A spectrophotometer which uses as a detector a silicon photocell and a phototube sensitive to short wavelengths. The light from the sample cell is divided into a smaller and a larger portion. The smaller portion is projected onto the silicon photocell and the larger portion, to the phototube. The two outputs are combined and amplified for indication. The whole of the light transmitted through the sample cell may also be projected to the phototube while the spectrophotometer operates in the short wavelength region and alternatively to the silicon photocell while the instrument operates in the long wavelength region.

This invention relates to a spectrophotometer which is capable of covering 
a wide spectral range from the ultraviolet to near infrared regions. 
When a spectrophotometer is to be designed which can measure wavelengths 
from, say, about 190 to 1000 nm or, from 320 to 1000 nm, no single 
detector suffices to cover such a wide spectral range. 
In the prior art, in order to cover a wide spectral range it has been 
customary to use a combination of two kinds of photocathodes which have 
different spectral responses, for example, the Sb-Cs (S-4 or S-5) or 
bialkali metal photocathode sensitive to wavelengths below 650 nm and the 
Ag-O-Cs (S-1) photocathode sensitive to longer wavelengths above 600 nm. 
In one case two separate phototubes of different spectral responses have 
been alternatively used, and in another a "wide spectral range" phototube 
has been used. The latter phototube is provided in a single envelope with 
two photosensitive surfaces, one for shorter wavelengths and formed as a 
semi-transparent layer deposited on the inner surface of the front window 
of the envelope and the other for longer wavelengths and formed on the 
rear surface of the envelope. 
The S-1 surface sensitive to long wavelengths, however, has a very low 
quantum efficiency of about 0.1 to 0.5% and consequently a very low 
sensitivity. In addition, it has a large amount of dark current. These are 
serious disadvantages. 
In recent years solar batteries or silicon photocells have come into wide 
use in various electronic measuring instruments. Generally speaking, the 
conventional phototube of the S-5 type has a high sensitivity in the short 
wavelength region while the silicon photocell has a high sensitivity in 
the middle and high wavelength regions. Advantageously, the silicon 
photocell has little or no dark current. 
Accordingly, the primary object of the invention is to provide a 
spectrophotometer wherein in order to cover a wide spectral range two 
detectors of different spectral-sensitivity or response characteristics 
are used, one of which is the silicon photocell highly sensitive in the 
middle and high wavelength regions and the other is a conventional 
phototube specifically designed for use in the short wavelength region. 
In one embodiment of the invention, both the silicon photocell and the 
phototube are simultaneously used. In this case, however, the light from 
the sample is divided into two portions of different quantities, which are 
distributed between the photocell and the phototube and the outputs from 
the two detectors are combined for indication. If necessary, the outputs 
from the two detectors are amplified individually by different 
amplification degrees before they are combined. 
In another embodiment of the invention, the silicon photocell and the 
phototube are alternatively used. While the spectrophotometer is used in a 
short wavelength region, the phototube is in operative condition and while 
the instrument is used in a long wavelength region, photocell is placed in 
operative condition, with the change from one of the two detectors to the 
other being effected at a suitable point intermediate the two wavelength 
regions.

Referring now to FIG. 1, which shows the actually measured values of the 
photoelectric currents produced by three different photoelectric elements, 
with a tungsten lamp as the light source above 350 nm and a deuterium lamp 
below it. The curves A and A' are obtained from a silicon photocell, the 
curves B and B', from a phototube having the S-5 (Sb-Cs) surface sensitive 
in a short wavelength region; and the curve C, from a phototube having the 
S-1 (Ag-O-Cs) surface for a long wavelength region. 
As is clearly shown, the silicon photocell has the same or higher degree of 
sensitivity than the phototubes in the region of 320 to 1000 nm. 
Particularly, above 600 nm the photocell has a very high sensitivity which 
is about one hundred times that of the S-1 phototube. Moreover, the 
silicon photocell has little dark current. In this respect the silicon 
photocell is superior to the phototube. 
