Stray light trap in a monochrometer

A monochrometer having a stray light trap which substantially directs stray light away from light of wavelength of interest and/or absorb the stray light so as to substantially reduce the stray light component in the light of interest. The monochrometer has internal surfaces each having one of several optical characteristics.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the production of monochromatic light, and 
in particular relates to a monochrometer for use in an analytical 
centrifuge. 
2. Description of Related Art 
The following United States Patents are incorporated by reference herein: 
U.S. Pat. Nos. 4,830,493; 4,919,537; and 4,921,350. These patents were 
issued to the inventor of the present invention and were commonly assigned 
to Beckman Instruments, Inc., the assignee of the present invention. The 
patents describe monochrometers designed for use in analytical 
centrifuges. 
A monochrometer is a device used to supply a collimated beam of light 
having some desired, narrow range of wavelengths. A monochrometer 
typically has the following component parts: (1) an entrance slit with a 
source of radiation of a wide range of wavelengths; (2) a prism or 
diffraction grating dispersing the incident radiation into a continuous 
spectrum of wavelengths; (3) some mechanism to rotate the prism or grating 
so that the desired spectrum of radiation is obtained; and (4) an exit 
slit selectively isolating a narrow band of wavelengths. For 
spectrophotometry studies, a detector is positioned at the monochrometer 
exit for detecting the radiation antennuation of a sample placed between 
the exit and the detector. Appropriate signal amplification circuit is 
provided in conjunction with the detector. 
Referring to FIG. 1, the arrangement of a prior art monochrometer in an 
analytical centrifuge as described in the patents is briefly summarized. 
The centrifuge 10 comprises a rotor 12 driven to rotate about an axis by a 
motor 14. The rotor 12 has several sample cells 16 having transparent 
windows to allow a monochromatic light beam from the monochrometer 18 to 
be directed vertically through each sample cell 16 as the cells rotate 
pass the beam. A detector 20 is positioned below the rotor 12 in line with 
the beam. As illustrated in FIG. 1, the monochrometer 18 comprises a light 
tube 22 folded along its length. Light from a source 24 is directed 
through the light tube 22 to a mirror 26 having diffraction rulings. The 
mirror 26 can be tilted to direct light of a particular spectrum range 
through the remaining length of the light tube towards the rotor. A slit 
(not shown) at the tube exit select a narrow wavelength band from the 
spectrum for transmission through the sample cells 16. 
The prior art monochrometer has certain limitations in obtaining a true 
monochromatic beam of light. Specifically, the diffraction grating mirror 
reflects light at wavelengths other than that of interest, including 
higher order wavelengths. The light exiting through the slit at 
wavelengths other than that of interest is often referred to as "stray 
light". The stray light is scattered at various angles which reflects or 
scatters off the internal walls of the light tube 22. Some of the 
reflected or scattered stray light is incident back to the diffraction 
grating mirror 26 which causes additional diffractions at the wrong 
wavelengths or is mixed in with the monochrometer output. The reliance 
upon a monochrometer output of a known wavelength of light is however 
critical to the results of the spectrophotometric study. 
SUMMARY OF THE INVENTION 
The present invention is directed to a monochrometer which substantially 
reduces the stray light component in the monochrometer output. The 
monochrometer has internal wall features that are designed to capture 
stray light, so that the stray light does not reflect or scatter back to 
the diffraction grating or mix in with the monochrometer output. 
According to the present invention, surfaces with different optical 
characteristics are strategically positioned along the internal walls of 
the light tube in a manner which will either absorb incident stray light 
and/or reflect stray light in a direction away from the diffraction 
grating and the exit of the monochrometer. In the described embodiment, 
light absorbing surfaces with three different characteristics are 
provided. Some of the surfaces are very smooth reflective semi-absorbing, 
some are semi-smooth reflective semi-absorbing, and some are rough 
scattering, highly absorbing. The placements of the surfaces at selected 
locations and orientations causes stray light to be trapped away from the 
diffraction grating and the exit of the monochrometer.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
The following description is of the best presently contemplated mode of 
carrying out the invention. This description is made for the purpose of 
illustrating the general principles of the invention and should not be 
taken in a limiting sense. The scope of the invention is best determined 
by reference to the appended claims. 
Referring to FIG. 2, the installation of a monochrometer 30 according to 
one embodiment of the present invention in an analytical centrifuge 32 is 
illustrated. The set up of the monochrometer 30 with respect to the 
centrifuge 32 is similar to those described in U.S. Pat. Nos. 4,830,493; 
4,919,537 and 4,921,350 which have been incorporated by reference herein. 
However, the design of the light tube 34 of the monochrometer 30 is 
significantly different in that the light tube 34 is structured to trap 
stray light. 
Referring to FIG. 3, the inside of the light tube 34 of the monochrometer 
30 is illustrated in simplified form. The light tube 34 is made up of two 
segments 36 and 38 coupled at an elbow 39 in which a diffraction mirror 40 
is disposed. Some structures have been omitted for clarity, the details of 
which are found in U.S. Pat. Nos. 4,830,493; 4,919,537; and 4,921,350. 
