Control of multi-position filter mechanism in an optical measuring system

A method of identifying an operating position of a multiple-position filter changing mechanism situated in an optical path of a spectrophotometer by detecting and measuring a spectral characteristic(s) of light passed along the optical path to the spectrophotometer detector for one or more operating positions of the filter mechanism and comparing the measured spectral characteristic(s) with stored spectral characteristic information for one or more operating positions of the filter mechanism.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the use of light filters in optical 
measuring systems and, more particularly, the operation and control of 
multiple position filter changing mechanisms in such systems. 
2. Background of the Invention 
Many optical measuring systems including but not limited to photometers, 
spectrophotometers, fluorometers, and the like, are adapted to perform 
optical measurements with various optical filters positioned in the 
optical measuring path. Depending upon the nature of the sample to be 
analyzed, the type of instrument to be employed, and the operating 
conditions to be satisfied, different filters or combination of filters 
are required to be positioned in the optical path. In order to simplify 
the filter positioning operation, various multiple position filter 
changing mechanisms have been developed which are controllable to 
automatically select the required filters. One common and mechanically 
simple form of filter changing mechanism is a filter wheel or disc 
supporting a plurality of filters or filter combinations at spaced 
circumferential positions around the disc. The disc is rotated or 
otherwise advanced to position various ones of the filters in the optical 
path sequentially. 
The control of such filter changing mechanisms has been effected in several 
basic ways. In one approach the filter mechanism is connected to and 
driven by mechanical linkages which slave the filter mechanism to the 
mechanical state of other system components. In another approach, a 
separate position sensing device monitors the position of the filter 
mechanism and sends filter mechanism position information via a closed 
loop feedback control network to a filter mechanism controller. While the 
above approaches function satisfactorily to control filter mechanism 
positioning, they require additional control elements and logic and 
generally are less versatile and more costly than desired. 
SUMMARY OF THE INVENTION 
The present invention resides in a novel method of controlling a multiple 
position filter changing mechanism which overcomes the drawbacks of the 
prior approaches. The method is simple in operation and straightforward in 
implementation and is particularly adapted for use in optical measuring 
systems of the type including a detector in an optical path to measure 
filtered light received from a sample situated in the path for 
measurement. 
The method of the invention in its broadest aspect contemplates measuring a 
spectral characteristic of the filter mechanism in one or more operating 
positions to determine an operating position of the filter mechanism. To 
these ends the invention includes the steps of (a) storing spectral 
characteristic information of one or more of the operating positions of 
the filter changing mechanism, (b) setting the filter changing mechanism 
in an operating position, (c) directing light of predetermined spectral 
characteristics along the optical path, (d) detecting and measuring a 
spectral characteristic of light reaching the detector, and (e) comparing 
the spectral characteristic measured in step (d) with the stored spectral 
characteristic information in an effort to identify an operating position 
of the filter changing mechanism. After identifying one operating position 
the filter changing mechanism can be advanced to a predetermined operating 
position or through a known sequence of operating positions for executing 
sample or other measuring operations of the spectrophotometer. 
In a preferred form of the invention, the method comprises the further step 
of repeating the steps (b)-(e) for one or more other operating positions 
of the filter changing mechanism. In this manner a spectral characteristic 
may be measured for each of successive operating positions of the filter 
changing mechanism, even if each measurement alone is not determinative of 
an operating position, to establish a sequence of measured characteristics 
which establishes an operating position of the mechanism. 
By virtue of the invention, the operating position of the filter changing 
mechanism can be identified using optical elements already existing in the 
instrument and is accomplished without the need for feedback position 
sensors or other complex position sensing and control elements of the 
prior art. Comparison of the known and measured spectral characteristics 
further serves to identify operational failures of the filter changer 
mechanism such as failure to change from one position to another or to 
stop in a given position as well as loss of a filter from the filter 
mechanism or degradation of its spectral characteristics with time.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings for purposes of illustration, the present 
invention is embodied for use in an optical measuring system, such as the 
spectrophotometer system illustrated in FIG. 1. The basic 
spectrophotometer system components are of a form conventional in the art 
and will be described only to the extent necessary to set forth use of the 
invention in the spectrophotometer. To this end, the spectrophotometer 
includes a light source 10, light energy from which is directed along an 
optical path or axis 11 serially through a filter changing mechanism 12, a 
monochromator 14, a sample 16 and thence to a detector 18. Source 10 is 
preferably a white light tungsten incandescent lamp. Filter changing 
mechanism 12 supports a plurality of filters 20a, 20b, 20c and 20d which 
are adapted to be positioned individually in the path of light issuing 
from source 10. Filter 20b is illustrated as a stacked filter combination 
of two individual filters. Each positioned filter 20, when operatively 
positioned in optical path 11, attenuates selected wavelengths of the 
light from the source and passes the attenuated light beam to 
monochromator 14. The monochromator, in turn, is controllable in a 
conventional manner to further restrict or limit the wavelength of light 
and passes only a selected narrow bandwidth of light to sample 16 and 
thence to detector 18. The detector provides a measure of the 
transmittance or absorbance of the sample by detecting the amount of light 
energy passed by the sample at the selected wavelengths. 
