DE2238662C2 - Monochromator - Google Patents

Description

ε =

-j- (C ₂₁ + 2 C ₄ )

results, where

W _{0 is} the grid width,

C ₄ , Qm and C22 are the 4th order aberration coefficients and

ρ is the ratio of the height to the width of the grid

are.

2. Monochromator according to claim 1, characterized in that a drive (25) for rotation of the diffraction grating (Λ) is provided.

3. Monochromator according to claim 2, characterized in that two exit slits (SO at different angles, based on the incident beam, are arranged and that for each of the two Austr '' ^ column (SO the condition T + T ' = f is met.

The invention relates to a monochromator with a concave spherical diffraction grating, a fixed entry slit and a fixed exit slit.

Such monochromators are known (DE-AS 10 56 854). They are used to make from polychromatic Weed out light with a narrow range of wavelengths. Through a spherical diffraction grating dispersion and imaging are effected at the same time.

The beam path of such a monochromator is shown in FIG. 1. The image of a source assumed in the entrance slit 5, which is located at a distance r from the grid, is dispersed in different directions on the grid. The diverging rays are bundled at a distance / ■ 'from the lattice apex. These distances are dependent on the wavelength X _1. The plane adjoining the lattice apex is characterized by the radius of curvature R, the number of lines iY per mm, the dashed width W and the vertical dashed height L. The direction of the line defines the local vertical. The axis of rotation of the grid runs parallel to the local vertical through the grid apex. The entrance slit S runs parallel to the lines of the grating. The exit slit S ' (or in FIG. 1 the exit slit S \ .S'"S'" door the different wavelengths) is arranged parallel to the direction of the astigmatic, tangential focal lengths. In general, the center of the entrance slit is in the horizontal plane containing the grid norm NO , and the exit slit runs parallel to the lines of the grating. Focussing is done either by moving a receiver depending on changes in the direction of the output beam or by rotating and simultaneously moving the grating.

In the case of a monochromator for practical use, the directions of the incident and failing rays must be solid. In addition, if only a simple grid rotation for reasons of manufacturing cost to be made, dana shows the classical theory that to achieve a good

ίο focusing a shift of the exit slit In this case, the tight displacement tolerances of the grid are dictated by the displacement tolerances the column replaced

An experiment by SEYA (Sei. Light, Vol. 2 [1952], p. 8) has shown that a monochromator with stationary slits can be realized if the angle 2 Θ at which you can see the two slits from the top of the grating closes 7O ° 3O 'is made. In this case one obtains fixed values for r and r ¹ which are made equal to R cos θ . However, the corresponding structure shown in FIG. 2 does not provide a satisfactory optical quality. It gives low resolution and low brightness. Since angle of incidence α and angle of refraction./? are large, there is also a strong degree of polarization and increased astigmatism. This result shows that when focusing is carried out with a rotating concave grating , it is necessary, on the one hand, to maintain a certain angle θ very precisely, and, on the other hand, to require resolution

jo and brightness still leave a lot to be desired. This is because the known monochromators the aberrations are not taken into account. One neglects the fact that without taking into account of the aberrations results in an unfavorable reference sphere. Therefore a correspondingly narrow Diffraction grating can be used.

In the prior art, the entrance slit and exit slit are arranged in such a way that the basic equation of focusing T + T'-O is fulfilled. Here 7 ″ represents the tangential focal length on the object side and T the tangential focal length on the image side. The tangential focal lengths are defined by the focus of the meridional bundle, i.e. that bundle that runs in the meridional plane (spanned by the main ray and the perpendicular in the lattice vertex) The tangential focal lengths T and T ' in the focusing condition T + Γ'— Ο are:

• x ·

cos σ

_T , _ cos ² ./? _ cosj?
r 'R

(H. Greiner and E. Schäffer in "Optik" 16, No. 5,1959, pp. 288-293.) In these equations, α means the angle of incidence, β the angle of reflection, r the distance: lattice vertex entry slit, / - 'the distance : Grid vertex image of the entrance slit (exit slit) and R the radius of the grid in the Rowland circle plane.