Similar to the monochrometers described in the patents, the mirror 40 has 
diffraction grating ruled on its surface. The mirror 40 is shaped about 
one axis (along the plane of FIG. 3) for the generation of collimated 
lights. The cross-section of the segment 36 is round and that of the 
segment 38 is rectangular. Incident light 50 from a light source 44 
directed at the mirror 40 diffracts into coherent lights of different 
wavelengths including 0, 1, and higher order components. About another 
axis (perpendicular to the plane of FIG. 3), the mirror 40 is provided 
with a varying curvature and varying spaced rulings so that tilting of the 
mirror 40 selectively directs light of a particular wavelength range 
through the exit end 42 of the light tube segment 38 towards the sample. 
An exit slit 48 is used to selectively isolate a narrow band of the light 
emerging from the exit end 42. A motor coupled with a gear mechanism (not 
shown) are provided to tilt the mirror 40 between two extreme positions A 
and B as shown in FIG. 3. 
Because the mirror 40 diffracts light at wavelengths not of interest (stray 
light) in different directions parallel to the plane of FIG. 3, the 
internal structure of the light tube 34 of the present invention is 
designed so that it does not reflect or scatter back stray light to the 
mirror 40 and cause re-diffraction at the wrong wavelength. The internal 
structure is designed also to prevent stray light from mixing with the 
exit light. This results in the light at the wavelength of interest 
passing through the monochrometer to have very little stray light. 
Typically, the wavelength of interest is in the 1 or -1 order. The zero 
order wavelength is typically not used for spectrophotometric studies 
because of too high intensity and polychromatic. Higher order wavelengths 
are also not of interest because of possible confusion (overlap) with the 
primary order wavelengths. 
To accomplish stray light trapping, the inner surfaces of the light tube 34 
is finished or otherwise lined with materials of certain optical 
characteristics. In the illustrated embodiment, the surfaces have one of 
three characteristics: (1) very smooth reflective semi-absorbing, (2) 
semi-smooth reflective semi-absorbing, and (3) rough scattering, highly 
absorbing. The first type of surface can be obtained by lining the inside 
of the light tube 34 using absorptive glass, Cat-A-Lac Gloss or Parson's 
Black surfaces. The second type of surface can be obtained by black 
anodising the smooth machined inside surface of the light tube 34. The 
third type of surface can be obtained by coating the inside surface of the 
light tube 34 using a special "optical black" coating developed by Martin 
Marietta Aerospace Corp. It has been found that for the three types of 
surfaces, light back scatter ten times less for the first type of surface 
as compared to the third type of surface, and the third type is ten times 
less than the second type of surface. The first type of surface, however, 
does have the highest reflected intensity at the complementary angle. 
The surfaces having the respective characteristics are strategically 
positioned along the inner walls of the light tube 34 as shown in FIG. 3. 
Generally, the surfaces nearer the mirror 40 has the first characteristic. 
Further down the segment 38 of the light tube 34 are positioned the 
surfaces having the third characteristic. Surfaces having the second 
characteristic are found near the exit end of the segment and any other 
surfaces not having the first and second characteristics. Because the 
diffracted light from the mirror is collimated parallel to the plane of 
FIG. 3, the side walls (parallel to plane of FIG. 3) of the rectangular 
section of the segment 38 (including the elbow portion having surfaces 112 
and 114) are exposed to very little stray light and thus can be left as 
machined surfaces or made to possess the second characteristic. 
Specifically, the surfaces (flat) 110, 112, 114 and 116 with a high view 
factor of the diffraction grating mirror 40 (i.e. ratio of direct view to 
the mirror and direct view to other structures), or that receive zero 
order light, have the first characteristic so as to minimize scattering 
and provide controlled reflection, as will become clear in the discussion 
below. For the first type of surfaces, about 10% of incident light is 
reflected, 0.5% of incident light is diffusely scattered, and the rest of 
the light is absorbed. Surfaces 120, 121, 122 and 124 which do not have a 
high view factor to the diffraction grating mirror (i.e. substantially not 
in direct view of the mirror) and which receives light reflected from 
other surfaces, have the third characteristic. Such surfaces absorb 
reflected light and/or diffusely scatter light but not to the mirror 
because of the absence of view factor to the mirror. The general 
characteristics of the third type of surfaces are reflectance of less than 
0.5% over the entire visible region (400-700 nm) and less than 0.8% from 
300 to 900 nm, and light absorbance over a large spectrum (0.27 to 20 
microns). Finally, the rest of the surfaces (e.g. 130, 132 etc. near the 
exit end 42 of the segment 38) which are exposed to light very near the 
wavelength of interest have the second characteristic. 
