JP6322952B2 - Wavefront measuring apparatus and wavefront measuring method - Google Patents

Description

  The present invention relates to a Shack-Hartmann wavefront measuring apparatus and a wavefront measuring method.

The Shack-Hartmann wavefront measuring apparatus generates an image in which a large number of focused spots are arranged by making a light wave incident on a lenslet array in which a large number of small lenses (lenslets) are arranged in a lattice pattern. Next, the Shack-Hartmann wavefront measuring apparatus converts the image on which the focused spots are arranged into an image signal, and calculates the uneven distribution of the wavefront from the imaging pattern. When calculating, the Shack-Hartmann wavefront measuring device detects the coordinates of each of the many imaging points with an accuracy that is less than the pixel size, and the coordinates of which condensing spots are caused by which lenslets. The correspondence is done. When the Shack-Hartmann wavefront measuring apparatus performs the association, a method having a processing table that associates a specific lenslet with a specific area on an image is generally used. However, in this method, one-to-one correspondence is made. For this reason, an arbitrary lenslet should not protrude a condensing spot from a specific region corresponding to itself. As a result, this method limits the dynamic range of wavefront distortion that can be measured.
For example, Patent Document 1 discloses a method for improving the dynamic range (ratio of wavefront distortion height to resolution) by providing two sets of lenslet arrays with different magnifications. .

JP 2004-113405 A

  In the conventional Shack-Hartmann wavefront measuring apparatus and wavefront measuring method, the dynamic range is expanded so as to cope with larger distortion of the wavefront. However, in the conventional Shack-Hartmann wavefront measuring apparatus, the greater the distortion of the wavefront that can be handled, the greater the difference in brightness between the captured light spots. For this reason, the conventional Shack-Hartmann wavefront measuring apparatus is susceptible to noise, and there is a problem that the calculation accuracy of the wavefront deteriorates.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a wavefront measuring apparatus and a wavefront measuring method that enable highly accurate wavefront measurement.

The wavefront measuring apparatus of the present invention is
A lenslet array in which a plurality of lenses are arranged to collect light, an exposure time control means for controlling the exposure time of the light, an imaging means for converting the light collected by the lenslet array into an image, and imaging Signal processing means for outputting the wavefront shape of light based on the image converted by the means,
The imaging means includes
Based on the exposure time controlled by the exposure time control means, the condensed light is converted into a continuous image,
From the converted image, a partial image is generated by extracting pixels belonging to a focused spot whose peak exceeds a threshold and does not exceed the upper limit of gradation,
A label is given to the partial image based on the connectivity of the pixels,
For the set of pixels having the same label, find a barycentric coordinate weighted with luminance,
Define a partial area of a predetermined size and shape around the barycentric coordinates,
By extracting pixels having a luminance exceeding a threshold smaller than the threshold for pixels in the partial area, pixels whose signal-to-noise ratio does not satisfy a predetermined condition are removed,
In a plurality of frames of the partial image, if the condensing spots overlap, leaving the one with the higher signal-to-noise ratio,
Combining the remaining partial images into one image,
The signal processing means includes
Using said combined image, wherein the imaging means and outputs a wavefront shape of the light receiving light.

  According to the present invention, highly accurate wavefront measurement can be performed.

The block diagram of the wavefront measuring apparatus of this invention. The figure which shows typically the condensing spot pattern which is an image signal in case the distortion of the wave front of light is small. The figure which shows typically the condensing spot pattern which is a screen signal in case the distortion of the wave front of light is large. The line profile figure which shows intensity distribution on the dotted line shown by A and A 'of FIG. Line profile of the focused spot image acquired in a shorter time than the set exposure time Line profile of the focused spot image acquired with the set exposure time Line profile of the focused spot image acquired over a longer time than the set exposure time The line profile of the condensing spot image which synthesize | combined the partial image cut out from FIG.5, FIG.6, FIG.7. 1 is a block diagram of a wavefront measuring apparatus according to Embodiment 1 of the present invention. The flowchart which shows the wavefront measuring method in Embodiment 1 of this invention. The block diagram of the wavefront measuring apparatus in Embodiment 2 of this invention. The flowchart which shows the wavefront measuring method in Embodiment 2 of this invention. The figure explaining the algorithm which extracts the condensing spot image in Embodiment 3 of this invention. The flowchart which shows the wavefront measuring method in Embodiment 3 of this invention.

