Note: Descriptions are shown in the official language in which they were submitted.
7:~
This invention relates to an op-tical system for
a spectrophotometer and in particular to double beam
spectrophotometer which can be easily transported for field
use.
Measurement of the spectral reElectance of growing
vegetation and terrain features has obvious relevance to
fields of applied science such as land-use survey and
camouflage. Methods used in the past fall roughly into
three categories, namely photogrammetric,multi-spectral
survey and spectrophotometric, each of which has its
disadvantages. Photogrammetric methods suffer from inadequate
spectral range, sensitivity and accuracy. Multi-spectral
survey is generally carried out from a complex and costly
installation in a satellite or aircraft, and gives insuffi-
cient data and image resolution for some purposes (e.g.
colour rneasurements on individual trees). Spectrophotometric
methods involvinginternal light sources are usuable only for
the very small areas of ground or for individual 1eaves
broughtinto the laboratory. The only spectrophotometer
using ambient lighting and remote imaging techniques known
to us which has a double-beam facility requires both the
sample and reference targets to be imaged within the same
telescope field which is undesirable.
Single beam systems are also known which could
possibly be used in the field but such systems suffer from
the problems of correctly identifing the measuring field
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in a viewfinder of the system and it is also necessary -to
direct the system firstly at a sample, take all the measurements
required at the wavelengths of interest and then manually
direct the system at a reference target. During the time
taken to redirect the sys-tem the ambient light may change
considerably thereby giving a false reading of, for example,
the reflectance of the sample as compared with that of the
reference target.
It is the object of the present invention to over-
, come some or all of these problems.
The present invention may be said to reside in an
optical system for a spectrophotometer said system having a
first optical branch for forminq an image from a sample and
a second branch for forming an image from a reference target,
lS a splitter means for directing a portion of said image formed
by the first optical branch to an eye piece and -to a detecting
means and for directing a portion of said image formed by
said second optical branch to said eye piece and to said detec-t-
ing means and a shutter means for sequentially allowing the
image formed by the first branch to be received by said
splitter means and the image formed by said second branch to
be received by said splitter means.
Accordingly the device of the present invention allows
a user to direct the first branch at a sample and the second
branch at a reference target and uses ambient lighting to
receive the sample and references images. The first and second
branches can be independently aimed and focused. The
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shutter means an~ splitter means allows the image in the
first branch to be directed to the eye piece and detecting
means so that the sample field can be identified. Since a
shutter means is utilized, only a small amount of time elapses
between the time the image in the first branch is received
by the detector and the image in the second branch is
received. Accordingly the first and second branches will
be subject to the same illumination for a series of
measurements. The first branch may be directed to obtain
~ an image of the foliage of a tree and obtain measure-
ments therefrom directly in the field and thereby provide
an accurate indication of the spectral reflectance of a
reasonably sized area from which to evaluate types of
camouflage or the like.
In a preferred embodiment of the invention the
splitter means comprises a segmented mirror, the segmented
mirror having a slit therein and portions which are
reflective and other portions which are transparent. The
mirror is arranged so that the portion of the mirror image
in the first branch received at the detecting means passes
through the said slit so that the image is not affected by
the mirror. The reflective portion of the mirror directs
a part of the image in the first branch to the eye piece.
A portion of the image in the second branch passes through
the transparent parts of the mirror to the eye piece and
the remainder of the image is reflected to the detecting
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means. In this embodiment the sample field will be
- missi~g in the eye piece thereby indicating that the
image corresponding to the missing area has been directed to
the detecting means.
S The invention may also be said to reside in a
system for collecting light from separate sources said
system comprising a first telescope means and a second
telescope means, said first and second telescope means
having a common primary focus.
~ The images of the common focus may be directed
by a mirror or the like to one or more locations such as
an eye piece or a detecting system for inspection or
measurement.
The invention may also be said to reside in a
mirror for a spectrophotometer, said mirror having:
(a) an aperture therein to allow light to pass
through the mirror to a first location;
(b) reflective portions to reflect light to a
second location; and
(c) transparent portions to allow light to
pass through said mirror to said second location;
(d) further reflective portions to reflect
light to said fixst location.
A preferred embodiment of the invention will be
described with reference to the accompanying drawings in
which:
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FicJure 1 is a diag~am of an optical system
embodying the invention;
Figure 2 is a view of a segmented mirror used in
the system of Figure l; and
Figure 3 is a diagram of the image seen in an eye
piece of the system of Figure 1.
