enormous acclaim. In 1985,

individual electro-

the first appliances fabricated in series were sup-nic components.

plied and immediately won the „IR 100“ Awardand the „Innovation Award of German Economy“.
The main fields of application of these spectrometer modules are the field of industrial analysis
and varnish and film thickness measuring technology.
During this period, the idea of a miniatures p e c t rometer for portable measuring instruments or for use as a general „spectral sensor“
was born. Sponsored by the German Ministry for
Research and Technology, Carl Zeiss developed amonolithic spectrometer using a glass body as its
basis. This module featured a further reduced
number of components. in 1993, the module
baptized MMS 1 (Monolithic Miniature Spectrometer) was presented to the public for the firsttime. In 1994, it won the „Photonics Award“
and was therefore considered one of the 25 bestproducts of the year.
A NIR version for the range of 0.9 to 1.7 µm
was introduced in 1997. Further versions of up
to 2.2 µm or 2.4 µm sensitivity followed or are
currently being developed.
This means that miniaturised spectrometer
modules, in other words expanded imaging gra-

3

The quasi-monolithic principle

Concept
The size of a spectral device is technically

Optical Components
The optical components are:determined by the mounts for the gratings, slits,■ imaging grating

detectors, imaging components etc. In terms of■ fibre cross-section converter as the optical input

physics, the size is only determined by the reso-■ diode array as the opto-electronic output

lution required. As many applications do not■ these are arranged around and cemented torequire high resolution, but a high repeatability,

a central body. The central body has a diffe-

small sizes are sufficient.

rent design for different versions. The components important for the interfaces, i.e. theThe concept of the MMS and MCS family isbased on reducing the opto-mechanical design

cross-section converter and the diode array,
continue to be used.

as well as the number of components to their
physical minimum and, in addition, to use as
many of the same parts for the individual versions as possible.

Central bodyIn the MMS 1, the central body is a lens-type
glass device made of UBK 7. The imaging grating
has been directly replicated on this glass body;In the meantime, a multitude of diff e re n t
MMS and MCS variations are on the market,

mally protected against dust and gases. The use

the specifications of which are described onof an optically more dense medium and thepage 22. They are different with regard to theirresultant larger aperture allows the use of small

design and parameters but they are all based on

gratings. This leads to fewer aberrations.

the same principle described further below. For
the purpose of simplification, the illustrations in
the following text are generally related to the
MMS 1.

4

the grating is thus permanently fixed and opti-

Grating for
MMS 1 andMMS UV-VIS:
366 l/mmin the centres,
t-number 1.6;
imaging ratio 1:1

MMS NIR 1.7
efficiency
for PDA andgrating

For reasons of transmission, the solid glass

dal profile. In addition to its dispersive function,body has been replaced in the UV-sensitive

the grating must image the entrance slit on themodules by a hollow body to which the grating

diode array. By varying the groove density andand the front plate have been cemented. The

using curved grooves, coma can be corrected

overall stability of the tube design comparesand the focal curve flattened (flat-field).

This

favorably to the glass body design because theensures that the focal curve is optimally adapted

wavelength drift due to temperature changes

to the flat detector structure. Even with thehas been reduced.

short back focal distance of the MMS 1, flat
spectra of a length of over 6 mm are obtained.

Grating
The gratings for the MMS family itself con-

The same grating design is used for the VIS

sist of so-called holographic blazed (optimised

and the UV-VIS versions. The master grating has

efficiency degree) flat field gratings. These gra-its efficiency maximum at approx. 215 nm. Duetings which are produced at Carl Zeiss in a sta-

to the higher optical density, the efficiency curve

tionary wave process achieve significantly higherof the VIS module is shifted by the factor of theefficiency degrees (for non-polarised light) thanrefractive index.

gratings with sinusoidal profile. This grating produced by Zeiss using the standing-wave technique achieves considerably higher efficiencies (fornon-polarised light) than gratings with a sinusoi-

MMS 1 including housing5

Cross-section converter

sure time, the stronger and sooner this effectA fibre bundle cross-section converter isoccurs. This means that the transmission can evenused to further optimise the light sensitivity of

be restricted in the range of more than 220 nm

the module. Single fibres in a linear configurati-

to 250 nm. This solarisation effect can only be

on form the entrance slit (slit height h determi-

partly cancelled by heating, but it is possible toned by the number of individual fibres; slit widthcorrect it by means of frequent reference measu-b ≈ core diameter). The diameter has been adap-

rements.

