Spatial modulation of light to determine object length
Spatially modulated light emanating from an object moving along a flow path is used to determine various object characteristics including object length along the flow direction. Light emanating from at least one object moving along in a flow path along a flow direction of a spatial filter is sensed. The intensity of the sensed light is time modulated according to features of the spatial filter. A time varying electrical signal is generated which includes a plurality of pulses in response to the sensed light. Pulse widths of at least some of the pulses are measured at a fraction of a local extremum of the pulses. The length of the object along the flow direction is determined based on the measured pulse widths.
1. A system, comprising:
a spatial filter having a plurality of mask features;
at least one detector positioned to sense light emanating from at least one object moving in a flow path along a flow direction, an intensity of the sensed light being time modulated according to the mask features, the detector configured to generate a time varying electrical signal comprising a sequence of pulses in response to the sensed light; and
an analyzer configured to:
measure a pulse width of at least some of the pulses at a fraction of an amplitude extremum of the pulses and to determine a length of the object along the flow direction based on the measured pulse widths, and
predict an expected pulse width measurement of a hypothesized mask feature of length x based on the measured pulse width.
2. The system of claim 1 , wherein:
the mask features include first features and second features; and
the first features are substantially transparent and the second features are substantially opaque.
3. The system of claim 1 , wherein the analyzer is configured to:
determine a function, f({p i },x) that fits a set of mask feature measurement points {p i }, each point given by p i =(x i ,y i ), where each x i is associated with a feature length of an i th mask feature and each y i is associated with an i th measured pulse width associated with the i th mask feature, the function predicting an expected pulse width measurement of a hypothesized mask feature of length x based on the set of mask feature measurement points {p i }.
4. The system of claim 3 , wherein the function is a linear function.
5. The system of claim 4 , wherein:
the mask features include first features and second features;
the first mask features are substantially optically transparent and the second mask features are substantially optically opaque; and
extrapolating the object length using the function comprises determining f({p i }, for x=0).
6. The system of claim 5 , wherein the mask features are arranged so that a length of the mask features changes linearly along the flow direction.
7. The system of claim 5 , wherein the mask features are arranged so that a length of the mask features changes non-linearly along the flow direction.
8. The system of claim 3 , wherein the function is an exponential or logarithmic function.
9. The system of claim 8 , wherein the mask features are arranged so that the length of the mask features changes logarithmically along the flow direction.
10. The system of claim 1 , wherein a frequency of the mask features is constant along the flow direction and corresponds to a fixed pitch value.
11. The system of claim 1 , wherein the fraction of the pulse extremum is in a range of about 10% to 40% or about 60% to 90%.
12. The system of claim 1 , wherein:
the mask features include a first section arranged in a first linear chirp or logarithmic chirp pattern and a second section arranged in a second linear or logarithmic chirp pattern, wherein the first pattern and the second pattern are symmetrical around a center line extending laterally across the spatial filter;
the first mask features are substantially transparent and the second mask features are substantially opaque and the transparent features have a first specified length proximate to the center line; and
the transparent features of the first pattern have a linear decrease in length of a second specified value along the flow direction and the transparent features of the second pattern have a linear increase in length of the second specified value along the flow direction.
13. The system of claim 1 , further comprising a light source configured to provide input light, wherein the light emanating from the objects is responsive to the input light.
14. The system of claim 1 wherein the intensity distribution of the input light is approximately Gaussian or Lambertian.
15. The system of claim 1 , wherein:
the mask features include first features and second features, a length of the first features is constant along the flow direction of the spatial filter and a length of the second features is constant along the flow direction of the spatial filter;
the analyzer is configured to:
measure pulse widths of positive going pulses;
measure pulse widths of negative going pulses; and
determine the length of the object based on averages of widths of pulse pairs, each pulse pair comprising a positive pulse and an adjacent negative pulse.
16. The system of claim 15 , wherein the analyzer is further configured to determine whether a velocity of the object is increasing or decreasing based on a slope of the averages of the pulse width pairs.
