A system for rapid-scan spectral analysis comprising a concave holographic diffraction grating continuously rotated at a substantially constant angular velocity to provide a rapid scanning monochromator (a monochromator is used to transfer nominal regions of wavelengths out of the continuous light source). The unique sampling circuitry uses an optical shaft encoder. The angular velocity and angular acceleration of the grating are calculated from time measurements, just before the first wavelength of interest falls on the detector. This information is used to control the Analog to Digital converter sampling rate across the region of interest. The samples as a function of time are stored in a memory buffer so that each data point corresponds to a wavelength.

BACKGROUND OF THE INVENTION 
The field of the invention pertains to spectrophotometry and, in 
particular, to rapid-scan spectrophotometers and the speed with which such 
instruments can scan the relevant spectra. 
In well known mechanical scanning spectrophotometers the entrance and exit 
slits are located on either side of the optical grating. A simple 
ellipticaly concave mirror is used as a collimating and focusing mirror 
intersecting and directing the light from an ultraviolet or visual light 
generator for a UV/VIS spectrophotometer. The light beams entering the 
monochromator strike the left side of the mirror, aare collimated and 
reflected to the grating. The diffracted radiation goes to the right half 
of the same mirror and is focused on the exit slit. The wavelength is 
selected by simple pivoting of the grating about the monochromator axis. 
The angle between the incident and diffracted rays remains constant. 
Either a manual or motor driven sine bar drive produces a direct 
wavelength readout on a linear scale. 
Since the useful range in UV/VIS spectroscopy lies typically within a few 
degrees about the grating optical axis, the grating is rotated back and 
forth over this range to scan the region of interest. Mechanical scanning 
of the desired spectrum is achieved through a device such as a stepping 
motor. The information from a shaft encoder thereattached is used to 
translate the angular position of the grating into a wavelength. 
Spectrophotometers that rely upon such electromechanically reversing 
arrangements for the grating cannot truly be considered rapid scanning, 
because they typically scan about 400 to 2400 nm per minute. The 
arrangement cannot be increased in speed because of the mechanical cam 
shaft follower drive and the need to determine the grating position 
accurately. 
Full electronic spectroscopy has been achieved with diode array 
spectrophotometers that scan the range of 200 to 800 nm many times per 
second. Such spectrophotometers require custom-made circuits with 
attendant high cost for limited production. Diode arrays have a limited 
spectral response, require a "reverse optics" configuration. Extension 
into the near and far infra red remains unavailable without arrays of 
hundreds or thousands of elements. 
High-throughout spectroscopy can also be accomplished with a fast 
mechanical scanner with all reflective optics. Scanning is achieved by 
vibrating a low-inertia grating or mirror as disclosed in U.S. Pat. No. 
4,225,233 and the paper by J. Stoijek and Z. Uziel, Pol. J. Chem., 53, 
1619 (1979). The mirror or grating (depending on the optical 
configuration) is mounted directly on the output shaft of a galvanometer 
type optical scanner, where the position is a function of the applied 
electric current. By changing the source, grating, and detector, a wide 
wavelength range can be covered. A commercial device based on U.S. Pat. 
No. 4,070,111 is available presently with a vibrating grating. 
Unfortunately, the scanning speed, although much greater than with the 
electro-mechanical scanner above, caused increased optical difficulties. 
To minimize intertia the grating or mirror is very small. A large number 
of optical elements, fixed magnification between the entrance and exit 
slits and a high energy input light source are required. 
U.S. Pat. No. 4,245,911 discloses a drum cam mechanical drive to oscillate 
the grating and means to adjust the scanning speed. U.S. Pat. Nos. 
4,264,205 and 4,285,596 disclose a conjugate cam mechanical drive to 
oscillate the grating. In both disclosures the mechanical drive is 
directed to retaining the accuracy of the mechanical drive and to 
eliminate backlash the mechanical parts thereby reducing noise in the 
measurements at high scanning speeds. All such mechanical oscillating 
drives for the grating are inertia limited because of the reversal of 
movement in each cycle. 
SUMMARY OF THE INVENTION 
Applicant has developed a rapid-scan spectrophotometer for UV/VIS 
spectroscopy (but not limited to) at scanning speeds comparable to diode 
arrays without the limitations of spectral resolution, "reverse optics", 
spectral range and high cost. Objectives of the new monochromator and 
spectrophotometer are to mechanically scan at high speed with a 
high-inertia, concave holographic grating, to minimize the number of 
optical elements and to integrate the data acquisition and system control 
into one function. 