FIG. 1 illustrates, in effect, a single-beam spectrophotometer in which 
sample material 16 and a reference material are inserted individually into 
the optical path 11 at different times for measurement. In a well known 
double-beam configuration, however, as is preferred with the present 
method, the sample is disposed in one path and a reference is disposed in 
a second reference path 11a illustrated in dashed outline. Light issuing 
from monochromator 14 is chopped in a conventional manner to be directed 
alternately along the sample beam path and the reference beam path and the 
thus formed reference and sample light beams are combined before reaching 
the detector. A demodulator (not shown) is associated with the detector 
for demodulating the combined sample-reference signal to derive a 
measurement of sample vs. reference. 
The signal generated by detector 18 is amplified by amplifier 22, converted 
to digital form by a-d converter 24, and supplied to a data acquisition 
and control system 26. System 26 can be either a hardwired controller or 
programmable microcomputer well known in the art. Control signals are 
supplied by the control system over respective control lines 28 and 30 to 
control the operation of filter changer mechanism 12 and monochromater 14, 
respectively. 
Filter changer mechanism 12 is illustrated as a conventional filter wheel 
or disc 32 mounted on a rotatable shaft 34 with the disc segmented into 
the plurality of filter sections, herein 20a, 20b, and 20c, and 20d. Shaft 
34 is rotated by a conventional solenoid 36 and ratchet 38, in response to 
the control signal received from control system 26 over line 28, to 
position selected ones of the filters 20 in the optical path 11 between 
light source 10 and detector 18. 
FIG. 2 illustrates an alternative and preferred form of filter changing 
mechanism comprising a pulse driven rotary solenoid 40 having a rotatable 
output shaft 42 on which a disc 44 is axially supported for rotation 
together with a linearly actuable slider 46 supporting three filters 20a, 
20b, and 20c. Slider 46 includes a vertical slot 48 adapted to receive an 
axial pin 50 extending from rotatable disc 44. With pin 50 in slot 48, 
energization of solenoid 40 rotates disc 44 and pin 50 causing slider 46 
to be driven linearly back and forth in a manner which positions each of 
the optical filters supported thereby in the optical path 11 in 
succession. Thus, beginning arbitrarily with filter 20a, the filters would 
be positioned in the sequence of 20a, 20b, 20c, 20b, 20a, 20b, 20c, etc. 
Since movement of slider 46 is linearly back and forth, middle filter 20b 
is positioned more frequently in the sequence than the remaining two. 
In accordance with a primary aspect of the present invention spectral 
characteristics of the filter changing mechanism 12 in one or more 
operating positions are measured to identify an operating position of the 
filter changing mechanism and to control positioning of the filter 
changing mechanism thereafter. For this purpose, control system 26 
communicates with a segment of storage 52 in which known spectral 
characteristic information of the filter changing mechanism 12 and the 
filters 20 therein is stored. The spectral characteristics stored in 
storage segment 52 relate to the optical transmission characteristics of 
the filters, i.e. the amount of light energy passed by the filters, as 
measured by detector 18, in an appropriate band or bands of the light 
wavelength spectrum. Storage segment 52 may take any convenient form of 
read-write memory. If the invention is practiced on a manual instrument, 
without a control system 26, the spectral information can be stored as 
positions or conditions of an impedance network or as potentiometer 
settings. 
With the spectral characteristics of filter changing mechanism 12 held in 
storage 52, and with the initial operating position of the filter changing 
mechanism unknown, the present invention contemplates measuring the 
spectral characteristics of light passing through one or more of filters 
20a-20c by means of spectrophotometer detector 18, and comparing the 
measured spectral characteristic with the stored spectral characteristic 
information to identify an operating position of the filter changing 
mechanism. In the double-beam arrangement, only light directed along 
reference beam path 11a is measured (i.e. the sample beam is effectively 
blocked) for each positioned filter. This avoids interference by sample 
containers inadvertently or inaccurately positioned in the sample beam 
path. Thereafter, with one operating position of the filter changing 
mechanism identified, and with the filter position orientation of the 
mechanism pre-established, the filter mechanism is advanced to any 
designated initial operating position by supplying the requisite number of 
control pulses over control line 28 to drive the solenoid and hence the 
filter changing mechanism to the desired initial operating position. 
Thereafter, with the filter mechanism appropriately positioned, a sample 
measuring operation is conducted in the usual manner. 
