Abstract:
A self-contained automatic computer controlled system for checking and recalibrating the wavelength scale of both monochromators of a fluorescence spectrophotometer is described. No operator adjustments are required. The system sets the wavelength of each monochromator to within 0.1 nanometers. Besides automatic indexing at turn-on the operation can be repeated at any desired time. The system is applicable to single or double monochromator instruments.

Description:
FIELD OF INVENTION 
     This invention relates to spectrophotometers and, more particularly, to automatic wavelength calibration systems for spectrophotometers. 
     BACKGROUND OF THE INVENTION 
     Spectrophotometers having a monochromator to provide light of a known wavelength to the sample are usually precalibrated by mechanical adjustments made during manufacture. However, in precision analytical or research work it is desirable to be able to recheck and, if necessary, to readjust the wavelength scale to correct for mechanical or temperature induced changes. Where two monochromators are involved in a single instrument, such as in a fluorescence spectrophotometer for example, this could be a very time consuming adjustment requiring changes of the light source and the insertion of special samples, filters, etc. for transfer of light from the excitation monochromator to the emission monochromator. 
     It is an objective of the present invention to provide a build-in wavelength standard with means to couple it to the monochromator for calibration check. 
     It is a further objective to provide microprocessor controlled means for effecting said wavelength check. 
     It is a yet further objective to establish by software control an accurate readjustment or indexing of the wavelength. 
     It is an objective for a fluorescence spectrophotometer to follow up wavelength indexing on one monochromator with software executed indexing of the second monochromator. 
     It is an accompanying objective to obviate the necessity of operator attachment or removal of any part or appurtenance of the instrument during the recalibration process. 
     Yet a further objective is to extend said software controlled indexing of wavelength to other reference wavelengths and to double monochromators, where used. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In higher performance fluorescence spectrophotometry, the wavelength accuracy and wavelength precision is considered one of the important parameters of the instrument. On the present instruments, wavelength calibration is performed by using emission lines of a known source or absorption bands of a known standard. This type of calibration is usually performed in the factory or by a skilled person and the instrument is expected to maintain that calibration for some time. But, due to mechanical instability or other unavoidable factors, that calibration may change in time, and periodic recalibration may be required. 
     In the described embodiment, the wavelength calibration is performed when the instrument is turned-on or it can be checked and recalibrated within 0.1 nm whenever the instrument is on. Also, in this system, no operator inserted calibration standard or reflector is required in a sample position, and the user can leave the sample in the cuvette or flow cell in the sample position. To achieve the wavelength calibration automatically and accurately, an inbuilt mercury lamp source is used and calibration is performed at an emission line of this source. 
     When calibrating, a software program first sets the emission monochromator by directing light from the mercury source into its entrance slit. Then, by monitoring its output the computer locates the wavelength setting corresponding to the peak of the reference mercury line. The value of this setting is stored in memory as index for further settings. 
     The excitation monochromator is then calibrated by directing light from its exit slit into the emission monochromator. The software then directs the computer to find the wavelength setting where the excitation monochromator agrees with the emission monochromator and to store that setting in memory as index for future wavelength settings. 
     There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention which will be described more fully hereinafter. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as the basis for the designing of other systems for carrying out the several purposes of the invention. It is important, therefore, that this disclosure be regarded as including such equivalent systems as do not depart from the spirit and scope of the invention. 
     Several embodiments of the invention have been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows a simplified block-like diagram of the mechanical parts of the calibrating system, according to the invention. 
    
    
     DETAILED DESCRIPTION 
     In order to provide a fully detailed description of the presently preferred embodiment, the automatic wavelength calibration system of the invention will be described as part of a fluorescence spectrophotometer having an excitation monochromator illuminated by a conventional Xenon arc source and an emission monochromator whose light source is the fluorescence of a cuvette-contained sample excited by the exit beam of the excitation monochromator. Such an arrangement is well known in the art and need not be described in detail except for aspects pertinent to the present invention. Each of these two monochromators is provided with a wavelength drive comprising a stepper motor which receives rotational commonds from a microcomputer in a Central Processor Unite (CPU) of the instrument. The number of steps counted into this motor starting from an initial count stored in computer memory determines the wavelength to which the monochromator is set. Details of the actuating software for such stepper motor wavelength drives are now well known in the filed and need not be described herein. Each of these monochromators is also provided with entrance and exit slits which are positioned with respect to width by further stepper motor drives similarly controlled. Each of these monochromators also has adjacent to one of its slits a rotatable filter assembly driven by a stepper motor similarly controlled. These will be further described hereinafter. 