ted to the pixel size of the diode array used andthe imaging and dispersive properties of the flat-

If however no measuring values below 225 nm

field grating. Thus, light intensities near the

are necessary, it is recommended to use a WG 225

theoretical limit are achieved. The cross-section

filter with a thickness of 3 mm until solarisation-

converter is an integral component of the spec-

free fibres are available!

trometer design and is therefore not easy tom o d i f y. There is a possibility, however, ofmodifying the length of the fibres and thedesign of the input.
In addition, it must be taken into accountthat quartz fibres which are used in older MMS
UV- (VIS-) modules form so-called solarisation
centres when they are exposed to deep UV light
below 220 nm. That means that the transmission of the fibres is reduced when they are exposed to high-energy light. The shorter the wavelength of the light (higher photon energy) andthe higher the intensity and the longer the expo-

Input∅ = 0.5 mm
NA = 0.2
approx. 30 single fibres

Outputb ≈ 70 µm
h = 2.5 mm

Detector
The silicon diode array S3904-256Q from
Hamamatsu is incorporated in the MMS family.
Only the MMS 1 NIR enhanced uses the Hamamatsu – Type S4874-256Q. This array is packed
in a shorter special housing which results in avery small split-off angle, allowing an efficient
MMS UV-VIS

6grating design (see page 15). When changing toSpectralsensitivity ofthe diode array
S3904-256Q,
number of
pixels: 256,a re a :25 x 2 5 0 0µ m 2 ,pitch: 25 µm

Spectralsensitivity ofstandard-InGaAs(1.7 µm) andextended-InGaAs( 2 . 2µ m )a different detector, this angle and the approxi-

Total package

mate spectrum length of 6 mm must be taken
into account. To suppress the second order, theThe whole configuration including a pream-

diode array has beeen directly coated with a di-plifier board is accommodated in a protective

electric cutoff filter.

housing. The grounding is important for the performance (signal-to-noise ratio) of the MMS andDue to the longer wavelength an Indium-

MCS modules.

Gallium-Arsenide-(InGaAs)-diode array is used in
the MMS NIR instead of the silicon diode array.

The basis for the high quality of the minia-

Standard InGaAs is sensitive between 0.9 toture spectrometers is the wide range of technical

1.7 µm, extended InGaAs on the other handexperiences gained in the Zeiss Group regarding

exhibits a sensitivity of up to 2.2 µm or 2.4 µm.

mathematical design, structuring (grating manu-

As all diode arrays are equipped with a Peltier

facture and replication), coating and materials

cooling, a favourable signal-to-noise ratio is also

processing. Furt h e rm o re, the expertise in cemen-achieved in this case.

ting technology provide for MMS modules which
are extremely insensitive to mechanical shock

Diode arrays have the following advantages overand temperature changes in particular. The wave-

CCDs:

length accuracy changes only by 0.012 nm/K in■ larger dynamic range (for non-cooled devices)

the MMS 1; the drift in the UV-modules is even■ better UV sensitivity

lower.

■ intensity-independent noise■ minimum cross-talk■ no trailing effect.

7

Back to the basics: The specificationsor The Jungle of Resolutions

The most important criterion when selecting
a spectrometer is the spectral range which must

Spectral resolving power
According to DIN, the Rayleigh criterion is

be covered by the spectrometer. In most cases,

relevant to the separation of spectral lines. The

this range is clearly defined. The other two

criterion indicates how wide the spectral dis-

important criteria of a spectrometer – the spec-

tance between two lines ∆λ Rayleigh must be totral and the intensity-related (dynamic) resoluti-

allow their recognition as separate lines. Here,on – are, however, very rarely clearly defined.

the spectral width of the individual lines ∆λline
must be markedly smaller than their spacing.

Spectral resolution

This is the really significant definition of spectral
resolving power.

The following four terms are often used todescribe „spectral“ resolution:

2 lines with Imax,1 = Imax, 2 are separated,

1. Rayleigh-criterion – ∆λRayleigh (DIN standard)

if ∆Idecrease ≥ 19%.

2. Line width, mostly full width at
half maximum – ∆λFWHM
3. Sub-pixel-resolution(also termed „software resolution“)
4. Pixel dispersion – ∆λ / Pixel.