The new spectrophotometer features a grating drive mechanism that enables 
the recording of ten complete absorption spectra per second with 
wavelength accuracy of better than Inm. This scanning mechanism can scan 
the UV, visible and even NIR wavelengths. As it uses conventional optics 
with both inlet and exit slits, the bandwith is well defined resulting in 
lower stray light and higher sensitivity. The use of a single detector 
element means that the UV enhancement of the detector surface is 
significantly better than a diode array. The unique scanning mechanism 
also allows for continuous dark current correction resulting in lower 
drift. All these features result in a high quality, low noise, low drift 
spectrophotometer. 
The spectrophotometer according to the present invention is provided with a 
data acquisition system which system is only dependent on the sensed 
position of the grating, provided by a shaft encoder and vertified by the 
naturally occurring effect of the zero order spectrum (white light) which 
is directly dependent on the position of the grating. Therefore the need 
for feedback circuits and associated servo systems to regulate the angular 
velocity of the rotation of the grating is eliminated. An approximately 
constant angular velocity may be provided by a suitable drive motor and 
smoothed out by the flywheel effect of the turntable attached to the 
grating. The exact angular position of the grating is calculated by 
measurements of the rotational speed and acceleration immediately 
preceding the angular section of interest. 
To accomplish the rapid-scan with a mechanical drive for the grating, the 
grating is affixed to a relatively high inertia fully rotatable turntable 
which continuously rotates the grating about its optical axis. The grating 
is rotated at a suitable angular velocity which should be constant. By 
adding mass to the turntable and thereby increasing the inertia of the 
rotating mass, the flywheel effect is increased. Thus, the sampling cycle 
may be termed "free flying". 
The optical region of interest is approximately 15 degrees (depending on 
application) of the rotation sweep as with the oscillating gratings, 
therefore means are provided to select and coordinate the scanning with 
the angular position of the grating. 
The (free flying) sampling cycle starts with the zero order light (white 
light) falling on the detector. This signal is easily distinguished from 
spectral signals (due to its intensity), giving an accurate position of 
the grating once per revolution. An index pulse (coming from the shaft 
encoder) is generated immediately after the zero order for synchronization 
purposes and verification of the shaft encoder position. 
Shortly before the first wavelength of interest appears at the exit slit, a 
suitable trigger circuit starts the sampling cycle; this may be easily 
determined from the above data. Once the sampling cycle is started, pulses 
from the shaft encoder, which may be spaced at angular positions 
corresponding to, e.g. 20 nm, are fed into a logic circuit. This logic 
circuit, using the principle that the velocity profile can be determined 
assuming constant acceleration, interpolates pulses corresponding to the 
desired wavelengths. If required, the grating equation may be used at this 
point to correct for slight nonlinearity between the angular displacement 
and wavelength. 
The pulses delivered by said logic circuit are fed to a sampling A/D 
converter circuit. This circuit also continuously receives the voltage 
value (suitably processed) from the detector measuring the intensity of 
the light falling thereon after passing through the sample cell. Said 
sampling - A/D converter circuit samples this continuous voltage input for 
each pulse received and optionally digitizes same. The output of this 
circuit is a function of the intensity of the light falling on the 
detector relative to the wavelength. Thus, the desired spectrum appears in 
digitized form. 
This is the primary output of the presently claimed spectrophotometer and 
may be stored in a suitable memory or possible displayed on a CRT (this 
may be done also by utilizing the sample voltages without digitizing 
same), or otherwise utilized in conventional ways. 
In order to take advantage of the fact that only a fraction of the time 
necessary for the rotation of the grating is utilized for measuring a 
spectrum, and the remainder is available as free time, further circuitry 
has been devised which may be optionally used with the spectrophotometer 
of the invention. 
This additional circuitry includes a scratch-pad memory, receiving the data 
for each revolution (digitized light intensity as a function of 
wavelength) in real time. These values are then stored for further 
processing at a slower rate and the scratch-pad memory cleared, enabling 
the scratch-pad memory to accept a new spectrum prior to the beginning of 
the next sampling cycle. 
This data handling/computing circuitry may be accessed as desired to 
retrieve any valuable information, and also programmed to transfer 
required information such as absorption values for pre-determined selected 
wavelenths, directly successive to processing units. Alternatively such 
information may be transferred to non-volatile storage or discarded. Of 
course, the successive processing unit may also be programmed to retrieve 
information from said non-volatile storage for comparison or computing 
purposes. 