If one sets r = R · e and r '= R ■ e', one obtains for the tangential focal lengths:

results, where

There are special mathematical methods for solving the equation Γ + T— 0. SEYA (Sei. Light, Vol. 2 [1952], p. 8) and NAMIOKA (J. Opt Soc. Am. Vol. 51 [1961], p. 4; 13) have shown that only one possible value ( 70 ⁰ SOO of the angle 2 Θ exists, under which you can see the two columns from the grid vertex, for which the image distance and the object distance are equal to R cos θ .

The invention is based on the object of creating a monochromator of the type mentioned at the beginning, in which the entrance and exit columns are arranged so that, taking into account the width and The highest possible resolution is achieved at the height of the diffraction grating.

To achieve this object, the invention provides that the entry slit and the exit slit are arranged with respect to the grating in such a way that the sum of the tangential focal lengths Γ and T on the object side and on the image side equals the value

, _W ₀ ^2/6

"TlT

(C ₄ + C ₀₄ ) + γ (C ₂₂ + 2C ₄ )

cosgl 1 sir

W _{0 is} the grid width,

C ₄ , C ₀₄ and C _{22 are} the aberration coefficients

4th order and ρ the ratio of height to width of the grid

are.
The focusing equation results from this

T + T '= ε,

where ε depends on α and β and on the aberrations of the image on a given line through a concave grating with a rectangular pupil. The monochromator according to the invention allows good focusing with a simple rotation of the grating and with narrow gaps, the resolving power being improved in particular. Alternatively, while maintaining the same resolution, the grid width can be reduced and the line spacing increased. Overall, in addition to the resolution, the brightness is also improved.

The aberration coefficients C ₀₄ , C ₄ and C ₂₂ result from the known general equation for the grating spectrum (e.g. Journal of the Optical Society of America, Vol. 40, No. 3, March, 1950, 153 ff., In particular 155). The aberration coefficients can be derived from this as follows:

cos ² j8 IT \

Γ 1 _ cosgH 1 Γ 1 _ cosjSH _ 1 Γ cos ² g _ cosg

Lr R J Ta ² " LV Τ *] 77 L ~ r ~ ~~ R ~.

SR ²

1_ Γ cos ² ^ _

8r 'L r' Ä J

_ 1 sin ² g fl cosffH 1 sin ² j8 Γ 1 -osjffH, 1 Γ 1 cos a ~ \

~ C? ₂ —r ~ —ϊ - - —r— + -τ- —τ> - —γ - —ξ— τ * - - - ~ - I

"2 r ² Lr tR J 2 γ ' ² L r' tR J

ΓΙ cosjSH 1 Γ ^cos2 C cos ο 4Ä //?

j Γ_1_ _ cosjgH _ 1 Γ cos ² g _ cosgH Γΐ _ cosgH

RtK [τ R J 17 L ~ r ~ ~ ίγ \ Lt "TjTj

_ _ \ _ Γ COS ² ^ S _ COSjgH Tj_ _ COS ^ H ₁

4 / - 'L τ''R J Lr''/ Λ. -r = ^J Γ ± - - £ 2 ££ f | + ί Γ-L -

⁴ St ² R ² Lr / Aj β / ² « ² Ly

Here Λ is the radius of the grating in the horizontal plane and tR for a toroidal grating is the radius of curvature in the vertical plane. For spherical grids / = 1.

The following considerations led to the invention:
The theoretical resolving power given by the refraction is equal to KNW, where K is the order of refraction and W is the dashed width. This resolving power is limited because as W increases , the aberrations increase and a tolerance must be determined which is tied to the changes in the image spot. The optimal resolution is therefore equal to KN W ₀ . In general, one also determines a practical resolving power R " that depends on the width / and / 'of the entrance resp.

'R J depends on the exit slit and is expressed by
/ ■ ' KNX

63 ' /' COS ^

In the case of minor / aberrations, the practical limit of the resolving power results from R _p = 0.8 KNW ₀ , whereby the value W ₀ of W is calculated according to the criterion of STREHL (Instrumentenkunde, VU. 22 [1902], p. 213). In the geometric hypothesis (strong aberrations), a quality factor results in a limit resolution <d X> for each value of the pair 'W, L , and thus the practical limit resolution power R _p = Xl <8 X>. This quality factor Q is defined as follows:

Ul WIl

_L Γ f (UL + UL) ²

WL JJV Bw dl J

-L / 2 -W / 7

dnd / S

A ' is the deviating optical path, which depends on the grid coordinates B' and / in the direction of the grid width W or the grid height L.