     Referring to the drawing the entrance slit 10 of an emission monochromator 11 of the fluorescence spectrophotometer illustrated has mounted adjacent thereto a rotatable disc or wheel 13 having apertures disposed therein at intervals which may be covered with optical filters 15 for stray light control or light attenuation in the usual manner. This filter wheel is rotated to any of several desired positions by a stepper motor 17, as directed by the CPU 18. On the wheel 13 is attached a reflector 19, which may be so positioned that it directs light from a fiber optics cable or light pipe 20 into the entrance slit 10 of the emission monochromator 11. 
     At the other end of the fiber optics light pipe 20 is mounted a small low pressure mercury lamp 21 contained in a suitable housing 22. This lamp is the reference source for wavelength calibration since it emits light having only a few known specific wavelengths. It is preferred to utilize the 435.84 mercury emission line, but it will be recognized that another line of the available spectrum could alternatively be used. Lines from the Xenon arc spectrum are preferably not used for calibration because they are too broad to give an accurate wavelength reference. Any of several well known small mercury lights such as the Penlite lamp is suitable. 
     If the calibration procedure being described were to be used to calibrate an absorption spectrophotometer with one monochromator the above described attachments would suffice together with the software routine to be presently described. For a fluorescence spectrophotometer with two monochromators a second similar filter wheel 24 with stepper motor drive 26 is mounted adjacent to the exit slit 28 of the excitation monochromator 29. This second filter wheel has a similar reflector 38 which may be so positioned by the software that it accepts light from the exit slit 28 and directs the light to a second light pipe 30 through which it is conveyed to reflector 19 and directed into the entrance slit 10 of the emission monochromator 11. These added elements make it possible for the wavelength of the excitation monochromator 29 to be calibrated referring to the previously calibrated emission monochromator 11 without the complication of special attachments in the sample carrier or manipulation by the operator. 
     When the instrument is first turned on, the microcomputer 18 under software control first coarse indexes the excitation and emission monochromators 29 and 11 by driving their wavelength stepper motors to location switch points of the mechanical-optical interrupter position sensors attached to their wavelength drives. This sets the monochromator wavelengths of both monochromators to within about 20 nanometers (nm) of the mercury reference line. The reference mercury lamp is then turned on, the Xenon source shutter is closed, and the emission filter wheel is rotated to the index position. Here, light piped from the mercury lamp 21 via the fiber optics cable 20 is reflected by the small reflector 19 mounted on the emission filter wheel 13 into the entrance slit 10 of the emission monochromator. The entrance slit is widely opened by the computer at this time. 
     The software routine then causes the microcomputer 18 to scan to the known reference line peak within ±10 nm. Data controlling this scan is provided by the photometric system from the output of the emission monochromator 11. The CPU locates the peak of the line by performing a series of scans. With each successive scan the emission slit widths are reduced to increase the accuracy in locating the reference line. In the final scan the emission monochromator 11 is indexed, i.e. set at that line to an accuracy of better than 0.1 nanometers. 
     After indexing the emission monochromator 11, the emission monochromator is used to index the excitation monochromator 29. The mercury lamp is turned off and the Xenon lamp shutter opened. The excitation filter wheel 24 located at the exit slit 28 of the excitation monochromator 29 is rotated to its index position. Here the reflector 38 located on the filter wheel 24 gathers light from the opened exit slit 28 of the excitation monochromator 29 and pipes it by another fiber optics cable 30 to the reopended entrance slit 10 of the emission monochromator 11. The microcomputer 18 then scans the excitation wavelength to the calibrated wavelength of the emission monochromator 11 and performs a series of scans of the excitation monochromator 29 to locate the point at which its wavelength equals that of the emission monochromator. This point is reached when the light passing through both monochromators reaches a peak of intensity. Both emission and excitation slits 10 and 28 are set at about 1 nanometers band pass or maybe reduced with successive scans to increase the positional accuracy. In the final scan the excitation monochromator 29 is calibrated and indexed to a precision of 0.1 nanometers. 
     It will be apparent that both monochromators could be indexed from the mercury source in the same manner, if desired, without departing from the teaching of the invention but at some increase in complexity. Our invention as described herein reduces the calibration complication by the simpler method of using the monochromator already calibrated to calibrate the remaining monochromator. 
     It will also be apparent that the scope of this invention may be expanded by the addition of suitable similar hardware, viz. reflectors, filter wheels and light pipes plus additional software steps to extend the invention to the calibration of instruments having double excitation and/or emission monochromators. While the automatic calibration herein set forth has been described as happening when the instrument is first turned on, it can also be repeated at any time at the option of the operator by touching a command button or it can be repeated at prescribed intervals or times. 
     Although certain particular embodiments of the invention are herein disclosed for purposes of explanation, various modifications thereof, after study of this specification, will be apparent to those skilled in the art to which the invention pertains, and reference should accordingly be had to the appended claims in determing the scope of the invention.