Although the silicon photocell has a higher sensitivity than the phototube 
over the whole wavelength range covered by the photocell, it has such a 
low sensitivity in the ultraviolet region that it is practically very 
difficult to use the silicon photocell in that region. In the visible 
region the silicon photocell gives so great a difference in sensitivity 
between the short and long wavelength regions that much stray light is 
introduced in measurement in the short wavelength region. In other words, 
although in the short wavelength region of 320 to 400 nm the silicon 
photocell has much the same degree of sensitivity as the phototube, it has 
a far higher sensitivity in the long wavelength region so that the long 
wavelength components of the stray light contained in the light being 
measured are exaggerated with a resulting relative increase in stray 
light. 
An experiment has disclosed that in the short wavelength region the output 
from the silicon photocell caused by stray light is about 10 times greater 
than the phototube with the S-5 cathode sensitive in the short wavelength 
region. Such an increase in stray light greatly reduces the measuring 
precision and accuracy of a spectrophotometer. 
In view of the fact that as shown in FIG. 1, above 600 nm the silicon 
photocell has a sensitivity about 10 times as high as the phototubes for 
the short wavelengths (such as S-5 type) and 100 times as high as the 
phototubes for the long wavelengths (such as the S-1 type), in one 
embodiment of the invention the amount of light to be measured is to be 
projected onto the silicon photocell is reduced to, say, about one-tenth 
of the amount to be projected onto the short wavelength phototube so that 
stray light may be reduced to about one-tenth of the amount which would 
otherwise be introduced into the light measuring system. 
In this arrangement, a loss of the amount of light to be measured is caused 
to the short wavelength phototube. However, such a loss is about 10% and 
is practically negligible since it is much the same as the fluctuation or 
variation of the sensitivity in individual phototubes. Since the silicon 
photocell has a very high sensitivity in the long wavelength region, the 
10% of the light from the sample distributed to the photocell suffices for 
proper measurement, and with the sensitivity having been reduced to 
one-tenth of the normal value, the adverse effect which the long 
wavelength components of the stray light have on the results of 
measurement in the short wavelength region is also reduced to one-tenth of 
that which otherwise be caused. 
Turning now to FIG. 2, which somewhat schematically shows one embodiment of 
the invention, the light from a source 10 is focussed by a lens 11 at an 
entrance slit 12 of a monochromator M, in which a plane mirror 13 reflects 
the light entered through the slit 12 onto a collimating mirror 14 which 
renders the light parallel and directs it onto a grating 15. The 
diffracted light is converged by a concave mirror 16 and then reflected by 
a plane mirror 17 to be passed through an exit slit 18. The monochromatic 
light is projected onto a sample cell 19. 
The light L transmitted through the cell 19 is led to a light measuring 
device D which comprises a beam divider 20, a silicon photocell 21 and a 
phototube 22 suited for the short wavelength region. The light hits on the 
divider 20, which reflects about 10% of the light received onto the 
silicon photocell 21 and transmits the remaining about 90% of the light 
onto the phototube 22. The outputs from the two detectors 21 and 22 are 
combined and applied to an amplifier 23. The divider 20 may comprise a 
simple glass or quartz plate. 
With the above arrangement, in the short wavelength region the silicon 
photocell 21 contributes little to the function of the light measuring 
device D while in the long wavelength region the phototube 22 contributes 
little to the function of the light measuring device D. This means that it 
is possible by mere combination or summation of the outputs from the two 
detectors 21 and 22 to automatically change from one of the two detectors 
to the other and thereby to make operative that one of the detectors which 
has a spectral response most suited for the wavelength region being used 
without the necessity of using any mechanical changing device. The two 
detectors 21 and 22 operate as if they were a single detector, and by 
appropriately selecting the reflective ratio of the surface of the divider 
20 it is possible to freely change or adjust the spectral response 
characteristics of the light measuring device D over the whole wavelength 
range of the light to be measured. 
The principle of FIG. 2 may be applied to a double-beam spectrophotometers 
as schematically shown in FIG. 3. The monochromatic light L from a 
monochromator M is reflected by a plane mirror 30 and then split by a beam 
splitter 31 into two beams LR and LS. The beam LR is directed by a concave 
mirror 32R onto a cell 19R adapted to contain, say, a reference material, 
while the other beam LS is directed by a concave mirror 32S onto a cell 
19S adapted to contain a sample material. 