If one makes the distance of the exit slit from the grating apex stationary for a given spectral region, one can assume that the deviations A (w, I) or aberrations of the optical path, which are given by the "first order equation", will be compensated Need to become. According to the invention, these deviations are derived from the quality factor Q , which also expresses that the aberrations

cos ² a . co

are compensated for when the reference sphere is shifted by C \ W + C ₂ W ¹ with respect to the initial conditions of the first-order equation mentioned, so that the generalized focusing equation (7 * + 7 "= ε) is sufficient, with the remaining deviation A ' in this case A (w, Γ) + C ₁ π- + C ₂ tv ^2. C | and C ₂ are coefficients, and under such conditions the basic focusing equation of a device is given

is easy rotation too

= cosa + cosjS + R ^ i- \ 4- (Q + C ₀₄ ) + 4 "(C ₂₂ + 2 C ₄ ) P ² I = H (X).

In this equation, Q ₄ , C _4, and C _{22 are} the aberration coefficients4. Order and ρ = -. In the "first-order equation" the first term would be the same (cos a + cos ß). °

On the basis of these results, practical implementation must enable a continuous band AX to be made as narrow as possible in a given direction, since the direction of the incident beam and the position of the slits are fixed. In particular, it must meet condition (2). This can generally be solved by iteration, since H (X) depends on the value of the parameters e and e '.

According to the quality factor Q defined above, the error c λ inst must be below a tolerable limit value t when it is carried out:

BX inst S t

X>

For the practical limit ^ resolving power R _p = XI <IX> and the slope ρ of the curve λ /? λ inst applies:

SSOJL

2/1

The practical limit resolution according to the STREHL criterion (Instrumentenkunde, Vol. 22 [1902], p. 213) is equal to R _p = 0.8 NW ₀ K, and ρ must be the condition

satisfy, where the second term of this inequality is independent of X.

One or the other structure of FIGS. 3a to 3d is chosen for a given angle θ Resolution R _{p is} sufficient In the borderline case, assuming very small values for W and L , condition (3) can still be fulfilled, but the corresponding devices will then not be of practical interest, since their light intensity would be very weak, which has an effect particularly in the spectral range of the distant ultraviolet, where the energies of the light sources are generally not very high compared to the other spectral ranges.

The use of spherical concave diffraction gratings, which are in the arrangement »in piano «steep, d. H. in which the centers of the entry and exit gaps are in a plane that contains the grid normal and runs perpendicular to the direction of the lines of the grid, the entry and exit gaps having a fixed width (ie independent of the wavelength X), so that there is no loss of flux in the plane of the exit gaps.

The monochromator according to the invention can optionally either with a fixed diffraction grating or with rotating diffraction grating. The second embodiment can be known as Spectrometer can be used. In this case is a Drive provided for rotating the diffraction grating. According to an expedient development of the invention it is provided that two exit slits at different angles, based on the incident Beam, are arranged and that for each of the two Exit column the condition Τ + Τ'— ε is fulfilled. This makes it possible to separate two different wavelengths at the same time with a single monochromator. The concave grids used can be such grids

be like this, which consist of lines applied to glass (or copies of such gratings) or, in general, optical elements that simultaneously ensure the focusing and diffraction of an incident wave, regardless of the method used this refraction is carried out under the condition that, if d is the grating which is constant over the entire pupil of the optical element, the basic equation for an arrangement of this type is:

(6)

In this expression, α and β are the angles of incidence and diffraction which, based on the lattice formula

len, N - -j is the number of lines per mm, K is the order of refraction and X is the wavelength; the negative sign applies to all broken beam

len that lie in the image space and are enclosed between the central spot (or = -ß) and the tangent of the grid.

These focussing takes place predominantly with holographic gratings of the 1-type, in which the lines of the grating are generated by a holographic process, and in which, as in the case of the classic line grazers, the first-order focusing equation is characterized by the relationship T + T ' - 0. Therefore the corresponding description is only ι ο based on the use of spherical concave grids, the optical characteristics of which are equivalent to those of the usual grids. Their lines are defined by the intersection of the surface of a concave mirror with parallel planes at the same distance.

