An entrance slit and exit slit of a monochromator is shaped so that the width of the slits becomes smaller from the center towards the ends of their height, where the width is a dimension in the direction of the separation of the light in the monochromator. An example of such shape is rhombic. When the total amount of light emitted from the monochromator and the resolution of the monochromator are set to be the same, the efficiency of light in measuring small samples is increased and the ratio of stray light in the light emitted from the monochromator becomes smaller.

The present invention relates to a monochromator, which is used in a 
spectrophotometer or other measurement apparatus. 
BACKGROUND OF THE INVENTION 
A monochromator includes an entrance slit plate having an entrance slit, a 
light separator (such as a diffraction grating, a prism, etc.), and an 
exit slit plate having an exit slit. Light coming into a monochromator 
through the entrance slit is separated (or dispersed) by the light 
separator into a series of component monochromatic lights, and an image of 
the entrance slit of every component monochromatic light is projected onto 
the exit slit plate. Thus a monochromated light of varying wavelength 
comes out of the exit slit while the monochromator scans through a preset 
range of wavelength. 
In conventional monochromators, the entrance and exit slits are both 
rectangular long in the direction perpendicular to the direction of the 
separation of light (the direction of the separation of light is 
hereinafter referred to as the lateral direction). A spectrophotometer 
that uses a monochromator equipped with slits of such shape has the 
following problem. 
When a sample is measured in a spectrophotometer, a bundle of light coming 
out of the exit slit of the monochromator is focused onto the sample where 
an image of the exit slit is formed, as shown in FIG. 1B. Thus the bundle 
of light 11 for measuring the sample 12 is shaped rectangular bearing the 
shape of the exit slit. When an ordinary box sample cell is used with an 
enough amount of sample in it, it is possible to set all the measurement 
light pass through the sample. When a flow sample cell is used or when a 
small-sized sample or small amount of sample is measured, however, it 
occurs that only a part of the measurement light can pass through the 
sample. Thus the efficiency of light in the measurement becomes low, 
since, as shown in FIG. 1B, the part 13 of the measurement light 11 
external of the sample 12 is not used for the measurement. 
SUMMARY OF THE INVENTION 
A monochromator of the present invention achieved for solving the 
above-mentioned problem is constructed as follows: 
the entrance and exit slits are shaped so that the width of the slits 
becomes smaller towards the both ends of their height (where the width is 
the dimension in the lateral direction). 
An example of such shape is shown in FIG. 1A in which the slit is shaped 
rhombic. When compared to the conventional shape shown in FIG. 1B, it is 
apparent that the efficiency of light is greater in FIG. 1A, where a 
larger amount of light is projected on a small sample 12 and less amount 
of light 13 is wasted than the light emitted from the conventional 
rectangular slit. This comparison is made under the condition that: a) 
brightness of the monochromator (i.e., total amount of the measurement 
light emitted from the exit slit) is the same, and b) the resolution of 
the monochromator is the same. The working principle of the present 
invention is described in detail in the following description of preferred 
embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The first embodiment of the present invention is a monochromator equipped 
with an entrance and an exit slit shaped rhombic as shown in FIG. IA. In 
monochromators, the entrance slit and the exit slit have the same shape 
since the image of the entrance slit is emitted from the exit slit. The 
effect of the rhombic shape of the slits is now described in comparison 
with that of the rectangular shape of the conventional slits as shown in 
FIG. 1B. 
When a monochromatic light enters through the entrance slit into the 
monochromator and the wavelength scanning is made by the monochromator, an 
image of the entrance slit moves on the exit slit plate in the lateral 
direction. When the image of the entrance slit moves in the lateral 
direction, it passes over the exit slit, and the portion 22, 27 of the 
image 20, 25 of the entrance slit overlapping the exit slit 21, 26 is 
emitted from the monochromator as shown in FIGS. 2A and 2B. Thus the 
amount of light emitted from the monochromator is proportional to the area 
of the overlapping portion 22, 27. The area increases parabolically in the 
case of the rhombic slits (FIG. 2A) and linearly in the case of the 
rectangular slits (FIG. 2B) as the distance x between the image of the 
entrance slit 20, 25 and the exit slit 21, 26 decreases to zero (at which 
the image 20, 25 and the exit slit 21, 26 coincide). 