Width of spectral lines
To enable the measurement of the width ofa spectral line ∆λlinie the expansion of this line by
the spectrometer must be smaller than the spec-

lt is the actual application which provides a

tral width of the line itself. To ensure this, it isuseful definition in this respect. There are mainly

important to know the expansion ∆λFWHM produ-

three different purposes for which a spectrometerced by the spectrometer. This property is related

is used (these can also occur in combination, ofto the Rayleigh criterion.

course):1. Separation of two or more lines within aspectrum – analysis of compositions
2. Determining the line shape mostly determining the width of a line or band (FWHM or
1/e2 width)
3. Measurement of a line with respect to its peakwavelength and intensity at the maximum.

8∆λFWHM = λ2 (Imax/2) - λ 1 (Imax/2)∆λFWHM ≈ 0,8 x ∆λRayleighWavelength accuracy
To determine the absolute spectral position λ– with a specific accuracy ∆λ ± – of a single line, aSpecial features ofdiode array spectrometers (DAS)

spectrometer with at least this absolute wavelength accuracy ∆λ± is required. This parameter isSpectral resolution

dependent on the accuracy of the positions of

Due to the fixed position of

the readout elements (pixels or slit/detector) or

the pixels with respect to thethe stability of these positions characterized bywavelength of the incident

re p e a t a b i l i t y. Contrary to this, the absolute

light, the resolution provided

wavelength accuracy only depends indirectly on

by DAS differs from that provi-

the dispersive and focal properties of the spectro-

ded by monochromators/spec-

meter and is not „resolution“ in the classic

trometers with moving com-sense.

ponents: resolution definedas the „separation of two ad-

The stability (or repeatability) of a spectral

jacent lines“ is dependent on

sensor is dependent on the mechanical stability

the relative position of these linesof the module and the wavelength drift due towith respect to the pixels:

temperature changes. The former is completely
uncritical in the MMS and MCS modules, andlf two adjacent lines are ima-

the drift can be more or less neglected.

ged on the pixels in such a way

Dispersion

central pixel (I2) and the maxi-

that the minimum falls on the
The term ∆λ / Pixel (= ∆λPixel) has nothing to

ma on the adjacent pixels (I1,I3)

do with spectral resolution; it is merely the linear

the lines can be separated if thedispersion of a diode array spectrometer. The

intensity displayed is I2 ≤ 0,81 x

pixel dispersion and the spectral resolution are

I1 (I3), ∆λ is then exactly two

related to each other via the width of the ent-

pixels (2 x ∆λPixel ). In this case,rance slit and the imaging properties of the spec-

it is sufficient to evaluate atrometer. lf the entrance slit is imaged on approx.

total of 3 pixels; the loca-

3 pixels, the triple of the pixel dispersion approx.

tions of the maxima corres-corresponds to ∆λ Rayleigh.

pond almost exactly to thecentral wavelengths of the∆λ Rayleigh ≈ 3 x ∆λPixel

pixels displayed.

9lf the maximum of a line is imaged on the

Sub-pixel-resolution or the Parabola Fit

separating line between two pixels (I1, I2) how-

To determine the peak wavelength λm a x

ever, a total of 4 pixels is required to be able to(and/or peak intensity Imax ) the spectral line to be

detect a clear reduction in the pixel intensities.

measured must be imaged on at least 3 pixels

Both pixels record about the same intensity, with(see below). Three pairs of values (intensity per

the result that a reduction to 81 % is not dis-pixel I1,2,3 and the related central wavelength of theplayed until in the next pixel (I3). Here, the actual

pixel λl,2,3) allow relatively easy fitting of the line

maxima are separated by fewer than 3 pixels; theto a parabola. The equation for the parabola

DAS displays a spectral spacing of 3 x ∆λPixel as athen gives the peak of the curve including thediode array can only detect discrete values using

data for the peak wavelength and peak intensi-

the stepwidth of the pixel dispersion. A total of

ty. The accuracy of this method largely depends

4 pixels is needed for processing.