Above it was stated that if one takes into account the aberrations (ie higher order terms in the equations derived from Fermat's principle) and if one takes into account the possibility of compensating for these aberrations by shifting the reference sphere, ie through a suitable choice of the image distance, monochromators can be realized in which the object and image distances are fixed, and this in a larger spectral interval, with rotation of the grating around an axis that goes through the vertex of the grating. It has been found that such arrangements, which have a low cost price, since there is no displacement of the grating, also have the significant advantage for the user that the direction of the incident and broken bundles is absolutely fixed. It has also been found that, contrary to the solution proposed by SEYA and then by NAMIOKA, which for ultraviolet is limited to a single angle close to 70 ° 30 ", it is possible to make arrangements with any values of 2θ . The objects - and Siid distances are given by a generalized focusing equation in the form of equation (2), in which higher-order expressions occur. The equation thus depends on the value of the dashed width W and the dashed height L (or on the relationship ρ = L / W ) from. It must be added that other values of 2 θ have been provided by SEYA and Namioka, but these were associated with a considerable reduction in the resolution (or light intensity) when the image distance remained firm, or with an additional mechanical Complication when the value r ^{1 was} adjusted experimentally as a function of the wavelength

The drawings show various embodiments of the known technology as well as curves and embodiments for explaining the invention. It demonstrate

F i g. 1 to 3 d schematic representations of monochromators according to the known technology,

4 shows a plan view of a monochromator, partially in section, with narrow, straight or curved columns,

F i g. 4a a vertical section, 4b the control of the grid rotation,

Fi g. 4c is a diagram corresponding to the monochromator of FIG

F i g. 5 the values of the reduced object and image distances e, e ' as a function of θ for different ss grids,

FIG. 6 is a diagram showing the value p for the various arrangements of FIG. 3, but with corri the yawed tangential focal length (for the case of weak deviations),

7 shows the values of the resolving power as a function of θ for a given grating (in the case of large deviations),

Fig. 8 (8a to 8d) the basic diagram of the various asymmetrical arrangements for focusing with a simple rotation of the concave Grid, where the values of the angles or and./? are recognizable for incidence and diffraction,

F i g. 9 is a schematic top view of a monochromator with straight columns of fixed width;

10a and 10b are diagrams showing the value of <S λ> and the value of the practically achieved resolution d λ _ρ = λ / Rp for the two examples in Table 1,

11 schematically shows a double monochromator with Z-shaped structure.

In the embodiment shown in FIGS. 4, 4a and 4b, the monochromator rests on a vacuum stand which carries a plate A on which the various elements according to FIGS. 4, 4a, 4b and 4c are arranged.

The vacuum stand, which is not shown, carries a plate A on which, on the one hand, a central block 2 is mounted, in which the concave grating R and the rotating mechanism of the grating are arranged. On the other hand, a block 4, which carries the entry slit, and finally blocks 5 and 5 'with the exit slits are attached to the plate A. The blocks 4, 5 and 5 'are connected to block 1 by pipes 6 which are provided with vacuum-tight bellows membranes 7. These make it possible to adjust the gaps by means of the screws 8 provided on each block 4, 5 and 5 'and allow the blocks to be displaced accordingly in guides of the plate A , not shown. In the case of each block 4, 5, 5 ', the width of the column can be adjusted by a device 9.

The disk A also carries a control mechanism 10 for controlling the wavelength, which will be described later.

According to FIG. 4 a, the block 2, which is fastened in the block 1 by nut bolts Γ, has a conical part 11 which is arranged with a vertical axial bore, the axis 12 of which forms the axis of rotation of the grid R. At its lower end, the axis 12 is firmly connected to the grid R , which is fixed in a bearing 13 which is mounted on the block 2 by suitable means. The axle shaft 12 is kept mechanically precisely aligned by two ball bearings. The first ball bearing 14, which is pressed onto the axis of rotation, is held in position by a shoulder 15 of the shaft and is pressed by a part 16 against an inner shoulder 17 of the conical part 11, while the other ball bearing 18 is pressed against an inner shoulder by two intermediate pieces 21 ' 19 of the part 11 and a shoulder 20 of the while is pressed. The clamping is ensured by the structure 21 with nut and brake 22. The free end of the axle 12 protrudes beyond the structure 22 and carries a horizontal ann 23 which is firmly attached to the shaft

The arm 23 here runs parallel to the tangent at the apex of the grid R (see FIGS. 4b and 4c), but it can be given any other desired or advantageous fixed direction with respect to the grid firmly connected to the plate A. A printing device 24 carries a drive 25 to advance the displacement of a roller 26 according to the axis of the printing device.

gain weight. The roller 26 acts against the arm 23. The contact between the roller and the arm is ensured by a return spring 27 acting on the arm. Under these conditions, the displacement of the roller 26 on its pressure device 24 acts on the arm 23 and causes a rotation through the angle γ, which enables the passage of waves through the exit gap. In addition, in this way one can measure the wavelength A with sufficient approximation, which is expressed in the following formula:

A = - cos θ siny.