The change in the amount of light emitted from the monochromator versus the 
distance x is shown in FIGS. 3A and 3B. Since the image 20, 25 is made of 
a monochromatic light, these graphs show a spectrum peak of a 
monochromatic light emitted from the exit slit. In FIGS. 3A and 3B, the 
peak position of the spectrum peak curve is set at zero for the 
convenience of the explanation. Actual peak position of a monochromatic 
light having wavelength .sub.o is at the wavelength .sub.o. 
The shape of the spectrum peak of FIG. 3A (by the rhombic slits) is 
formulated as follows: 
##EQU1## 
The shape of the spectrum peak of FIG. 3B (by the rectangular slits) is 
formulated as follows: 
##EQU2## 
The resolution of the monochromator depends on the width of the slits. 
Since the resolution is normally defined by the halfvalue width of the 
spectrum peak, the relationship between the width w of the rhombic slit 
(FIG. 1A) and the width a of the rectangular slit (FIG. 1B) for obtaining 
the same resolution is: 
EQU w=a/(2-.sqroot.2).apprxeq.1.7.multidot.a, (3) 
which is deduced from the equations (1) and (2) by equalizing the halfvalue 
widths of the spectrum peaks of FIGS. 3A and 3B. 
For obtaining the same brightness (amount of light emitted from the 
monochromator) with the resolution maintaining the same, the relationship 
between the height H of the rhombic slit and the height b of the 
rectangular slit is: 
EQU H=3.multidot.(2-.sqroot.2).sup.2 .multidot.b.apprxeq.b, (4) 
which is deduced from the equations (1) and (2) by equalizing the areas of 
the spectrum peaks of FIGS. 3A and 3B. 
Equations (3) and (4) show that the width of the rhombic slit is 1.7 times, 
and the height is approximately the same, as those of the rectangular slit 
for obtaining the same resolution and the same brightness. Thus, as seen 
by comparing FIGS. 1A and 1B, the light emitted from the rhombic slit 10 
ca cover larger part of a small sample 12 than that emitted from the 
conventional rectangular slit 11. That is, by using a measurement light 
produced by the monochromator of the present embodiment, a greater 
efficiency of light can be achieved in a measurement using a flow sample 
cell or when a small sample is measured. 
Next the stray light is considered. The amount of stray light in a 
monochromator is proportional to the amount of light entering into the 
monochromator through the entrance slit, i.e, proportional to the area of 
the entrance slit. Since the stray light disperses uniformly within the 
monochromator, the amount of stray light emitted from the exit slit is 
further proportional to the area of the exit slit. Therefore the amount of 
stray light emitted from the monochromator is proportional to the product 
of the areas of the entrance slit and the exit slit. The important thing 
about the stray light is not its absolute amount but the relative amount 
(ratio) C among the light emitted from the monochromator. For the rhombic 
slit of FIG. 1A, the ratio C.sub.A is: 
EQU C.sub.A ={)w.sup.2 .multidot.H.sup.2)/4}/{w.sup.2 
.multidot.H)/3}=(3/4).multidot.H, 
and for the rectangular slit of FIG. 1B, the ratio C.sub.B is: 
EQU C.sub.B =(a.multidot.b).sup.2 /(a.sup.2 .multidot.b)=b, 
where (a.sup.2 .multidot.b) is the amount of light emitted from the 
monochromator. 
If the amount of emitted light and the resolution are assumed to be equal, 
the ratios C.sub.A and C.sub.B of the two types of slits have the 
relation: 
EQU CA/CB=3/4 (5) 
since H.apprxeq.b as described above. Equation (5) means that the ratio of 
stray light in the light emitted from the monochromator is reduced to 3/4 
if the rhombic slits are used. Thus the S/N (signal to noise) ratio of the 
spectrophotometer using the monochromator of the present embodiment is 
increased, and the sensitivity and accuracy of the measurements by the 
spectrophotometer is improved. 
In the above explanation of the comparison of the stray light, the two 
types of slits are compared assuming the same levels of brightness and 
resolution. If, on the other hand, the same resolution and same stray 
light level are assumed, the brightness of the monochromator of the 
present embodiment can be larger than the conventional monochromators, or, 
if the same brightness and the same stray light level are assumed, higher 
resolution can be obtained. 
The narrowing manner of the width of the slit according to the present 
invention is not limited to the example as shown in FIG. 1A. The width can 
be reduced stepwise from the center to the ends as shown in FIGS. 4 and 5, 
and the same arguments as above applies to any slits embodying the present 
invention as long as the width narrows towards the ends.