The amount of light measured by detector 18 at a given operating position 
of the filter changing mechanism 12 at given wavelength setting of the 
monochromator 14 can identify the positioned filter with respect to the 
remaining filters if the detector output signal level has a value unique 
to such filter (e.g. is above or below a given threshold or is between 
predetermined upper and lower threshold limits). In such case measurement 
of light passed in a single operating position of the filter changing 
mechanism serves, when compared with the stored spectral characteristic 
information, to identify the operating position of the filter changing 
mechanism 12. 
If the measured spectral characteristic for one filter operating position 
is ambiguous or is similar to characteristics of other filter positions, 
it is still possible to measure further spectral characteristics of such 
one filter operating position which identify the one filter from among the 
others. In this respect a filter in a single operating position can be 
identified by controlling monochromator 14 to pass energy at selected 
wavelength settings in sequence, detecting the light energy passed at each 
wavelength setting, and measuring the ratios of detected light at the 
different wavelength settings. Such ratios calculated for appropriate 
wavelength combinations form a profile characteristic of one and only one 
filter in the set of filters and, accordingly, when compared with the 
stored spectral characteristic information in storage segment 52, identify 
the positioned filter. 
To develop the nature of the spectral characteristics stored in storage 52 
for the preferred filter changer embodiment of FIG. 2, FIG. 3 illustrates 
superimposed graphical plots of the normalized detected radiant power as a 
function of wavelength (nanometers) passed by the respective FIG. 2 
filters 20a, 20b and 20c when positioned in the path of light from source 
10. Filter 20a has a radiant energy pass band from about 400 to about 675 
nanometers. Filter 20b has a pass band between about 360 and 460 
nanometers. Filter 20c has a radiant energy pass band at about 350 
nanometers and between about 690 and 740 nanometers. It will be observed 
from these waveforms that at about 546 nanometers, for example, neither 
filter 20b nor 20c will pass light but filter 20a will. Thus, for the 
preferred embodiment, the initial operating position of the filter 
mechanism 12 is determined by directing light from source 10 through one 
or more of the filters 20a-20c while restricting the light energy reaching 
detector 18 to energy which only one filter will pass (i.e. energy at 546 
nanometers). This restriction is achieved by tuning monochromator 14 to 
pass light only in the narrow band at 546 nanometers. 
If the filters are positioned sequentially in the light beam and the energy 
passed by each is detected and measured by detector 18, beginning at an 
unknown operating position, eventually the filter operating position for 
filter 20a will be reached which will be the first and only filter 
position to pass a significant amount of light at 546 nm to generate a 
"high" signal which, when compared with stored spectral characteristic 
information in storage 52, indicates that the filter changing mechanism is 
in the operating position for filter 20a. However, such a "high" signal 
might also be indicative of an improperly positioned or missing filter or 
may indicate some other malfunction or failure of the filter mechanism. 
Accordingly, in the preferred embodiment, an operating position of the 
filter mechanism is determined with more certainty by measuring the 
spectral characteristics for sequential operating positions of the filter 
mechanism until a predetermined sequence of filter positionings is 
attained. For the filter changing mechanism of FIGS. 2-3 a sequence is 
ultimately executed in which virtually no energy is passed at three 
consecutive operating positions (corresponding to positioning of filters 
20 b, 20c and 20b in sequence) followed by passage of a substantial amount 
of light at 546 nanometers in the fourth position for filter 20a. Thus, a 
succession of three "low" readings followed by a "high" reading of 
detected light energy indicates that the filter changing mechanism 12 is 
then in the operating position which positions filter 20a in the optical 
path. 
In the preferred embodiment the detected light energy passed for each 
position of filter mechanism 12, as detected by detector 18, is amplified 
at 22 and converted to digital form at 24 and supplied to control system 
26. The control system, in turn, operates in a conventional fashion to 
store the readings at each operating position and to compare each reading 
and/or the resulting sequence of readings with the stored spectral 
characteristics of the filter mechanism stored in storage segment 52. 
FIGS. 4a-4d illustrate the algorithm or method steps performed in 
implementing the present method with control system 26 in the form of a 
conventional microprocessor with storage segment 52. It is understood that 
the particular hardware arrangement of such a digital processor is well 
known in the art and forms no part of the present invention. In general, 
however, such general purpose computers include a central processing unit, 
a programmed sequence of memory instructions (a read only memory), an 
uncommitted block of useable memory (a read/write memory), and various 
input and output interfacing capabilities. Instructions can be executed 
from the read-only memory. Data can be transferred into or out of the 
read/write memory and into or out of the central processing unit. The 
central processing unit is configured to fetch and/or execute data and/or 
instructions to and/or from the memories and to the various input and 
output control devices. Programming such a computer for automated method 
implementation and operation and coordinating information processing is 
straightforward and well established in the art and the algorithm set 
forth in FIG. 4 is programmed in such a conventional manner. 