(7)

The simple movement described results from an impact on the arm 23 by means of the roller 26, which is formed by a ball bearing with the radius u , which moves linearly as a function of time in the direction HoZ and an angle θ with respect to a parallel to the direction the normal for the value γ = 0 of the rotation, ie door forms the central spot (A = 0).

In the following, the cases of two grids will be examined in detail, viz

/{,(Radius"
A ₂ (radius R

■■ 500 mm, / V ■■ 500 mm, N

1831.8 lines / ^mm); 1221.2 lines / mm).

The corresponding structures were made with straight columns 10 mm high and wide

rNKX
cos a R.

r'NK cosßR.

- f for the entrance slit and

= / 'for the exit slit

for simple monochromators and those with multiple outputs. There are values of 2θ (approximately 28 °) at which the width / 'of the exit slit can remain the same for all wavelengths without light being intercepted by the slit edges.

With single monochromators you first have the results and conditions according to the STREHL criterion checked.

The values of e and e ' for the two grids R ₁ and R ₂ are shown in FIG. 5 as a function of θ. If you change the radius of curvature, the values of e and e ' practically do not change.

With a dashed height of 25 mm, the values of W are between 10 and 14 mm for changes in B between 6 and 45 ° and a reference wavelength of 75 nm. It should be remembered that the STREHL criterion is indeed for any value of A indicates other values of W For a practical implementation, one chooses an average value corresponding to a certain value of the wavelength. It will be found that the STREHL test beyond θ = 50 ° is inadequate for the dotted height considered, and that below θ = 6 ° the values of e ' (structures Fig. 3a, 3d) or of e (structures F i g. 3 b, 3 c) are too high to lead to practical implementations. F i g. 6 gives the value ρ = f (ß) for the grid R ₁ and the various arrangements. The solid curve shows R _p / 1.54, the broad bar determining the range θ for the

the limit resolution R _{p is reached} in a spectral range from 20 to 320 nm. In the arrangements of FIGS. 3a and 3d, the range extends approximately from 32 ° to 36 ° 30 ', while in FIGS. 3b and 3c it extends from 26 ° to 36 °. In the case of gilter R ₁ , the range θ is between 30 ° and 40 ° for the structures in FIGS. 3a and 3d.

Condition (5) thus limits the possible door area to the angle 0, which otherwise corresponds to the values of e and e ' , which are completely compatible with the handy realization of a monochromator.

If one is satisfied with a resolution of the order of 5000, then the arrangements of FIG. 3 a and F i g. 3 d (grid R _x ) between 27 ° 3O 'and 39 ° 30', the other two between about 20 ° 30 'and 39 °.

In summary it can be said that the use of the grating "S ₁ rnii of an optimal dashed area of 11 x 25 mm" allows a limit resolution of 7500 door 26 ° < θ < 36 ° 30 'to be obtained. But it is over For reasons of brightness, it is preferable to take the quality factor Q into account.
Therefore, the results and conditions are examined according to the figure of merit Q.

The calculation shows that the values of e and e ' according to FIG. 5 for W = 30 mm and L = 54 mm (p = 1.8) are still practically valid. The limit resolving power Λ, for the various values of ρ is shown in FIG. 7 in

ίο Dependence on θ for the two selected wavelengths A ₁ and A / are shown. Their values result in δ λ inst, minim, in the observed spectral range. For ρ = 1.8 and for θ between 6 and 50 ° the practical limit resolving power would be 30 to 75 nm and 800 to 250 nm. As before, the range of validity is limited by the desired value of R _p. If the above values are to be obtained, the calculations show that, on the one hand, the interval of θ increases when one moves from the arrangement of FIG. 3b to the arrangement of FIGS. 3 a passes, and on the other hand with p. For p = 1.8 the grating R ₁ can be used for values of θ between 22 ° and 40 ° and grating R ₁ between about 20 ° and 50 °. In summary: the use of the grid R ₁ with an optimal dashed line

An area of 30 x 54 mm ² allows a limit resolution between about 3000 and 8000 to be obtained for 22 ° <θ <40 °. With the same resolution, the second solution turns out to be much more interesting because it corresponds to a brightness gain of a factor of 5.9.

