Self-scanned photodiode array with selectively-skipped pixels

Abstract

A self scanning photodiode array allows low signal pixels to accumulate charge for multiples of predetermined exposure time t 0 before being read. The pattern of exposures, the integers M i where I runs from 1 to N the number of pixels in the array, is chosen such that the pixels of interest accumulate as much charge as possible without exceeding saturation.

What is claimed is: 
     
       1. A device having a self-scanned photodiode array wherein charge from individual pixels is switched from each pixel sequentially onto at least one output video line after a predetermined exposure time (t 0 ) comprising:
 at least one light source;   at least one sample cell having means for receiving light from said at least one light source;   a photo-diode array having pixels for collecting light transmitted through said at least one sample cell, wherein the intensity of said light may vary across the spatial extent of the array;   means for reading said pixels;   means for skipping the reading of selected pixels for one or more additional exposure times t 0  allowing said selected pixels to be exposed for specified integer multiples (M) of said predetermined exposure time t 0 , thereby allowing said selected pixels receiving less light to accumulate additional charge before being read out and thereby reducing the number of read cycles and improving the signal-to-noise ratio.   
     
     
       2. The self-scanned photodiode array of  claim 1  wherein different pixels are exposed for the same or different integer multiples (M) of said predetermined exposure time. 
     
     
       3. The self-scanned photodiode array of  claim 1  in which said predetermined exposure time t 0  does not exceed saturation of the pixel or pixels accumulating charge at a highest rate within a predetermined range of pixels. 
     
     
       4. The self-scanned photodiode array of  claim 1  wherein specified integer multiples M are chosen such that each pixel signal, within a predetermined range of pixels approaches but does not exceed saturation. 
     
     
       5. The self-scanned photodiode array of  claim 1  wherein said exposure time of individual pixels, Mt 0 , does not cause saturation of pixels from which charge is measured. 
     
     
       6. The self-scanned photodiode array of  claim 1  wherein said exposure time of each pixel, is an integer multiple M of said predetermined exposure time t 0 , wherein the lowest value of M is greater than one. 
     
     
       7. The self-scanned photodiode array of  claim 1  wherein a sample time is defined as a time taken for one or more complete measurements of the full or selected portion of the photodiode array, said individual pixel exposure times Mt 0  being submultiples of said sample time. 
     
     
       8. The self-scanned photodiode array of  claim 1  wherein said predetermined exposure time t 0  is established when said photo-diode array receives said light according to a reference condition. 
     
     
       9. The self-scanned photodiode array of  claim 3  wherein the predetermined range of pixels includes the full array. 
     
     
       10. The self-scanned photodiode array of  claim 4  wherein the predetermined range of pixels includes the full array. 
     
     
       11. The self-scanned photodiode array of  claim 1  wherein the value recorded when a pixel read is skipped is set to zero to avoid the addition of unnecessary read noise. 
     
     
       12. A method of improving signal to noise ratio of measurements made using a self-scanned photodiode array to detect light in the ultraviolet, visible and infrared portions of a light spectrum comprising the steps of:
 exposing pixels of said photodiode array to light received from a reference condition and measuring the spectrum of a short exposure time causing no saturation of said pixels; 
 measuring a dark spectrum of said pixels with shutter closed using said short exposure time; 
 calculating a dark corrected reference signal from each pixel; 
 calculating an exposure time for each pixel such that its accumulated charge would reach a predetermined level, close to but below saturation; 
 establishing a predetermined exposure time t 0 , short enough that no pixel in a specified range of interest will saturate; 
 assigning integers M such that individual pixels are exposed for integer multiples of the predetermined exposure time Mt 0 , such that after time Mt 0  said individual pixels have accumulated charge close to but not exceeding saturation; 
 re-measuring dark spectrum with shutter closed using exposure pattern determined by integers; 
 re-measuring reference spectrum using exposure pattern determined by integers, thereby creating a dark-corrected reference spectrum; 
 Measuring sample spectra using the same exposure pattern determined by integers and creating dark corrected sample spectra; 
 combining reference and sample spectra to determine absorption characteristics of sample and thereby identify and quantitate same with improved signal-to-noise ratio. 
 
     
     
       13. A self-scanned photodiode array wherein charge from individual pixels is switched from each pixel sequentially onto at least one output video line after a predetermined exposure time (t 0 ) comprising:
 an array of photodiode pixels for collecting light, wherein the intensity of said light may vary across the spatial extent of the array;   means for reading said pixels;   means for skipping the reading of selected pixels for one or more additional exposure times t 0  allowing said selected pixels to be exposed for specified integer multiples (M) of said predetermined exposure time t 0 , thereby allowing said selected pixels receiving less light to accumulate additional charge before being read out and thereby reducing the number of read cycles and improving the signal-to-noise ratio of the measured light.   
     
     
       14. A method of improving signal-to-noise ratio of measurements made using a photo-diode array including a plurality of pixels, the method comprising:
 exposing, for a first reference exposure time, the plurality of pixels of the photo-diode array to light received from a reference light source;   measuring a first reference spectrum of the light received by the plurality of pixels during the first reference exposure time, the length of the first reference exposure time selected such that no saturation occurs in the plurality of pixels;   closing a shutter of the photo-diode array for a second reference exposure time equal in length to the first reference exposure time;   measuring a first dark spectrum of the plurality of pixels during the second reference exposure time;   calculating, using the first reference spectrum and the first dark spectrum, a first dark-corrected reference signal from each pixel from the plurality of pixels;   calculating an exposure time for each pixel from the plurality of pixels such that accumulated charge would reach a predetermined level below saturation;   determining a unit exposure time t 0  that is less than each of the exposure times associated with the plurality of pixels;   assigning integer multiples of M to each pixel from the plurality of pixels such that, for each pixel, the time equal to M integer multiples of the unit exposure time t 0  is less than the exposure time for the particular pixel; and   measuring a second dark spectrum for each pixel from the plurality of pixels with the shutter closed, wherein the second dark spectrum for each pixel is measured for a time period equal to the unit exposure time t 0  times the assigned integar M assigned to the pixel being measured.   
     
     
       15. The method of claim 14 further comprising:
 measuring a second reference spectrum of the light received by the plurality of pixels from the reference light source, wherein the second reference spectrum for each pixel is measured for an exposure time period equal to the unit exposure time t 0  times the assigned integer M assigned to the pixel being measured;   calculating, using the second reference spectrum and the second dark spectrum, a second dark-corrected reference signal from each pixel from the plurality of pixels;   exposing the plurality of pixels of the photo-diode array to light received from a sample source;   measuring a sample spectrum of the light received by the plurality of pixels from the sample source, wherein the sample spectrum for each pixel is measured for an exposure time period equal to the unit exposure time t 0  times the assigned integer M assigned to the pixel being measured; and   determining absorption characteristics of the sample source using the sample spectrum and the second dark-corrected reference signal.