on the absolute accuracy of the central wavelength. In a diode array spectrometer, this wavelength can be determined, in principie, to almost
any accuracy required.
lf necessary, each pixel can be individuallycalibrated. However, this will only make sense if
the module features the necessary stability.
Otherwise, the wavelength specification will only
remain valid until the next shock or temperaturechange.
lf the imaging performance (and the dispersion) of a DAS has been chosen such that fewer
than 3 pixels are illuminated, no extrema can be
determined, resulting in a parodox: An apparently ideal situation – a line is very narrow at theoutput – leads to considerably increased inaccuracy. lf, for example, a line is only imaged on asingle pixel, the spectral inaccuracy is ∆λ Pixel inthis case.
Parabola-equation
I (λ) = a x λ 2 + b x λ + c
Coefficientsa = (I3 + I1 - 2I2) / 2∆λ2
b = (I3 - I1) / 2 ∆λ - 2a x λ2
c = I2 - a x λ22 - b x λ 2
Maximum at λmax = -b/2a10Determining the half-width

Accuracy

The parabola fit also supplies qualitative

Measurements of minimal changes and stabi-data on the halfwidth. For this, I max/2 must only

lity are directly dependent on each other and are

be inserted in the parabola equation. There aremainly limited by the noise present in the electro-

only minor differences between the halfwidth ofnics, as the stability of the „light path“ is ensured

a parabola fit and that of a Gaussian fit.

in most spectrometers. As with all parameters, it isimportant how a value – here in the true sense of

The half-width which is displayed by a DAS

the word – is determined. For the data provided byis also dependent on the position of a line relati-

the MMS, for example, an integration time of 10 ms

ve to the individual pixels. Our specifications areis set and the standard deviation ∆σ is computed

valid for worst-case values.

using 20 recordings. This supplies a measure of
the accuracy ∆l with which an intensity value canMore adequate, but more complex, are fits

be determined.

to Gaussian or Lorentz curves which correspondbetter to the actual spectral distributions. These∆l = Inoise = ∆σfits also have the advantage that the half-width
calculated from them is not dependent on its
position relative to the pixels.

Dynamic range and intensity changes
The dynamic response is defined as the ratio of∆λ FWHM = 2[(b/2a) - (c - Imax )/a]2

1/2

the saturation value lsat and the noise Inoise ↔ ∆σ andthus corresponds to the signal-to-noise ratio S/N.
(The usable range is reduced by the dark cur-lntensity resolution

rent.) S/N does not only depend on the detector,
but also on the digitization determining the

To measure intensity, the following pro-smallest step width into which a measured signal

perties which are dependent on each other are

can be decomposed. The weakest link in theof interest:

chain, of course, determines the signal to-noise
ratio to be achieved.

Relative:■ smallest detectable change

Dynamic range = S/N = Isat / Inoise■ signal stability■ detection or dynamic range■ linearity
Absolute:■ lowest detectable amount of lightor sensitivity.

11

It should be noted here that a wide dynamic
range is only obtainable if the PDA (photodiodearray) is near the saturation limit. The aim isalways to reach high light intensity – here, thehigh sensitivity of the MMS modules is a benefit.
Dynamic range = ADC/∆σ range

Linearity
The previous remarks will be completely accurate only if the detector and the post-detector
electronics provide ideal linearity. For quantification, the admissible deviation must be specified.
Fortunately, the behaviour of modern semiconductor detectors is almost perfectly linear within a
For instance, when using a 14bit converter –wide range. Before saturation (the extreme casethis corresponds to 16,384 steps or incrementsof non-linearity) is reached, however, the increase

and a noise of ∆σ = 1 count, a signal (full-scaleof the current (carrier of the intensity information)

display) can really be divided into 16,384 incre-

supplied is no longer linear to the number of pho-ments. Hence, the lowest measurable change istons striking the photosensitive material. For this

1/16,384 of the saturation signal. At a noise ofreason, the range of linearity is smaller than the

4 counts an uncertainty of 4 counts also exists,

dynamic range.

i.e. a change of 4/16,384 of the saturation signal can only be definitively measured or the signal divided into 4,096 increments.

External influences
As the graphic shows, a change of the temperature T does not cause any change in sensitivity of silicon, the sensitivity in fact increases
slightly in the range of up to 1100 nm when thetemperature is raised. In the case of InGaAs photo
diode arrays, the sensitivity also changes by lessthan 1% in the range of 1 to 1.55 µm with temperatures ranging from -50 and +50 °C. Merely
outside the specified range, does the different
coating lead to an increased influence of the temperature. (Falling temperatures lead to reduced
sensitivity on the band edge).