Embodiment 1
FIG. 1 is a block diagram showing the configuration of a wavefront measuring apparatus according to the present invention. In FIG. 1, 1 is a wavefront of light to be measured, 2 is a wavefront detection unit of a wavefront measurement device, 3 is a lenslet array constituting the wavefront detection unit, 4 is an imaging means constituting the wavefront measurement device, and 5 is wavefront measurement. A signal processing unit of the apparatus, 6 is an image signal output from the wavefront detection unit 2 to the signal processing unit 5, 5A is a frame grabber that takes the image signal 6 into the signal processing unit 5, and 7 is an imaging unit from the signal processing unit 5. 4 is a control signal output to 4. The wavefront detection unit 2 outputs an image signal 6 in which a large number of focused spots are arranged from the wavefront 1 of light based on the same principle as a conventional Shack-Hartmann sensor. The signal processing unit 5 is a computer on which an operating system such as Windows (registered trademark) or LINUX (registered trademark) operates, and a frame grabber 5A that is an interface for storing an image signal in a storage (hard disk or the like) of the computer. It is equipped with.

  FIG. 2 is a diagram schematically showing a condensing spot pattern 6B which is an image signal 6 when the distortion of the wavefront 1 of light is small. Since the distortion of the wavefront 1 of light is small, the brightness of each focused spot is uniform. On the other hand, when the unevenness difference of the wavefront distortion is large, the variation in brightness becomes large at each focused spot. FIG. 3 is a diagram schematically showing a condensing spot pattern 6A that is an image signal 6 when the distortion of the wavefront 1 of light is large.

Hereinafter, in order to make the description easy to understand, description will be made using the intensity distribution of one line of light on a two-dimensional image. The light intensity distribution refers to the light intensity distribution input to the wavefront measuring apparatus. FIG. 4 is a line profile showing the intensity distribution on the dotted line indicated by A and A ′ in FIG. The vertical axis represents intensity, and the horizontal axis represents position. Reference numerals 10 and 11 denote lines indicating the upper and lower limits of the gradation of the image signal 6A. Reference numeral 12 denotes a line profile of the image signal 6A. In the line profile 12, a plurality of peaks stands at a place where the condensed spot exists. The intensity profile does not become zero due to noise generated in the electric circuit of the image pickup means even at a place other than the focused spot. This noise is also included in the peak of the focused spot. Therefore, when the peak of the focused spot is below a certain level, it is affected by noise, and the SN ratio (Signal-Noise Ratio, hereinafter referred to as SN ratio) deteriorates, making it impossible to measure with a predetermined accuracy. Reference numeral 13 denotes a threshold value indicating the lower limit value of the peak that guarantees the accuracy of the measured value of the wavefront depending on the SN ratio. That is, 13 indicates a predetermined value with which the signal processing unit 5 can guarantee measurement accuracy.
The threshold 13 is set according to the accuracy required for the system, and is set to, for example, about 20 times the SN ratio. In the example of FIG. 4, there are only two condensing spots whose peaks exceed the threshold 13 among the nine condensing spots, and a state where a desired measurement accuracy is not obtained is shown.

  FIG. 9 is a detailed block diagram for explaining the configuration of the wavefront measuring apparatus in the present embodiment shown in FIG. 1 in more detail. In FIG. 9, the wavefront detection unit 2, the lenslet array 3, the signal processing unit 5, the image signal 6, and the control signal 7 are the same as those in FIG. In FIG. 9, reference numeral 40 denotes an exposure time variable means, 50 denotes a plural data storage / save unit, 51 denotes an image cutout unit, 52 denotes an image composition unit, and 53 denotes a wavefront calculation unit.