The light LR transmitted through the reference cell 19R enters a light 
measuring device DR while the light LS transmitted through the sample cell 
19S enters a light measuring device DS. The two light measuring devices 
are of the same construction and operate in the same way as the device D 
shown in FIG. 2, with the corresponding component elements being 
designated by the same numerals suffixed by R and S, respectively. 
The outputs of the light measuring devices DR and DS are applied through 
logarithmic amplifiers 26R and 26S, respectively, to a suitable circuit 34 
designed to produce an output corresponding to a difference between the 
logarithmic of the amplifiers 26R and 26S. The output of the amplifer 34 
is used for indication as is well known in the art. 
A modified form of the light measuring device D of FIG. 2 or 3 is shown in 
FIG. 4, wherein the outputs from the photocell 21 and the phototube 22 are 
amplified by individual amplifiers 24a and 24b before they are combined. 
This arrangement enables adjustment of any difference between the absolute 
sensitivities of the two detectors 21 and 22 by appropriately setting the 
gains of the individual amplifiers 24a and 24b by means of feedback 
resistors Ra and Rb. The resistors Ra and Rb may be of the variable type 
so that the gains may be changed as desired. For example, if the absolute 
sensitivity of the silicon photocell 21 is sufficiently high, the divider 
20 first optically distributes the light from the sample cell 19 to the 
phototube 22 and the photocell 21 by the distribution ratio of 9 to 1, and 
then electrically the amplifiers 24b and 24a amplify the outputs from the 
phototube 22 and the photocell 21 individually, with the ratio of the 
amplification degree of the amplifier 24b to that of the amplifier 24a 
having been set to 10 to 1. This results in the overall weighing ratio of 
the phototube to the photocell becoming 90 to 1. 
By using the separate amplifiers for the outputs of the two detectors 21 
and 22 before they are added it is possible to further reduce the 
influence of stray light caused by the silicon photocell. Although there 
is a great difference in the internal resistance between the photocell and 
the phototube, the provision of the two separate amplifiers enables use of 
the two elements in combination. 
In the embodiments of FIGS. 2 to 4, the reflective ratio of the glass plate 
divider 20 is about 10% and the reflected light is applied to the 
photocell while the transmitted light is applied to the phototube. On the 
contray, it is possible to use a mirror having a reflective ratio of 90% 
so that the reflected light is projected to the phototube and the 
transmitted light, to the photocell. 
If the two detectors which have different sensitivities (that is, 
spectral-sensitivity characteristics and absolute sensitivities) from the 
detectors of the above example are to be used, the light distribution 
ratio by the divider 20 is different from the above-mentioned ratio of 9 
to 1 and is determined to an appropriate ratio in view of stray light and 
the sensitivities of the detectors. 
Turning now to FIG. 5, which shows still another embodiment of the 
invention, the light L from the sample cell is projected onto a reflecting 
mirror 25 which is so arranged as to be selectively moved into and out of 
the path of the light L. 
Both the output of a phototube 22 and the output of a silicon photocell 21 
are connected to the inverting input terminal of an amplifier 26. A 
positive potential is applied to the anode of the phototube and both the 
negative electrode of the silicon photocell and the non-inverting terminal 
of the amplifier 26 are grounded. 
The amplifier 26 has a negative feedback circuit in which a transistor 27 
is connected for logarithmic transformation of the output of the amplifier 
26. The logarithmic transformation provides a measure by which the 
subsequent operation can be performed by mere addition and subtraction. 
With the negative feedback, the inverting terminal of the amplifier is 
hypothetically grounded and at zero level, so that the output current is 
taken out from the photocell 21 with the potential thereacross being zero, 
that is, under the short-circuited condition of the photocell. Such 
circuit connection provides a very good linearity of the output of the 
photocell with respect to the amount of incident light thereon. 
When the spectrophotometer is used in the short wavelength region, the 
mirror 25 is positioned in the path of the light L to reflect the same 
onto the phototube 22. At a predetermined wavelength, say, 600 nm the 
mirror 25 is moved out of the optical path of the light L so that the 
light L is projected onto the photocell 21 in the long wavelength region 
above 600 nm. 