12

In addition, the signal-to-noise ratio of theSensitivity

photodiode array used does not degrade withThe „smallest detectable change“ is a relati-

increasing T. lt is only the dark current Idark whichve specification. Much more difficult to specify isincreases with rising temperature, resulting in a

the lowest detectable amount of light or: how

reduction of the dynamic range. There f o remany photons are needed for the detection elec-

detectors, in particular InGaAs diode arrays, aretronics to record a change. The difficulties resultoften cooled.

from determining the light intensity of a lightsource and the coupling efficiency. Furt h e rm o re ,Idark (T+7K) = 2Idark (T) (Silicon)

these parameters are wavelength-dependent.

Idark (T+10K) = 2Idark(T) (InGaAs)
There is, on the one hand, a direct depenIn this context it should be mentioned that

dence, as all components feature wavelength

the amounts of light to be measured are also

dependent efficiencies – including the coupling –subject to fluctuations. The instability of the lightin device; on the other, there is a dependence, assource is often the limiting factor.

the bandwidth is of decisive importance for sensitivity measurements. The simplest case is a lightsource with a very narrow band, as displayed bymost of the lasers. lf the bandwidth of the lightsource used is markedly smaller than the bandwidth of the spectrometer used, the situation isclear. The MMS value of more than 1013 counts/Ws
has been measured with a red HeNe laser. At
1550 nm, the MMS NIR still exhibits a sensitivityof more than 1014 counts/ Ws (in the case of a
14 bit conversion).
Scattered light
The specification of scattered light data is
only useful in connection with the measuring

13

instructions. Scattered light data are determinedchanges in the radiation used only affect thefor the MMS program using three different light

dynamic range in proportion to the scatteredsources to measure the different spectral compo-

light present: for example, a change of 10 % in

nents of scattered light: a deuterium lamp forthe radiation used causes a change of 10-4 if theUV, a xenon lamp for VIS and a halogen lamp for

scattered light component is 0.1 %. lf the radia-

VIS-NIR.

tion causing the scattered light is not used, theamount of scattered light can be further reduced

The level of scattered light is defined as theby filtering this radiation. A blocking of 103

ratio of the respective measurement using an

results in a change of 10-7 in the case described.

OG570 or KG3 filter to the maximum useful sig-Thus, the measurement of minimal changes is

nal and is therefore specified for the short wave-

only impaired to a very limited extent, as noise islength range. This reveals that the main compo-

the bigger problem in most cases. In addition, if

nents of scattered light in the MMS modules

the signal causing the scattered light is known,

come from the NIR range. These spectral compo-

the scattered light component can be eliminatednents are easy to filter out as they are „far away“

by computation.

from the spectral range of interest.
Optical interface
The scattered light value for the MMS NIR is
reduced to 0.1 % as transmission of 10 mm waterat 1450 nm, exposure with halogen lamp.

Interfaces must be mechanically and opticallydefined. A useful mechanical interface for opticalsystems is the SMA connector as used in the

Scattered light reduces the dynamic range,

MMS. Together with the well-defined light gui-as the full range is no longer available. However,dance factor of a fibre bundle, this results in aunique interface.
Light guidance factor
The light guidance factor G is the product of
the light entrance area F and the aperture angleΩ of the light beam, with the refractive index nalso having to be taken into account. The first
factor corresponds to the cross-section of thefibre bundle, the second factor is derived fromthe numerical aperture NA. In the case of thee.g. MMS 1 family, the fibre optical light value iscalculated at G = 0.157 mm2sr.

14

G = F x Ω x n2
Ω = 2π x (1-cos α)
α = arcsin NA

For the optimum adaptation of an existing
light source (whether fibre, illuminator, imaging

Optimization of a diodearray spectrometersystem) it is recommended to determine the respective light guidance factor. A comparison of

In addition to the selection of the mostthe factor obtained for the light source with theefficient components possible (blazed gra-

MMS light guidance factor permits an estimate

ting, cross-section converter, sensitive diodeto be made of the possible coupling efficiency.

a rray), dispersion, imaging pro p e rt i e s ,In addition, Fresnel losses of 4% (index jump at

entrance slit and pixel size must be mat-glass fibre) must also be taken into account.

ched to each other. To obtain maximum
light sensitivity it is important that – withlncrease in transmission

monochromatic light – only just a little

lf round light spots are assumed, the use of

more than the 2 pixels are illuminateda cross-section converter CSC results in increa-

which are required for spectral resoluti-

sed transmission ηFF, CSC/ηFF, Slit compared with theon. In a first approximation, the gra-classic slit. This increased transmission can beting provides a 1:1 ratio image, i.e.