17. The system of claim 1 , wherein:
a length of the first features is constant along the flow direction of the spatial filter;
a length of the second features is constant along the flow direction of the spatial filter;
the at least one object comprises multiple objects;
the detector is positioned to sense light emanating from the multiple objects moving in a flow path along the flow direction;
the analyzer is configured to:
measure pulse widths of positive going pulses;
measure pulse widths of negative going pulses; and
identify a number of the multiple objects traveling together along the flow path based on the pulse widths of the positive pulses and the negative pulses.
18. The system of claim 17 , wherein the analyzer is configured to determine a distance between two or more objects traveling together based on the pulse widths of the positive pulses and the negative pulses.
19. The system of claim 1 , wherein the analyzer is configured to determine an instantaneous velocity of the object as the object travels along the flow path relative to the mask features based on the measured pulse widths.
20. The system of claim 1 , wherein the analyzer is configured to determine, on a mask feature-by-feature basis, whether the object is speeding up or slowing down as the object travels along the flow path based on the measured pulse widths.
21. A method, comprising:
sensing light emanating from at least one object moving in a flow path along a flow direction of a spatial filter, the spatial filter having a plurality of mask features comprising first features alternating with second features along the flow direction, the first features having first light-transmission characteristics and the second features having second light transmission characteristics, different from the first light transmission characteristics, an intensity of the sensed light being time modulated according to the mask features;
generating a time varying electrical signal comprising a plurality of time modulated pulses in response to the sensed light;
measuring pulse widths of the pulses at a fraction of a maximum extremum of the pulses;
determining a length of the object along the flow direction based on the measured pulse widths; and
predicting an expected pulse width measurement of a hypothesized mask feature of length x based on the measured pulse width.
22. The method of claim 21 , wherein determining the length of the object comprises:
determining a function, f({p i },x) that fits the set of mask feature measurement points {p i }, each point given by p i =(x i ,y i ), where each x i is associated with a feature length of an i th mask feature and y i is associated with an i th measured pulse width associated with the i th mask feature; the function predicting an expected pulse width measurement of an hypothesized mask feature of length x based on the mask feature measurement points {p i }; and
extrapolating the object length using the function.
23. The method of claim 22 , further comprising removing outliers in measurement points {p i } before determining the function, the outliers corresponding to erroneous measurements.
24. The method of claim 22 , wherein extrapolating the object length using the function comprises determining f({p i }, for x=0).
25. The method of claim 21 , wherein:
a length of the first features is constant along the flow direction of the spatial filter;
a length of the second features is constant along the flow direction of the spatial filter;
measuring the pulse width of each of the pulses comprises:
measuring pulse widths of positive pulses;
measuring pulse widths of negative pulses; and
determining the length of the object along the flow direction comprises determining the length of the object based on averages of widths of pulses in pulse pairs, each pulse pair comprising a positive pulse and an adjacent negative pulse.
26. The method of claim 21 , wherein:
the at least one object comprises multiple objects;
sensing the emanating light comprise sensing light emanating from the multiple objects;
measuring the pulse widths comprises:
measuring pulse widths of positive going pulses;
measuring pulse widths of negative going pulses; and
identifying a number of the multiple objects traveling together along the flow path based on the pulse widths of the positive going pulses and the negative going pulses.
27. The method of claim 26 , further comprising determining a distance along the flow direction between two or more objects traveling at a velocity based on the pulse widths of the positive pulses and the negative pulses.
28. The method of claim 21 , further comprising determining an instantaneous velocity of the object as the mask moves along the flow path past the mask features based on the measured pulse widths.
29. A system, comprising:
a spatial filter having a plurality of mask features;
at least one detector positioned to sense light emanating from at least one object moving in a flow path along a flow direction, an intensity of the sensed light being time modulated according to the mask features, the detector configured to generate a time varying electrical signal comprising a sequence of pulses in response to the sensed light; and
an analyzer configured to:
measure a pulse width of at least some of the pulses at in a range of about 10% to 40% or 60% to 90% of an amplitude extremum of the pulses,
predict an expected pulse width measurement of a hypothesized mask feature of length x based on the measured pulse width, and
determine, based on the pulse widths, one or more of:
a length along the flow direction of the object;
an instantaneous velocity of the object;
whether the object is accelerating or decelerating.