As can be seen in FIG. 3, along the second segment 38 of the light tube, 
some of the surfaces (e.g. 110, 116) protrude towards the axis of the 
segment 38. The protruding surfaces defines a clearance for passage of 
light 46 of the desired wavelength reflected from the diffraction mirror 
40. 
The stray light trapping mechanism of the surfaces will be described 
separately in FIGS. 4 and 5. As mentioned, the zero order white light 
component of stray light is of particular concern because of its high 
intensity. It is noted that the particular orientation of the mirror 40 
between positions A and B (FIG. 3) would not affect the analysis of the 
stray light trapping mechanism. The light rays shown in the figures are 
only representative of the paths of light reflected from the mirror 40. 
The analysis is applicable to all order of light rays reflected from the 
mirror 40 at any mirror orientation. 
Referring to FIG. 4, the worst case of stray light having zero order white 
light component is shown. The zero order ray 52 reflects off of surface 
114 which is positioned such that the reflected ray 54 misses the mirror 
40 before reaching surface 116. The ray 54 is reflected off of surface 116 
in a ray 56 towards surface 120. About 10% of incident light is reflected 
at the surfaces 114 and 116. About 0.5% of incident light on surfaces 114 
and 116 is diffusely scattered. The rest of the incident light is absorbed 
by these surfaces. Thus, the ray 56 is about 1% of the incident ray 52 
diffracted from the mirror 40. This 1% of light after being absorbed by 
highly absorbing surface 120 will have very little light intensity. Other 
light rays from mirror 40 directed at surface 114 will follow a somewhat 
similar path leading to a substantial reduction of intensity. 
The ray 58 which is directed at surface 112 just beyond surface 114 is 
reflected along ray 60 and clears the mirror 40 before being reflected at 
surface 116. The reflected ray continues to be reflected between the 
surfaces 112 and 116, wherein the intensity of the stray light is reduced 
by 90% each time it is reflected off the surfaces 112 and 116. The surface 
112 is set at a slightly diverging angle to surface 116, so that the 
multiple reflections do not converge back to the mirror. The transition 
point between surfaces 114 and 112 is determined by letting the ray 58 
just clear the mirror. Other rays from the mirror 40 to the surface 112 
will also experience multiple reflections, except beyond ray 62 which is 
reflected at surface 112 to the surface 121 (ray 64). The high absorbing 
surface 121 substantially absorbs the intensity of ray 64. Ray 66 is 
reflected at surface 112 to the inner walls of the light tube segment 36. 
The multiple reflections occurring in the segment will substantially 
diminish the stray light intensity. 
Stray light 67 from the mirror 40 is reflected at surface 116 to surface 
121 (ray 68) and to surface 122 (ray 69) which is substantially absorbed. 
Stray light ray 70 is reflected at surface 110 to surface 122. Any diffuse 
scattering from surface 122 does not affect the mirror 40 since surface 
122 does not have a view factor to the mirror 40. In any event, the 
diffusely scattered stray light from surface 122 is of low intensity which 
is further diminished when the scattered light reaches the adjacent light 
absorbing surfaces. To maintain the clarity of FIG. 4, the reflected or 
scattered low intensity light rays are not shown in the figure. 
Referring now to FIG. 5, the stray light trapped at the lower end of the 
segment 38 is now discussed. Ray 71 from mirror 40 is reflected at surface 
130 to surface 122 where the light is substantially absorbed. Similarly, 
ray 74 is reflected at surface 132 to surface 124 (ray 76) and absorbed. 
The light rays 78 and 80 which are very near the wavelength of interest 
(ray 46) are respectively reflected at surfaces 138 and 140 to surfaces 
134 and 136, respectively. The protrusions of surfaces 134 and 136 keep 
light from reflecting back to the diffraction mirror 40. In the 
configuration shown in FIG. 5, an incident light detector 82 can be placed 
at the shoulder defined between surfaces 116 and 122 to receive light ray 
84 reflected from surface 140. This reflected light provides a reference 
light intensity at a wavelength very close to the desired wavelength of 
ray 46 for the spectrophotometric analysis. The protruding corners of 
surfaces 134, 136, 138 and 140, and corners 142 and 144 shield the mirror 
40 from the light rays 90 and 92 reflected from the window 150 of the 
sample cell in the rotor. These corners also shields the incident detector 
82 from light reflected from the window 150. 
The exact dimensions of the internal wall structure can be derived without 
undue experimentation given the described functions of each of the 
surfaces. 
While the invention has been described with respect to the preferred 
embodiments in accordance therewith, it will be apparent to those skilled 
in the art that various modifications and improvements may be made without 
departing from the scope and the spirit of the invention. For example, the 
surfaces may be substituted with surfaces of other characteristics which 
can also accomplish the same stray light trapping tasks e.g. substituting 
the third type of surfaces for the second type, except that the third type 
of surfaces are more expensive to make. Accordingly, it is to be 
understood that the invention is not to be limited by the specific 
illustrated embodiments, but only by the scope of the appended claims.