calculated from the ratio of the amount of lightthe entrance slit should be 2 to 3

transmitted by the CSC to the amount of lightpixels wide. lf more pixels are illu-

transmitted by a rectangular slit.

minated, the signal-to-noise ratio
and the sensitivity will worsen

In the CSC, the transmitted amount of light(1 pixel does not cover the optimal

is given by the fill factor ηFF, QSW. The fill factor isbandwidth). lf fewer than 3 pixels

defined as the quotient of the optically effective

are illuminated, the wavelengthsurface Aeff and the overall area Aapt illuminated.

accuracy will worsen. The selec-

In the CSC, Aeff is the product of the fibre coretion of 70 µm individual fibre s

cross-section and the diameter dfiber and the num-(effective slit width approx. 60µm)

ber of fibres N; in the slit, Aeff is the area obtained

for the MMS 1 CSC, for example,

from the slit width b and slit height h. The total

is thus ideal for a pixel width ofarea is the circular area with the diameter dSpalt = h.

25 µm. The number of fibres
is obtained by dividing theηFF, CSC = N x dFiber / d
2

2
Apt

pixel height by the outerηFF, Slit = 4 b/(π x dSlit)

diameter of the individualη FF, CSC / ηFF, Slit = 16 (MMS)f i b re s .

15Control
Diode array

incident light. The information on the amount of

Diode arraylight is gained in the subsequent charging proThe spectrometer modules of the MMS/

cess. The time integral over the charging current

MCS family are equipped with silicon diodeis proportional to the light intensity. Thus, thearrays of the type S3904-256Q or S3904-512Q

diode array operates as a so-called – „self-scan-or even S3904-1024Q from the company Hama-ning“ multiplexer. The individual pixels are read-matsu, with 256 – 1024 pixels. Only the MMS 1

out one after the other. The external clock fre-

NIR enhanced uses the Hamamatsu Type S 4874-

quency f determines the frequency for switching

256 Q, which exhibits a higher degree of sensiti-

from pixel to pixel or, in other words, the timevity for longer wavelengthes (up to 1100 nm). A

per pixel tPixel. This way, the minimum possible

special housing which is shorter than the stan-

integration time tint, min is determined – even in thedard housing permits the detector to be positio-

case of the Hamamatsu diode array. This also

ned very closely to the optical input. The small

determines the minimum possible integration

split-off angle thus obtained results in optimised

time tint, min.

grating efficiency. For this reason, only detectors
with a similarly small housing can be used.

tPixel = 1/f
tint, min = (N+1) x tPixel

Indium-Gallium-Arsenide(InGaAs)

diodefmax = 2 MHz → tint ≥ 127 µs

arrays of different manufacturers are used for all(256 silicon diode arrays)

MMS NIR. However, they are all equipped with afmax = 2 MHz → tint ≥ 64 µs

Peltier cooling. Zeiss supplies a matching control(128 InGaAs diode arrays)

board for the control of the Peltier elements.
The individual N pixels of a diode array
which represent capacitors are discharged by

The integration time itself is selected via thetemporal distance of the sequential start pulses.
The start pulse triggers the charging process. The
integration time is realised technically over thetime up to the following start pulse (the time
until the following process of selection).

16

In the case of InGaAs diode arrays, the situation is slightly more complex. Here the process ofControl electronics for
MMS family and MCS

exposure is separated from the process of selection. If the exposure time, i.e. the time during

All MMS/ MCS modules are equipped withwhich the diode array is sensitive, can be as shortpre-amplifier electronics. An operating electronic

as required, this results in a connection for theis required to control and read-out of the diodeselection time comparable to the silicon arraya rr a y, which also executes the transformation

cell. It is relevant for spectroscopy that the detec-from analogue to digital.

tor is insensitive during the process of selection.
Blind periods of up to 3.5 ms need to be taken

The deciding factor in this electronic system

into account. (See also Operating Instructions ofis the resolution in the intensity axis. Currently,

the MMS NIR).