In the first embodiment of the present invention, the imaging unit 4 uses a two-dimensional imaging device such as a CCD camera (Charge-Coupled Device Camera) or a CMOS image sensor (CMOS Image Sensor). Such an imaging unit 4 can acquire a plurality of images with different brightness by changing the exposure time of imaging.
The signal processing unit 5 is programmed in advance to automatically detect that the captured image is an image whose measurement accuracy cannot be guaranteed as shown in FIGS.
For example, when there is even one condensed spot whose peak does not exceed the threshold 13 shown in FIGS. 4 to 7, or when there is even one condensed spot whose peak exceeds the upper limit 11 of the gradation, It may be determined that the image cannot be guaranteed.
In such a case, the signal processing unit 5 acquires a plurality of frames of images from the imaging unit 4 while transmitting the control signal 7 to the imaging unit 4 so as to continuously increase the exposure time.

Here, a case where a total of three images are acquired for each exposure time value by using three different exposure times selected by the program based on the image shown in FIG. 4 will be described.
5, FIG. 6, and FIG. 7 show the line profiles of the focused spot images acquired at different exposure times and continuously in time. 5, 11 and 13 in FIG. 5, FIG. 6, and FIG. 7 are the same as FIG. 4, 12A in FIG. 5, 12B in FIG. 6, and 12C in FIG. 7 are different exposure times (hereinafter referred to as M0, M1, M2). This is a line profile of a focused spot image picked up by (1).

  As a method for setting the exposure times M0, M1, and M2, for example, the range of the exposure time that can be set by the imaging means 4 is divided into 10 stages, and measurement is performed at each exposure time. The exposure time when the peak falls between the threshold 13 and the upper limit of gradation 11 is M0, and the exposure time when the peak of the highest peak of the condensed spot falls between the threshold 13 and the upper limit of gradation 11 is M2. , M1 may be an intermediate value between M0 and M2. Further, during the measurement, when a condensed spot is generated whose peak does not exceed the threshold 13 even when M0 is used for the exposure time, or conversely, even if M2 is used for the exposure time, the peak exceeds the upper limit 11 of the gradation. When a condensing spot appears, the value of M0 or M2 may be automatically increased or decreased to cope with real time.

FIG. 10 is a flowchart showing the operation and flow of the apparatus for acquiring a plurality of image signals with different exposure times shown in FIGS. 5, 6, and 7 and outputting a wavefront measurement result based on these image signals. is there. For example, assume that the exposure time M0 is used in FIG. 5, the exposure time M1 is used in FIG. 6, and the exposure time M2 is used in FIG.
(Step 1)
The multiple data storage / save unit 50 sends a control signal 7 for setting the exposure time to the exposure time variable means 40. The exposure time varying means 40 that has received the control signal 7 sets the exposure time of the imaging means 4 (1300).
The imaging unit 4 outputs a captured image using the set exposure time. The multiple data storage / save unit 50 receives the image signal 6 sent from the imaging means 4 and stores it in the storage (1310).
(Step 2)
Next, the image cutout unit 51 extracts, from the image signal 6 stored in the storage, pixels that belong to the condensed spot whose peak exceeds the threshold 13 and does not exceed the upper limit 11 of the gradation. Thereafter, the image cutout unit 51 cuts out the extracted focused spot as a partial image and stores it in the storage (1320). In the examples of FIGS. 5, 6, and 7, the portions indicated by the thick lines 12 </ b> A, 12 </ b> B, and 12 </ b> C correspond to the extracted focused spots (partial images).
The multiple data storage / saving unit 50 checks whether the processing has been completed for all exposure times (in this example, all of M0, M1, and M2) (1330).
If not completed, the processes from the setting of the exposure time (1300) to the generation and storage (1320) of the partial image are repeated.
(Step 3)
If completed, the image composition unit 52 then composes the partial images stored in the storage (1340). The portions indicated by thick lines 12A, 12B, and 12C in FIGS. 5, 6, and 7 are combined as shown in FIG.
(Step 4)
Next, the wavefront calculation unit 53 performs a wavefront calculation output on the image synthesized by the image synthesis unit 52 by performing signal processing similar to that of the conventional wavefront measurement device.