It should be understood that the operation of a
spectrophotometer in which -the present system may be embodied
is well known and accordingly will not be described in full
detail herein.
Referring to Figure 1 the ~tical system includes a
sample branch formed of an aiming mirror 10 and an objective
lens 12. A reference branch includes an objective lens 14.
An erecting lens 16 andan eye piece 19 are provided and the
lens 12 and the lens 16 and eye piece 19 form a first telescope
with the lenses 14 and 16 and eye piece 19 forming a second
telescope. As shown the first and second telescope have a
common primary focus. Located at the common primary focus is
a segmented mirxor 20.
A rotational shutter 18 is provided which is
generally cone shaped and is driven by an electric motor
(not shown) to rotate as shown by arrow A. As shown the
mir~'or 20 projects into cone 22 of the shutter 18 and the
cone 22 is provided with an opening 24 so that as the cone
is rotated by the electric motor, light from the lens 12 passes
through the opening to mirror 20. Then as -the cone continues
to rotate there is a period when the mirror 20 receives no
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light ~Intil the opening 24 allows the light from lens 14 to
pass therethrough to the mirror 20 and then there is a further
period of darkness until the opening again allows light to
pass from lens 12 to mirror 20.
Referring now to Figure 2 which shows the
segmented mirror 20, it will be seen that the mirror is
provided with a slit 30, two semi-circular portions 32 and
one rectangular portion 32 which are aluminised to provide
reflecting portions. The remaining portion 34 of the mirror
is transparent and is generally rectangular in configuration
so that a lower portion 36 may be used to mount the mirror
in a desired configuration.
The mirror 20 is mounted such that light passing
from lens 12 through the opening 24 is reflected from the
aluminized segments 32 and the remainder passes through slit
30 and portion 34 to alignment mirror 40. Accordingly reflected
light from the aluminised segment 32 passes through erecting
lens 16, rectangular reticle 17 and eye piece 19. The parts of
the sample image corresponding to aluminised segments 32
are therefore seen in the eye piece 19. These are the
portions Sl, S2 and S3 in Figure 3. The portions Rl, R2 and
R3 are transmitted to the mirror 40 and will appear black
when the opening 24 allows light to pass from lens 12. Light
from the lens 14 which passes through opening 24 passes
through the transparent portion 34 and slit 30 of the mirror 20
to lens 16 and then to eye piece 19 and the light reflected
'
from the aluminised portions 32Of the mirror 20 is reflected
to mirror 40. A user w~ll therefore see the portion of -the
reference image labeled R1, R2 and R3 in the eye piece 19
with the image corresponding to the portion S2 being reflected
to the mirror 40.
Light passing through the transparent portion 34 of
mirror 20 from lens 12 is reflected from mirror 40 onto the
sides of an entrance slit of monochromator 46 and is absorbed
by the sides of the monochromator. Only the light passing
through slit 30, which is reflected from mirror 40 enters the
monochrornator from lens 12 as will be hereinafter described.
With the shu-tter 18 rotating as shown by arrow A
the field of view through eye piece 19 consists of a mosaic
of parts of the sample and reference as shown in Figure 3
with the parts of the image from lens 14 at S2 and the image
from lens 12 at R2 being transmitted to mirror 40. These are
easily checked by alternately capping the sarple and reference
objective lenses wherein images will each be observed as semi-
circles, with missing strips corresponding to the input to
the detecting system. The measured field of view is further
defined by the rectangular reticle 17.
The mirror 40 is arranged so that light passing through
the slit 30 from lens 12 is re~lected through relay lens 42,
visible order sorting filter 44 to a lower half of the
entrance slit in the monochromator 46. Light of particular
wavelengths is sequentially obtained from monochromator 46
and passed through an infra-red order sorting filter 48 to
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a movable ~irror 53, an infra-red pass filter 52, cooled lead
sulphide detector 54 or to photomultiplier tube 56 as in a
conventional spectrophotometer. It should of course be noted
that mirror 50 is moved to allow light to pass to the detector
54 or is left in place to direct light to photomultiplier tube
56. Similarly light from rectangular portion 32 of mirror 20
and lens 14 is reflected to mirror 40 and enters the top half of
the entrance slit of monochromator 46 and then to photomulti-
plier tube 56 or detector 54 in the same manner as described
immediately above. The light from the o~her aluminised
portions 32 is reflected by mirror 40 to the sides of the
monochromator entrance slit where it is absorbed.