From these last-mentioned findings, based on the use of the selection criterion ρ = / (θ), it follows that the arrangement of F χ g. 3a alone is to be maintained and that there is a limit to the range θ , which essentially depends on the desired value R _p , the latter becomes for each value of θ between 22 ° and 40 ° for grid R ₁ and 20 ° and 50 ° for grid Be maximal.
The following table shows the various values of the parameters W, L, e and e ' for the grids R ₁ and R ₂ and the two particular values of Θ.

Grid / ?,

If ₀ = 30 mm

I ₀ = 54 mm

θ = 40 °
θ = 30 °

β = 0.80386
e = 0.82789

e '= 0.72306
s' = 0.91332

Grid /? 2

-Q = 30 mm

54 mm

Θ = 50 °
θ = 20 °

e = 0.76897
f = 0.85117

e ' = 0.51620
e ' = 1.0557

The advantages of such a focusing technique are numerous. In optical terms: the maximum Performances of the grating in this arrangement can be achieved. The invading and broken ones Bundles are solid and it is therefore possible to attach an additional optic in front of or behind the gaps, their Axis does not have to be changed depending on the wavelength.

In mechanical terms: a simple rotation is always easier to perform than a translation, especially dar.p., if the tolerances of the psrleüsmus are narrow, as is the case with the diffraction gratings. In particular, the structure is favorable when grids with large radii of curvature can be used.

For monochromators with multiple outputs, d. H. with a fixed direction of the incident beam and several fixed directions that are different at the same time Supply wavelengths, one can ensure that

a) the simultaneous realization of two exit slits with high resolution in each of the two spectral ranges is possible,

b) the simultaneous realization of two entry slits for light of different spectral ranges and with a single exit gap is possible, and that

c) Simultaneous photometric measurements in different spectral ranges with one device:

ίο are possible.

a - Examination of the curves of Figure 5 shows that two values of θ correspond to one value of e. A good resolution can be achieved for these two values if the values of the image distances are those given by the curve e ' = / (©). If a resolution of 0.03 pm is required (grid λ | in the arrangement of FIG. 3a), this object distance ^{can be used with an object distance v} Qn O ₋ RP R

2SS two outputs correspond to one at an angle of 2 θ = 44 ° (f - 1.08 R) and the other at an angle of 2 θ = 68 ° (r ¹ = 0.84 R).

b - If one takes an object distance r = Re equal to 0.81 Λ for the grating Λ, in the structure according to Fig. 3a, one can choose θ to cover a spectral range of 8 to 340 nm for the values dd, e, e \ θ as follows:

Dashed area 30 x 54 mm ² Θ = 14 ° θ = 35 ° θ = 75 ° e = 0.81 0.81 0.81 e '- 1.391 0.833 0.4203 Medium resolution 0.5 nm 0.03 nm 0.5 nm Spectrum range 550-430 nm 3u-juu nni 80-75 nm 'all of photometric or reflective this case

For polarization measurements, a resolution of a few 0.1 nm is acceptable. On the other hand, it is very often advantageous to carry out these measurements in a large range of wavelengths, which in fact requires the use of two different set-ups with different characteristics that are difficult to compare. The grating A ₁ in the structure of FIG. 3a can operate between 150 and 430 nm when 2 θ = 29 ° (r ¹ = 1.44 R) and between 20 and 200 nm when 2 θ = 100 ° (/ ■ = 0.518 R), with a single object distance of 0.77 R. In addition, a similar setup can prove to be of interest, on the one hand for measuring the grating performance depending on the interference order and on the other hand for calibrating the sources and receivers.

Since the values of e in structure 3a (or 3d) are equal to those of e ' in structure 3 b (or 3 c) and are reciprocal to e' , a structure with two inputs and one output can be implemented in the same way.