The FIGS. 4a-4d algorithm steps are numbered 1 through 14. In step 1 system 
26 issues an appropriate signal over line 30 to set monochromator 14 to 
pass light in the narrow band at 546 nanometers. 
In step 2 a counter (not shown) is initialized for counting the number of 
filter measurements made. The initial value is set to be a maximum number 
of measurements (e.g. 10) to be allowed before it is assumed that the 
algorithm has failed to identify an operating position. 
In step 3 a "low" reading counter is initialized for counting the number of 
light intensity measurements by detector 18 below a predetermined 
threshold value. Each measurement below this threshold is considered to be 
a "low" measurement. 
In step 4 a control signal is issued over line 28 to position the filter 
changing mechanism in the next of its operating positions at which the 
intensity of 546 nanometer light passing through the filter is then 
measured. Simultaneously the step 2 counter is decremented one count. 
In step 5 if the step 2 counter has been decremented to zero, then the 
maximum allowable number of filter measurements (e.g. 10) has been made 
without identifying an operating position, and the operator is directed to 
step 14 which generates an error or failure flag. 
If the step 2 counter has not been decremented to zero, then, in step 6, a 
determination is made whether the the measured spectral characteristic 
light intensity (of light directed along reference beam path 11a) is 
greater than a predetermined value (e.g. 64 ADC counts). If it is then 
such measurement is considered a "high" reading, and the operator is 
directed bak to step 3 to again initialize the "low" reading counter. 
If the measured intensity in step 6 is "low", then the "low" reading 
counter is decremented one count in step 7. 
In step 8 a determination is made whether the "low" reading counter has 
been decremented to zero. If not, then there have not yet been three 
successive "low" readings, and the operator is directed back to step 4 to 
move the filter mechanism to the next operating position. 
In step 9 if three successive "low" readings have been found, then the 
program stores the intensity of the third "low" reading so that it may be 
compared with the following reading. 
In step 10 the filter mechanism is moved to the next operating position and 
the intensity of light at 546 nanometers passing through the filter is 
again measured. The step counter is decremented one count. 
In step 11 if the step 2 counter has been decremented to zero, then the 
maximum allowable number of filter measurements (e.g. 10) has been made 
without identifying an operating position, and the operator is directed to 
step 14 which generates an error or failure flag. 
If it is determined in step 11 that the step 2 counter is not zero, then 
the question is posed in step 12 whether the measured light intensity is 
greater than the last "low" reading, i.e. the last "low" reading was the 
third of three successive "low" readings stored in step 9. If the measured 
intensity is not greater by a predetermined amount (i.e. by an amount 
which defines a "high" reading, e.g. 32 times), then the low-low-low-high 
sequence has not been established and the operator is directed to step 3 
to again initialize the "low" reading counter. 
If step 12 determines that the measured intensity is greater than the last 
"low" reading by the predetermined amount, i.e. if it is a "high" reading, 
then the four position sequence has been established and the operating 
position of the filter changing mechanism has been established. Such is 
flagged in step 13 and the operator is returned to the main program. 
The foregoing algorithm is straightforward to implement and a conventional 
microcomputer is readily and easily programmed by a programmer of ordinary 
skill in the art to execute the steps of the algorithm. 
The method of the invention is implemented by storing spectral 
characteristics for one or some or all of the operating positions of the 
filter changing mechanism. The operating position is determined by 
performing measurements in one or more operating positions. The measured 
characteristics are compared with the stored spectral characteristic 
information for one or more or all of the operating positions, as desired. 
If compared to stored information for one operating position, then 
measurements are made in sequential operating positions until the one 
operating position is located. If information is stored for more than one 
operating position, then measurements may be made in sequential operating 
positions, even if and after a single measurement identifies an operating 
position, to verify an operating position by deriving an established 
sequence of measurements. Moreover, a measurement may be made each time 
the filter mechanism operating position is changed to identify and verify 
the new position of the mechanism. By identifying filter mechanism 
position the present invention further serves to identify any failures or 
malfunctions of the filter changing mechanism such as failure to properly 
change from one position to another, failure to stop at one or more 
operating positions as well as a loss of a filter from the filter changing 
mechanism, or degradation of a filter's spectral characteristics with 
passage of time. 
It should further be understood that in some applications the filter 
changing mechanism 12 may include one or more filter mechanism operating 
positions in which little or no light is attenuated and even may be 
configured with a totally blocking filter or comparable blocking shield or 
a totally transmissive filter or comparable aperture with no filter at all 
and that the present invention as described and claimed includes the use 
and positioning of filter changing mechanisms as thus configured. 
Moreover, while a preferred embodiment of the invention has been 
illustrated and described, modifications may be made therein without 
departing from the scope of the invention as defined in the appended 
claims.