Zeiss offers operating electronics with a resolution of 12, 14 and 16 bit. If the measurement is

Preamplifier

carried out with the highest resolution of 16 bit,
this results in a separation into 65 536 (216) indi-

The electronic control of the Hamamatsu

vidual steps (counts) with a driving ratio of 100 %.

diode array is described in the corresponding

It is possible to detect changes of intensity withdata sheet. The installed Zeiss pre-amplifier gua-this of less than 10-4 .

rantees a low-noise amplification of the video
signal to 3 VSS. In addition, it converts external
TTL levels for the array management to the levels
required by the diode array. All InGaAs arrays are
read-out with pre-amplifiers which supply a voltage of 4 V SS on the differential amplifier output.

17

Applications

The flexible design of the MMS makes it sui-Emissiontable for use in many applications. They can beclassified according to the measurement princi-

To determine the spectral emission of a lightples used, the fields of use or the materials to besource, part of the light is directed to the spectro-

analyzed. The MMS modules using cross-section

meter module. In view of the high light sensitivi-

converters as optical inputs offer specific bene-

ty of the MMS modules, it is in many cases suf-

fits in practical work. lt is very easy to connect

ficient to bring the fibre near the light source.

other fibres using the SMA connector.

For optimization, an (achromatic) collective lenscan be used.

In addition, the round input offers considerably better conditions for most illumination situa-

Examplestions than the classic entrance slit. However, the■ Checking illuminators (aging)most important asset of the MMS modules is■ Determining the wavelength of LEDs

their compactness, light sensitivity and insensiti-or (tunable) lasers (see page 9)vity to external influences. This allows them to■ Luminescence, fluorescence

be directly integrated in processes; for example,■ Monitoring the solar spectrum, burns,

the MMS modules can be built into printing
machines for the on-line inspection of paper ordischarges or plasma■ Determining the temperature T in accordancecolour quality. In most of the applications mentio-with the Wien displacement law. e.g.:

ned in the following, an on-line check is highly

3000 K ↔ 9 6 6n m

interesting.
Requirements
Measurement principles
1. Emission

The wavelength accuracy which is very highconsidering the size of the module allows an

2. Diffuse reflection

exact determination of the wavelength of light

3. Reflection

sources which emit a single line, such as LEDs

4. Transmission – absorption(calibration), using the sub-pixel resolution pro-

5. Interference spectrum

cedure. The MMS modules are not suitable forthe analysis of emission radiation containing
spectrally adjacent lines which are too close
together (see page 8).

λmax x T = 2.8978 x 103 m x K

18

Diffuse reflection
The diffuse reflection of scattered light (fromrough surfaces) supplies information on the colourof the surface. Important for this procedure (inaddition to the spectrometer used) is the lightsource and the position of the detector (angle
with surface normal). In most cases, a light source with a wide-band emission, e.g. a halogen

Reflection

lamp, is used. Here, too, it is often sufficient tobring the input of the cross-section converter

Reflection is the directionally reflected lightclose to the coloured surface to be measured

thrown back from (low-scatter) surfaces. In addi-

without using additional optics.

tion to the sensor, a light source is needed. ltshould be noted, that reflectivity is strongly de-

Examples■ Colour measurement of different
surfaces (materials), e.g.
■ Condition of coatingspendent on the angle α. The most simple setup
for measurements at α = 0° can be obtainedusing a special light guide which supplies thelight and also directs it to the detector.

■ Analysis of paper quality■ Determination of fat contents in meatand sausages■ Determination of the humidity content incereals, food and cellulose

Examples■ Coatings in general, antireflection coatingsof surfaces using metals or dielectric coatings■ Elipsometry■ Plastic identification for recycling and disposal

Requirements
The MMS 1 module has been specially desi-

Requirements

gned for colour measurement. Its high repeatability

Many reflection spectra do not display any

and light intensity combined with moderate spec-

marked structures. For this reason, high absolute

tral resolution exactly meet the demands made inwavelength accuracy is considerably more impor-this field.

tant in many cases than good spectral resolution.

α=βInterference spectrum
When white light is incident on opticallyTransmission
The transmission of material with a thickness d(partially) transparent layers, interference occurs,

supplies information on the spectral dependence

as the path difference between specific wave-of the absorption constant α(λ) (I0 : incident inten-

lengths is exactly a multiple of the optical layersity, l(d) : transmitted intensity). Immersion probes

thickness n x d (λ1, λ 2: position of the extrema,connected to a light source and an MMS module

spaced at a cycle). lf the refractive index n isusing fibres is the simplest system for measuring

known, the geometric layer thickness d can be

the concentration c of liquids. The concentration

calculated. The fibre Interface ensures easy cou-

is related to the absorption constant via the ab-pling to microscopes or the flanging to coating

sorbance coefficient ε. In other cases it is advisa-

systems. lf the layer thickness d is known, disper-

ble to set up a collimated beam path. Measure-sion n (λ) can be determined.

ments where the input of the cross-section converter is in direct contact with the object to be

Examplesmeasured can also be perf o rm e d .