If the wavelength observed on the first beam

Κι λι = 2 siny ccis Θ _Χ ΙΝ
the one on the second bundle must be in

K ₂ X ₂ = 2 sin γ cos Θ ₂ / Ν = Κ _λ A, cos 8 ₂ lcos θ _χ

be. A wavelength λ can be in order 1 (Ki = 1) and in order 2 (K ₂ = 2) at the same time

so observed when cos Q ₂ = 2 cos θ \. The use of the grid R ₂ in the structure of FIG. 3a, with an object distance r = 0.6 R, allows this condition to be implemented if approximately 0, = 60 ° 45 ' (f = 0.38 R), and if θ ₂ = 4 ° (r ¹ = 2.1 R) , with only a wide, continuous (frequency) band (S 1 nm) being required.

The embodiment of FIG. 9 largely corresponds that of Fig.4, but is only one Block 4 with an entry gap and a block 5 with an exit gap.

Below are some results that can be achieved with Single and double monochromators according to FIGS. 9 and 4 were achieved with the described Mechanisms and fixed entry and exit slits are provided.

In Table 1 below, two examples are given for which the value of θ with / · = 10 μπι has been determined

N »Ό 13th L ₀ Γ 22 38 662 H 14th X min λ max xA Str / mm well mm mm mm nm ηπί ι Table 1 1200 38 30th 152.28 6th 5 110 1 R. 1221.2 54 54 412.06 2Θ 8th mm 35 400 I. mm mm 0.195 1 400.7 94.47 144 ° 52 ' 0.007 500 594.51 28 °

1. Single monochromators

In the F i g. 10a and 10b are shown for the two cases under consideration (Table 1) on the one hand the value / 'as a function of the wavelength and on the other hand the values of the theoretical resolution <δλ> and the practical solution δ λ _ρ

With the grid of 500 mm (FIG. 10b), which works with 2Θ = 28 °, the difference between <δλ> and δ λ _{ρ is} negligible for the gap heights considered, although the opening is large.

In the case of the 400.7 mm grating (Fig. 10a) the difference is only about a tenth of the limit resolution, although it is between <δ λ> and δ λ _ρ and larger (-0.05 nm), the aperture 1 / 4 is. This aperture is very large for the spectral region under consideration in comparison to that of the current commercially available devices, where it is equal to 1/75. It can therefore be seen that it is possible to realize simple monochromators in which

the incident and flexed bundles are fixed, the entry and exit slits are fixed in position and Width are

the grid is excited by a rotary movement by a mechanism that simultaneously controls the Wavelength measurement enables.

With a rotation γ of the grating, at the exit slit of width / J, continuous bands δ X _1n whose length depends on / 'and whose intensity maximum lies in the direction of β + Aß:

C ₁

cos ;? Δ β = - (c _m + - ^ p- A - ^ - = KNA X,

which corresponds to a wavelength K {λ + AX) .

In practice, AX cannot be observed in the weak dispersion structures, and wavelengths are observed for a given rotation of the grating

X, j, j first, second and third order. It surrenders

thus a superposition of the orders in the plane of the exit slit. This phenomenon that is known can only be avoided, either by using filters or by a structure that ensures pre-filing, i.e. H. especially with double monochromators.

2. Double monochromators

Since only a limited number of filters are available for ultraviolet, it is necessary to implement double monochromators in order to separate the orders. The exit slit S \ of the first structure plays the role of the entry slit of the second structure (Aus step S ₂ ). This clearly reveals the interest shown for the single monochromators in such a structure, in which the central gap S \ is fixed in position and width. You can therefore create structures in "Z" shape with two grids R ₁ , R ₂ (Fig. 11), where a construction M (Fig. 8a) is added to a construction M] (Fig. 8c), or by connecting a construction M ₂ (Fig. 8b) with a construction Λ / ₄ (Fig. 8d). So it is possible to use double monochromators too build, whereby the separation of the orders is guaranteed and the spectral unit is increased. One works with very high brightness and increased resolution and with an angle of 2θ between the two angles which are considerably below 70 °. Furthermore who the strong loss of intensity due to astigmatism and Polarization avoided.

In addition 7 sheets of drawings

Claims (1)

Patent claims: 1. Monochromator with a concave spherical diffraction grating, a fixed entrance slit and a fixed one! Exit slit, characterized in that the entry slit (S) and the exit slit (SO with respect to the grating (R) are arranged in such a way that the sum of the object-side and image-side tangential focal lengths Γ and T ' equals the value