Although disclosed herein with respect to UV/VIS spectroscopy, the 
Applicant's spectrophotometer can be adapted to infra red spectroscopy 
with a suitable source, detector and sample cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Illustrated in FIG. 1 is a light source comprising a light generating 
device 10 and an elliptically curved concave mirror 12. The light 
generating device 10 provides polychromatic light in the ultraviolet, 
visual or infra-red region as required. For the more common uses of a 
spectrophotometer, a visual range or ultra-violet range light source is 
selected. Recently availabe are combined ultra-violet/visual range light 
sources (UV/VIS). Such a light source provides a complete spectrum over 
the UV/VIS range most commonly used. The light reflected from the concave 
or elliptical mirror 12 is directed through a slit 14 and an optional 
filter wheel 16 to impinge upon a rotating optical grating 18. The optical 
grating 18 rotates about its optical axis with a preselected angular 
velocity. The light reflected from the optical grating 18 is then directed 
to and through a second slit 20 and through a sample chamber 22 to finally 
impinge upon a detector 24. The rotating optical grating 18 is mounted on 
a relatively heavy turntable 26. 
As shown, the filter wheel 16 is located just beyond the entrance slit 14. 
However, the filter wheel may be positioned ahead of the entrance slit 14 
or on either side of the exit slit 20. The purpose of the filter wheel is 
to delete certain wave lengths from the spectrum sample and is a well 
known option in the spectroscopy field. Typically, the light generating 
means 10 is either a bulb providing the visual spectrum or a bulb 
providing the ultra-violet spectrum. However, a new bulb new being 
introduced by Hamamatsu Instruments of Japan provides the full range of 
ultraviolet and visual wavelengths from a single point source. 
The supporting wheel or turntable 26 for the optical grating 18 is 
preferably relatively heavy so as to act as a flywheel. The optical 
grating 18 itself may be suitably heavy because the increased inertia in 
the rotating assembly is advantageous in this spectrophotometer. Increased 
mass helps to retain a relatively constant angular velocity. Referring to 
FIG. 2 the optical grating 18 and flywheel 26 are mounted on a vertical 
shaft 28 in turn mounted in bearings 30 and 32. The bearings as shown 
schematically at 30 and 32 are in turn mounted in a rigid supporting 
structure 34 and 36. The bearings 30 and 32 are selected for rotational 
accuracy to eliminate any possibility of translational vibration being 
transmitted to the optical grating 18. Illustrated next to the flywheel 26 
and optical grating 18 is a drive motor 38 which is connected 40 to the 
flywheel 26. A suitable drive system may be a floppy disk drive motor or 
tape drive motor, selected from the many available for computers. Such a 
drive in combination with a relatively heavy fly wheel 26 will assure a 
sufficiently constnat rotational velocity for the optical grating 18. Also 
affixed to the shaft 28 is an optical incremental shaft encoder 42,44 such 
as the HEDS 5000 by Hewlett Packard, which provides 500 pulses per 
revolution and an index pulse. 
To the left side of FIG. 3 is a brief schematic of the spectrophotometer 
apparauts shown in FIG. 1. The light source to the lower left is the 
combination of the light generating means 10 and elliptical focusing 
mirror 12. As schematically shown in FIG. 3, the position of the index on 
the shaft encoder 42,44 is shown as a dark spot 46 that is about to enter 
the optical head 44 of the encoder. 
Referring again to FIG. 3 the pulses coming out of the shaft encoder 
assembly 42, 44, 46 are fed into the velocity and acceleration calculator 
48, the output of which is used to control A/D logic circuit 50. This 
logic circuit 50 comprises: timing circuitry adapted to measure the time 
intervals between successive pulses delivered by the optical head 44 of 
the encoder; memory for storing such successive time intervals (a number 
[n] of such time intervals being measured); computing means for 
extrapolating from the above data the expected time interval [n+1] for the 
angular distance between the two following markings and circuitry 
interpolating pulses corresponding to the angular positions representing 
the selected wavelengths. 
The wavelengths interpolated in time interval [n+1] are preferably equally 
spaced. This is based on the assumption that the angular velocity of the 
grating in the time interval corresponding to a few shaft encoder markings 
is sufficiently constant to justify such extrapolation and interpolation. 
In each revolution, the naturally occurring phenomena of the zero order 
spectrum (polychromatic light), directly depends on the position of the 
grating. One instant each revolution, the grating acts as a simple mirror 
reflecting polychromatic light onto the detector. This signal, together 
with the index pulse initiated by dark spots 46 of the shaft encoder, 
which occurs once per revolution, is used by the calibration algorithm, to 
verify the angular position of the grating. The zero order is a strong 
signal easily distinguished from the subsequent spectral data. 
The index pulse is also fed to the filter wheel drive 54. The purpose of 
this filter depends on the location thereof, either to eliminate undesired 
wavelengths from the incident light (when located between the entrance 
slit 14 and the grating 18) or to prevent higher order spectra from 
reaching the detector (when located between the grating 18 and exit slit 
22). 
By way of example, if the spectrum is to be scanned from 200 to 600 nm, it 
is possible to scn the section from 200 to 370 nm in one scan, advance the 
filter wheel 18 and then the section from 370 to 600 nm is scanned in the 
next revolution. This necessarily lowers the time resolution of the 
instrument. 