■ Layer thickness measurements ofphotoresists, films and dielectric layers

Examples■ Measurement of filters

Requirements
To ensure exact thickness measurements, highabsolute accuracy of the wavelength is again required. The maximum measurable thickness is linked to the spectral resolving power (separation of
two interference maxima), the minimum thickness
to the spectral range to be covered (recording of(colour filters, interference filters)■ Measurement of the concentrations of liquids■ Thickness measurement if absorption
coefficient is constant■ Determination of the sugar and alcohol
contents in beverages■ Quality control in the petrochemical industry

at least one half-cycle). The measurement of eventhinner layers (evaluation of less than a halfcycle)
requires that absolute intensity values be known.

Requirements
Here, too, very high spectral resolution is lessimportant than a very good wavelength accuracy

E.g. MMS 1, n = 1,5

and high dynamic resolution – as provided by thedmax, ≈ 25 µm, dmin, ≤ 0,2 µm

MMS modules,

Lambert-Beer-Law
I = I0 x e-α x d
α=εxc

2 n x d = λ1 x λ2 / (λ 1 - λ2)

20

Applications/fields of use
Measurement of■ colours■ layer thickness■ temperature■ wavelengths■ luminescence■ fluorescence■ concentrationofin■ LEDs■ monitoring■ lasers■ process control■ illumination■ quality control■ cloth■ calibration■ paints■ recycling.

■ liquids
Benefits of the MMS modules
All spectrometer components are permanently connected with each other. This results ina very high insensitivity to mechanical shock,hence ensuring high reliability. In addition, thewhole configuration is maintenance-free, i.e. recalibration is not necessary.
The following benefits result directly from themonolithic design used:■ repeatability – reliability■ wavelength accuracy – calibration standard■ compactness – ruggedness■ high sensitivity■ versatility■ usability in industry■ easy-to-use interfaces

21

Specifications
Past Modules
ModulesSpectral range [nm]MMS 1

MMS UV-VIS

MMS UVMMS NIR 1.7 t1

MMS NIR 2.2 tc

MCS

310 – 1100

190 – 720

195 – 390

950 – 1700

900 – 2200

190 – 1100

oror

different

250 – 785

1500 – 2200

rangesSpecified

360 – 900

spectral range [nm]400 – 1100

or

(NIR enh.)

250 – 785

<10Spectral resolution

220 – 720

(Depening on(Depening on

the number

the numberof pixels)of pixels)

220 – 390

950 – 1700

see above

220 – 1000

<7

<3

<18

<18

<3

a p p r ox . 3.3

approx. 2.2

approx. 0.8

a p p r ox . 6

approx. 6

approx. 0.8

0.3

0.2

0.2

0.6

0.6

0.3

InGaAsInGaAsS3904-512Q[nm]
Pixel dispersion [nm]∆-precision [nm]Diode array

Hamamatsu S3904-256Q with 256 pixels
2of 25 x 2500 µm

S3904-1024Q

S4874-256Q for NIR enhanced
Optical input

Fibre bundle cross-section converter

see MMS 1

see MMS 1∅ = 0.5 mm∅ = 0.5 (0.4) mm; NA = 0.22;∅ = 0.2 mm∅ = 0.2 mm

NA = 0.22; SMA

input slit

mounted in SMA plug
output slit

7 0µ m x 2 5 0 0µ m

0.1 x 1.0; 0.5; 0 . 2 5m m2

8 0µ m x 2 4 0 0µ m

(Depending on diode split)
Electronic output

Pre-amplifier, video output 3 Vs s

4 Vs s

4 Vs ssee MMS 1

Strip management

In acc with Hamamatsu data sheet,

see

see

see MMS 1

operatingoperating

instructions

instructions

TTL level

Carl Zeiss Jena GmbHOEM – Spectral Sensors

D – 07740 Jena
Phone: +493641642838

Fax:

493641642485+

E-Mail: info.spektralsensorik@zeiss.de
Internet: http://www.zeiss.de

For further details, please contact: