Abstract:
An instrument for processing light information to assay chemical or biological molecules is made more flexible by the use of a replaceable filter module. Several light filters, or in one embodiment dichroic mirrors, are mounted in a common module in fixed position relative to each other. The whole filter module can then be removed, and a different one inserted in its place, to change the wavelength bands of light detected by the instrument, as needed, for different applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/100,308, filed Sep. 15, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     Many optical systems utilize spectrum separation techniques and light diversion systems to select one or more wavelength bands of light and to direct the selected light to a sensing device. In many different types of instrumentation, often a specific spectrum of light is what is selected, because the signal sought to be detected is spectrum limited. For example, in many chemical analytical instruments, the distinguishing characteristics of a chemical mixture are detected by sampling the wavelengths or wavelength bands of light originating from a chemical or biological sample. In a simple example, a single distinct species of molecule that emits light in a characteristic wavelength can be identified by a detection system that selectively collects and transmits that wavelength band to a photodetector. When mixtures of chemicals, each with a specific light-emitting characteristic, are mixed, it then becomes necessary to enable multi-color transmission and detection, as well as separation. 
     Instrument designs to accomplish multi-color detection rely on optical elements such as color filters, gratings and dichroic mirrors to separate an incident light signal into one or more light signals that each encompasses a characteristic wavelength band. Light signals contained within selected wavelength bands can then be analyzed by photodetectors to determine the intensity of light present in the incident light beam in one or more wavelength regions. Such analysis can reveal the chemical components of the sample under study. 
     Multi-color systems are known which use dichroic mirrors to define the wavelength regions to be analyzed. FIG. 1 illustrates a simple system, known to the prior art, that uses three dichroic mirrors in a row to separate incident light into three primary colors. This system is used in electronic color imaging such as described in U.S. Pat. No. 4,654,698 to Longworthy. FIG. 2, also illustrating a prior art system, illustrates the type of system used in U.S. Pat. No. 3,7944,407 to Nishimura that includes two dichroic mirrors and three photodetectors. This systems uses an additional filter in front of each photodetector to further define the wavelength region being detected. Systems using three dichroic mirrors and four photodetectors, exemplified by U.S. Pat. No. 4,776,702 to Yamaba and U.S. Pat. No. 5,538,613 to Brumley, are illustrated schematically in FIGS. 3 and 4 respectively. 
     In all of these systems, the dichroic mirrors are used to reflect a desired wavelength band of light to a detector or group of detectors while the non-selected wavelength bands are directed to other dichroic mirrors. In all of these designs, the dichroic mirrors are fixed in place, thus limiting the instrument to the detection of light patterns of certain defined characteristics. In short, the instruments are dedicated to a defined type or pattern of light sensing. 
     In some applications, notably for the detection of fluorescently tagged DNA molecules, it would be desirable in different applications to be able to filter and detect different wavelength bands. For DNA sequencing procedures, a set of fluorescent dyes are commonly used to tag the DNA molecules so that the sequence of the DNA molecule can be detected by optical reading. For other DNA analysis procedures, for a variety of reasons not important here, the use of different fluorescent tags which have a different spectral characteristic, are more desirable. Therefore, an instrument which is capable of altering its optical characteristics for the particular application would be adapted for use in more applications, as contrasted to one that was invariable in it optics. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is summarized in an apparatus using light signals to test chemical properties of a sample, that the apparatus includes a reasonable filter module which includes a plurality of dichroic mirrors each of which is fixed in place relative to the other mirrors in the filter module, so that filter modules can be changed in the instrument to permit the optical characteristics of the instrument to be changed without the need for mirror adjustments of the mirrors relative to each other. 
     It is an advantage of the present invention that it enables devices intended for analysis of DNA molecules to be used for a variety of different applications which use a variety of fluorescent tags. 
     It is a feature of the present invention that it enables optical instruments of greater versatility for chemical or biochemical analysis. 
     Other objects, advantages, and features of the present invention will be apparent from the following specification, when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the optical path for a simple prior art instrument. 
     FIG. 2 is a schematic illustration of the optical path for a prior art instrument. 
     FIG. 3 is a schematic illustration of the optical path of another prior art instrument. 
     FIG. 4 is a schematic illustration of the optical path of another prior art instrument. 
     FIG. 5 is a schematic illustration of the optical path for the instrument of the present invention. 
     FIG. 6 is a partially expanded perspective view of a filter module according to the present invention. 
     FIG. 7 show, top plan, side plan, bottom plan and cross-sectional views respectively of the filter manifold of the embodiment of FIG.  6 . 
     FIG. 8 is a schematic illustration of an alternative optical path for an instrument according to the present invention. 
     FIG. 9 is another schematic illustration of an alternative optical path for an instrument according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention makes use of a removable filter module for use in an instrument for detecting fluorescence from molecules, or for other instruments utilizing multispectrum light for information gathering. In its initial application, the present invention is intended to be an improvement to the type of device described in U.S. Pat. No. 5,538,613 to Brumley, the specification of which is hereby incorporated herein by reference. In summary, an instrument of that type is one in which fluorescently labeled DNA molecules move within an electrophoresis gel and are then detected optically. The light from the DNA gel is transmitted through a series of dichroic mirrors. As used herein, a dichroic mirror is one in which light of a limited range of wavelength, typically defined as a center wavelength of transmission with a certain full width at half-maximum, is transmitted through the mirror while wavelengths outside of this transmission band are reflected off of the dichroic mirror&#39;s surface. Wavelengths of light that are not transmitted through the dichroic mirror are reflected with high efficiency, exceeding 98%. So the dichroic mirror is, in essence, a sort of band-pass filter permitting light of a defined wavelength band to pass through it while the remaining light is reflected. 
     As shown in FIG. 1 of the aforementioned U.S. Pat. No. 5,538,613, an apparatus for detecting fluorescently-labeled molecules can include an excitation light source, such as a laser, connected through optical fiber to be directed onto a gel separation medium such as an electrophoresis gel. Light which originates from the gel is then picked up by collecting optics and conveyed to an array of dichroic mirrors. What is described in this specification is a filter module sub-component, useful in the type of devices disclosed in said U.S. Pat. No. 5,538,613, or other similar optically based instruments in which light separation is desired. The removable filter module permits convenient changing or swapping of the dichroic mirrors so that the same instrument can be used to detect and analyze more than one fixed set of fluorescent labels. 
     In essence, the replaceable filter module of the present invention is intended to allow an instrument to be utilized for more than one purpose. In essence, a filter module assembly incorporating a series of dichroic mirrors can be used for one specific purpose, such as analyzing the fluorescent tags normally used with DNA sequencing. Then, the entire filter module assembly can be removed from the instrument and replaced with a second filter module assembly having dichroic mirrors of different wavelength configurations. This would enable different fluorescent tags used for other purposes to be read by the same instrument. In essence, the purpose of changing the filter modules in an instrument of this kind is to change the frequencies which are analyzed by the instrument so as to make the instrument capable of detecting fluorescence activity at any defined wavelength bands and permitting switching from one set of tests to another simply by changing a filter module corresponding to the light wavelength bands which are sought to be detected. 
     The basic optical set up for the preferred style of instrument is illustrated in FIG.  5 . Light emanates from a light source  12 . The light source as illustrated in FIG. 10 would, in an actual instrument, refer to the light collected from the sample, such as the fluorescence light pattern originating from an electrophoresis gel assembly, which is then directed to the optics of FIG. 6 by other conventional optical elements. The incoming light is directed to a first dichroic mirror  14 . Light which is able to pass through the first dichroic mirror  14 , that is which contains within it spectrum of light signals corresponding to the transmission wavelength band of the dichroic mirror  14 , pass linearly through the dichroic mirror  14  and on to a photodetector  16 . Light which is reflected from the dichroic mirror  14  passes to a next dichroic mirror  18 . Any light incident upon the dichroic mirror  18  which fits within the wavelength band of light transmitted by the mirror is passed through the mirror to a photodetector  20 . The remaining light is reflected by the dichroic mirror  18  to a dichroic mirror  22 . Again, light which is incident upon the dichroic mirror  22 , and which fits within the wavelength band of light which is transmitted by the dichroic mirror  22 , passes on to a photodetector  24 . Light which is reflected by the dichroic mirror  22  passes through a bandpass filter  26  and on to a last photodetector  28 . 
     Thus, each dichroic mirror selectively transmits a defined wavelength region onto its respective photodetector and reflects the unused remaining wavelengths of light onto the next dichroic mirror or to the bandpass filter. The bandpass filter can be used for the last stage, instead of a dichroic mirror, since no further use is required for the rejected light. This allows simply detection of four separate color spectrums and the concept may quite easily be extended beyond four color detection by continuing the dichroic mirror cascade. Note that the first three dichroic mirrors are each angled at 45° relative to the incident light, and this geometry can be maintained indefinitely. This system does, however, require precise precisioning of the mirrors with respect to each other and with respect to the photodetectors. This is desirable because the transmission properties of the dichroic mirror are very sensitive to the angle of incidence of light incident on the mirror. In order for the dichroic mirrors to perform according to their design parameters the mirrors must be oriented at precisely 45° with respect to the incident light beam. Similarly, the optical properties of the bandpass filter are also dependent upon the angle of incidence of the fluorescent signal. As a result, the module was designed to minimize angular variation of the positioning of the filters and the module within the apparatus, by firmly fixing the dichroic mirrors in their physical relationship to each other. This is done by fixing the positions of the dichroic mirrors and the bandpass filter relative to each other in a sub-assembly. This sub-assembly can then be housed in a larger instrument in such a fashion that replacing the filter module does not alter the physical relationships between the mirrors. In other words, when it is time to swap out optical elements, the whole assembly is swapped as a fixed unit and the critical physical relationships between the mirrors are not altered. 