FIG. 6 schematically shows a double-beam spectrophotometer to which the 
principle as shown in FIG. 5 is applied. The monochromatic light L from a 
monochromator M is reflected by a plane mirror 30 and then split by a beam 
splitter 31 into two beams LR and LS. The beam LR is directed by a concave 
mirror 32R onto a cell 19R adapted to contain, say, a reference material, 
while the other beam LS directed by a concave mirror 32S onto a cell 19S 
adapted to contain a sample matrial. 
The light beam LR transmitted through the reference cell 19R is directed by 
a concave mirror 33R onto a plane mirror 25R which is movable selectively 
into and out of the optical path, thereby causing the beam LR to hit 
either a silicon photocell 21R or a photocathode 22'R. In a similar 
manner, the light beam LS from the sample cell 19S is directed by a 
concave mirror 33S onto a plane mirror 25S movable selectively into and 
out of the optical path, thereby causing the beam LS to impinge on either 
a silicon photocell 21S or a photocathode 22'S. The photocathodes 22'R and 
22'S are encased in a single envelope to form a composite phototube 22. 
The outputs from the two detectors are applied to logarithmic amplifiers 
26R and 26S, respectively, the logarithmic outputs of which are applied to 
a subtracting circuit 34 to provide a ratio between the intensities of the 
light beams transmitted through the reference and sample cells. 
A wavelength setting device 35 sets the grating 15 to a predetermined 
wavelength in a manner well known in the art, and in association with the 
device 35 a mirror control device 36 controls the movement of the mirrors 
25R, 25S into and out of the path of the light beams LR, LS. 
FIG. 7 is a schematic showing of a single-beam spectrophotometer to which 
the principle of FIG. 5 is applied. The construction and operation of the 
arrangement of FIG. 7 can be easily understood from the foregoing 
description so that no explanation will be given to FIG. 7. 
FIG. 8 shows by way of example a mechanical arrangement of the mirror 
control device 36 used in FIG. 7. The mirror 25 is fixed to the upper end 
of a lever 40 swingable on a shaft 41, and a slot 42 is formed in the 
lower end of the lever 40. A lever 43 pivoted at 44 has its one end 
engaged in the slot 42 of the lever 40 and is provided at its opposite end 
with a cam follower 45 held in contact with the periphery of a cam 46. 
A manually operable knob 47 is turned to set the grating 15 to a wavelength 
through the device 33 and at the same time to rotate the cam 46 to swing 
the lever 40 thereby to position the mirror in and out of the path of the 
light beam L as will be easily understood. 
FIG. 9 shows by way of example the mirror control device 36 which can be 
used in FIG. 6. The mirrors 25R and 25S are fixed to the opposite ends of 
a lever 40 which can be turned on a shaft 41 by means of a knob 47 so as 
to position both of the mirrors 25R and 25S in and out of the path of the 
light beams LR and LS, respectively. 
Suppose that in FIGS. 5 to 7 the silicon photocell be replaced by a 
conventional phototube sensitive in the long wavelength region. Although 
the long wavelength phototube is not operative while the instrument is 
used in the short wavelength region, it produces a great dark current to 
be added to the outout of the operating short wavelength phototube, with 
resulting introduction of great errors into the result of measurement. 
Therefore, separate amplifiers would have to be provided for the outputs 
from the long wavelength and short wavelength amplifiers, respectively, 
with resulting increase in the number of circuit components and 
consequently the manufacturing cost of the instrument. 
On the contrary, in accordance with the invention, the phototube designed 
for the short wavelength region is employed for detection of short 
wavelengths and the solar battery is employed for detection of long 
wavelengths. Since these two detectors produce little or no dark current, 
the output from one of them is not influenced by the dark current which 
would otherwise be produced by the other which is not in operation, so 
that introduction of errors into the result of measurement can be avoided 
and the number of the circuit components becomes smaller than otherwise 
with resulting reduction of the manufacturing cost of the instrument. In 
particular, the use of the silicon photocell as a detector enables direct 
connection of a logarithmic amplifier to the output of the detector 
because the silicon photocell has little or no dark current. This is a 
great advantage.