The monochromator is thus provided with an input
in a four-part cycle of darkness then reference image from
lens 14 then darkness then sample image from lens 12 as in a
conventional spectrophotometer.
The spectral information generally required is in
the visible and near infra-red spectrum in the range of
380 -1800mm. It should also be noted that the shutter 18
may be provided with small holes (not shown) on flange 21.
Photodetectors are arranged to provide synchronizing pulses
corresponding to darkness and light as the shutter rotates
so that the detecting equipment is able to receive a signal
which can be used to distinguish an image from the sample
field and the image from the reference target. This would
be well known to those familiar with spectrophotometers and
will not be described herein.
As would be well understood to prevent ambient
light, other than that entering the lenses 12 and 14, from
entering the system the system would be located in a
housing schematically shown by the reference numeral 60.
In use the spectrophotometer embodying the optica]
system can be easily transported to a site for use, in the
back of a Land ~over or the like. The system would be set
up with the mirror 10 directed at a sample target and the
lens 14 directed at a reference target. It is desirable
to place the reference target within about 5 of the sample
target. By placing a lens cap over thelenses12 and 14 alter-
natively and with the shutter 18 rotating,a user can ensure
correct alignment by merely viewing through the eye piece
19 .
Since the paths of the images through the lenses
12 and 14 are different it is desirable to calibrate before
use. The first step in calibrating the instrument is to
point the lens 14 at a reference point coated with a barium
sulphate/ethyl cellulose based white paint. The alignment
mirror 10 is used to align the image of lens 12 with the
same part of the board, and a scan of wave lengths is ma,de.
This calibration scheme gives the ratio of efficiencies of
the two paths (reference and sample) for wave lengths of
interest. Since the double~beam ~eometry~is constant
further calibration scans at the geographic location should
not be needed. Calibration ratios are saved as resident
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calibration data for example in a micro-processor rnemory.
- The bias voltage applied to photomultiplier
tube 56 and the amplifier gain of detector 54 may be under
micro-processor control to ensure optimumsignal to noise
ratio for the detection system output over a wide range of
viewing conditions. This output can be fed to a micro-
processor together with a wavelength signal from the
monochromator 46 and synchronizing pulses from the shutter
18. The photodetector output is sampled at 5/nm intervals
in the visible range and at 10/nm intervals in the infra-red
range. At each wavelength of interest, the first reference
synchronizing pulse allows the reference branch output to be
sampled four times, summed and stored. The dark synchroniz-
ing pulse initiates similar sampling of the output during
the following dark period, and the sample branch output is
similarily sampled. The dark output is subtracted from
both stored branch sums yieldingdark corrected branch values
The entire sampling sequence is repeated for the next
eleven chopper cycles, the corresponding branch results
are averaged, stored and divided to give a reflectance
value.
As noted above to use the instrument the lens 14
is aligned with the reference board, the mirror 10 with
the object to be measured and a scan of wave lengths is
carried out. The board should, of course be subject to the
sameillumination as the ob~ect to be measured. The
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computed values are correc-ted by the stored calibration data
to give the reflectance of the sample seen relative to the
reference board, independent of changes of ambient light during
-the measurement. This is because the sample and reference are
measured for a particular wavelength and very li-ttle time
elapses between the two measurements. Should the ambient light
change for measurements at another wavelength this will have no
realeffectsinceit is the ratio between measurements of a
particular wavelength which are of interest. These ratios can
be plotted against wave length on an XY recorder for immediate
field evaluation. ~ facility for recording the stored data on
a digital tape cassette for later processing by computer
programme which contains corrections for the knownreflectance
spectrum of the reference board can also be provided. This
allows determination of absolute spectral reflectance curves
and colorimetric parameterssuch as the CIE color-coordinates
x, y and Y(%).
It will therefore be evident that the present inven-
tion provides an optical system for a spectrophotometer which
uses ambient light and which can readily be assembled together
with the remainder of the spectrophotometer whilst in the field.
Results may therefore be plotted in the field and can be
assessed to ensure that the results do accord with what would
be expected before the results are fully analysed by computer
or the like.
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The present systenl also allows -the light sampled
by the monochromator to be easily and unambigously identified
and at the same time provides a double-beam capability for
accurate reflectance measurement independently of the level
of illumination~
Whilst we have described in the foregoing
description preferred forms of our invention, we do not wish
to be limited to the positive terms employed therein, since
it will be understood ~hat many modifications and/or
alterations may be made without departing from the spirit
and scope of the present invention.
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