     This is accomplished by using a design as illustrated in FIG.  6 . In FIG. 6, a detection box  30  is a housing into which the replaceable filter module, illustrated in exploded fashion in the lower right portion FIG. 6, is inserted and removed to change filter modules. Note that the filter module is constructed on a filter manifold  32  onto which each of the dichroic mirrors are independently and fixedly mounted. The filter manifold is also illustrated in greater detail in FIGS. 7A,  7 B,  7 C and  7 D which shows the exterior views and a cross-sectional view of the filter manifold. The filter manifold  32  has a light passage  42  through its interior and four mounts for optical elements. The mounts  44 ,  46  and  48  are oriented at a 45° angle with respect to horizontal, and each is parallel to the others so that all the dichroic mirrors received in those mounts are parallel and aligned to transfer rejected light from each mirror to the next. The last optical element mount  50  is oriented at an angle of 45° to the other mounts, to receive the band pass filter element. The filter manifold  32  is bolted securely to side plates  34  which are in turn connected to a front plate  36 , which has a handle  38  mounted on it. A suitable recess is provided inside of the detection box  30  so that the side rails  34  can support the assembled filter module, composed of the filter manifold assembly  32  with the mounting side plates  34  and the front plate  36 , as it is slid in and out of the detection box  30 . The detection box  30  includes mounting locations for the photodetectors and suitable conventional optic (not shown) to direct the outputs from the dichroic mirrors to the photodetectors. It is an advantage of this design that the filter manifold includes a series of fixed mounts, each of which is precisely located and sized so as to receive there within one of the dichroic mirrors in a fixed and specific relationship with regard to other dichroic mirrors and the other optics of the instrument. Since the dichroic mirrors are fixed in position, the only alignment which has to be done is the alignment of the whole filter module with the detection box to ensure that all the optics are in proper alignment. 
     Note that the top edges of the side plates have cuts in their top edge to make contact with spring loaded bolters inside of the detection box  30  so that the filter module can be locked in place inside the unit. This holds the filter module securely inside the detection box in use and holds the filter manifold in a fixed relationship with regard to the other optical elements contained within the detection box  30 . 
     Removal of the filter module is accomplished simply by pulling on the handle  38 . Since the slope of the detent formed in the side panels is a little steeper when withdrawing than when inserting, more force is required to remove the module to insert it. By balancing parameters such as the plunger tension the slope of the detent on the side panels, a variety of holding pressures can be achieved until one that is comfortable and optimal can be obtained. 
     It can be readily seen that once the filter manifold is created and each of the mirrors is mounted on to it, the dichroic mirrors and filters are assured of proper alignment with respect to one another. This is superior to a design in which each of the filters is positioned independently inside the detection apparatus since placement of each mirror is subject to positional and angular error. By attaching the filters all to a common aligning manifold, in a manner in which the mirrors can fit into the manifold in only one location and at one orientation, only the alignment of the entire module with respect to the detection apparatus needs to be of any concern. 
     FIG. 8 illustrates the optics of an alternative embodiment of the invention in which bandpass filters are added to enhance the performance of the dichroic mirror. Since bandpass filters can be manufactured to have a very narrow band of transmission, the addition of bandpass filters to the dichroic filter module would allow more precise control over which wavelengths of light are passed onto each of the detectors. Alternatively, the dichroic mirrors could be replaced with beam splitters to separate the light signal prior to wavelength filtering. This design would require a slightly more complex part for the filter manifold, but would offer the same advantages of ease of changing optics and fixed alignments among parts. The bandpass filters are illustrated at  40  in FIG.  8 . Otherwise the reference numerals refer to the same elements as in the embodiment of FIG.  5 . 
     The embodiment of FIG. 9 illustrates a version where the dichroic mirrors have limited reflection bands and broad transmission bands. In this embodiment, the dichroic mirrors would reflect a limited range of wavelengths to a photodetector and pass the remaining wavelengths to subsequent mirrors. A disadvantage of this system is that the transmission of light through the dichroic mirrors is not efficient as light reflection. As a result, a significant loss of light intensity would incur if a cascade were very long. This embodiment does permit a relatively straightforward light path through the instrument however, as illustrated in FIG. 9, which would make the appropriate filter manifold relatively easy to make.