The filter wheel control 54 causes the filter wheel motor 37 to rotate the 
filter wheel 16 in synchronization with turntable 26. As said above, the 
location of the first wavelength of interest may be easily calculated from 
the position of the zero order signal and the index pulse. It is important 
that the utilization of the zero order signal, which is independent of the 
shaft encoder and dependent only on the actual angular position of the 
grating for continuous calibration, eliminates possible errors stemming 
from changes in the relative position of the grating and the shaft 
encoder. In effect, the grating angular position is then recalibrated with 
each revolution. 
The electric signal from detector 24 (an analog value) is fed to the 
pre-amplifier 56. The A/D circuit 52 samples this electrical signal for 
every pulse received and digitizes the sample values, if desired. 
The result of this intermediate information processing stage is the desired 
spectra in the form of light intensity values as a function of wavelength. 
This intermediate result may be utilized directly in conventional ways, as 
above said, but the preferred embodiment of the invention is to transfer 
the output of the A/D circuit 52 in digital form to a scratch-pad memory 
58 in real time for temporary storage. 
A computer and a program may be provided for transferring the data through 
the data bus of the computer to a fixed memory location, at a rate 
adequate for transfereing the data representing a single scan, thus 
preparing the scratch-pad memory for receiving a fresh burst of data 
representing the following revolution and scan. The unit for receiving the 
data and transferring the data for processing or to more permanent memory 
locations, possibly after appropriate selection and/or calculation is 
herein termed a data acquisition unit 60. The data acquisition unit 60 is 
followed by data processing unit 62 and data storage unit 64, two units 
which are interconnected and may be programmed in a variety of ways. 
Another unique feature of the monochromator of the present invention is the 
dark current measurement and compensation. 
A well known problem with detector such as those of the silicone diode type 
used herein is that due to thermal effects, an output signal arises 
despite the lack of incident light. This, termed the dark current, must be 
compensated. Heretofore a most common way to accomplish such compensation 
is to activate a shutter to block the light and then measure the dark 
current. 
According to the present invention, an almost continuous correction may be 
applied by measuring the detector output during the time for each 
revolution at which the back surface of the grating faces the detector. 
This back surface may be made non-reflecting by anodizing and coating with 
a non-reflecting material. The dark current output of the detector may be 
utilized at any suitable stage of the data acquisition process to correct 
the measured detector output 
The most suitable point may be the data acquisition unit 60 where this dark 
current value can be temporarily stored in an appropriate memory location 
since this value is not concurrent with the spectral measurement data. 
However, since the time interval between measuring the spectrum and the 
dark current in the same revolution is very short, there is a distinct 
advantage. The possible error due to a drift in the dark current value is 
minimized, thereby decreasing thermal instability effects and reducing 
long term noise. 
The applications of applicant's new spectrophotometer are manifold. For 
example, for routine measurements and slow scan, the rapid scan of the 
instrument permits each spectral element to be represented as a value and 
corresponding statistical uncertainty. In kinetics, wherein the 
concentrations of various species are followed as a function of time, to 
study their dynamic behavior, the new spectrophotometer is capable of 
acquiring at least ten complete spectra per second (200 nm to 800 nm) with 
one nm resolution. The new instrument can be used for 
spectroelectrochemistry, stop flow kinetics and as a detector for liquid 
chromatography (HPLC) or gas chromatography. 
Measuring changes in absorbance in samples that are highly heterogeneous is 
difficult due to scattering. To correct for the scattering effect, light 
of two different wave lengths is used, one where there is absorbance in 
the sample and one where there is no absorbance to thereby detect the 
attenuation caused by scattering. Because a complete spectrum can be 
collected in a fraction of a second, information coming from two different 
wave lengths can be considered to be acquired virtually simultaneously, 
and the dual wave length spectroscopy correction can be applied with a 
minimum of error. 
Other modifications of the spectrophotometer can be added. Because added 
mass is of benefit to the accuracy of operation of applicant's device, as 
shown ghosted in FIG. 3, a second grating 18' mounted back to back with 
the first grating and a second triggering means 46' on the rotating 
flywheel or shaft permit a double sampling rate at the same angular 
velocity. Secondly, as shown ghosted in FIG. 1, filters 16' can be mounted 
on the rotating turntable or flywheel 26 in front of the optical grating 
18 to remove second order and other harmonic wavelengths without any 
weight penalty. Thus, features that cannot be added to the prior art 
oscillating and vibrating optical gratings without severly impairing the 
scanning speed, can be added to the continuously rotating optical grating 
of applicant not only without penalty but with the added advantage that 
the increase in inertia is of benefit to the accuracy of applicant's 
spectrophotometer.