  If overlapping occurs when the partial images are overlapped, a pixel that can perform the most accurate calculation in the subsequent wavefront calculation unit 53 is selected. For example, the luminance of overlapping pixels may be compared and the highest luminance may be selected.

Since the wavefront measuring apparatus and the wavefront measuring method according to Embodiment 1 of the present invention are configured as described above, the following effects can be obtained.
Even in a state where only one image having a brightness value greatly different for each condensing spot can be obtained in one image, the use of a plurality of images can reduce variation in the brightness value for each condensing spot, and thus high accuracy. Wavefront measurement is possible.

Embodiment 2
In the first embodiment, an image of a plurality of frames is acquired by changing the exposure time, but other means may be used as long as the brightness of the acquired image can be varied. For example, when the light to be measured is laser light, the laser light may be varied. A variable attenuator that attenuates transmitted light may also be used.
A block diagram of the wavefront measuring apparatus in the present embodiment is shown in FIG. FIG. 11 has a light intensity control device 41 instead of the exposure time varying means 40 of FIG.
FIG. 12 is a flowchart showing a signal processing flow for outputting the wavefront measurement result in the present embodiment.
FIG. 12 is a flowchart of FIG. 10, in which the processing of 1300 for setting the exposure time is replaced with the processing for setting the exposure time 1301, and the processing for 1330 for determining the end of the entire exposure time pattern is replaced with 1331. Is.

Embodiment 3
In Embodiment 1 of the present invention, pixels exceeding an arbitrary threshold 13 are extracted from a plurality of images with different exposures and combined into one image. However, when the peak of the focused spot substantially coincides with the threshold 13, there is a problem that the number of pixels constituting the focused spot is small and the calculation accuracy deteriorates. The third embodiment of the present invention improves this problem.

  FIG. 13 is an explanatory diagram for explaining an algorithm for extracting a focused spot image according to Embodiment 3 of the present invention. In FIG. 13, 30 is a binary image in which a part of the image signal 6A is enlarged and a portion exceeding the threshold 13 is displayed in the same color, 32 is a pixel exceeding the threshold 13 of the binary image 30, and 31 is a threshold 13 or less. However, it is a pixel necessary for obtaining a predetermined accuracy in signal processing.

  Next, signal processing contents will be described. FIG. 14 is a flowchart showing the operation of the apparatus for outputting a wavefront measurement result from the plurality of image signals shown in FIGS. 5, 6, and 7 and the flow of signal processing. In step 1, the plurality of image signals shown in FIGS. 5, 6, and 7 are stored in the storage of the signal processing unit 5. In step 2A, a label is assigned to the extracted partial image based on pixel connectivity. Here, “pixel connectivity” means that when adjacent pixels both exceed the threshold 13, they are determined to be pixels that form the same condensing spot. When n focused spots are detected as a result of the processing in step 2A, pixels with n labels are extracted. Here, it is assumed that this label is Sk (k = 1 to n). Next, in step 2B, the barycentric calculation is performed on the coordinates with the luminance weight applied to the pixels to which the label Sk is assigned, and the barycentric coordinates are obtained. A pixel label Sk is assigned to the barycentric coordinates and stored. Next, in step 2C, a partial area having a predetermined size and shape is defined around the barycentric coordinates to which the label Sk is assigned, and the label Sk is assigned and stored. Reference numeral 32 in FIG. 13 is an example of a partial area defined by a perfect circle. The size and shape of the partial region can be defined by measuring in advance or calculating how much the condensing spot is widened. Next, in step 2D, pixels having luminance exceeding a certain threshold 33 are extracted from the pixels in the partial area to which the label Sk stored in step 2C is assigned. The threshold 33 is set smaller than the threshold 13 and larger than the noise level. By step 2D, it is possible to remove pixels whose SN ratio (signal noise ratio) does not satisfy a predetermined condition. Next, in step 2E, the partial image extracted in step 2D is referred to and searched for a condensed spot extracted in duplicate in a plurality of frames. If there are overlapping spots detected, the higher S / N ratio is left and the lower one is discarded. In step 3, the pixels extracted from FIGS. 5, 6, and 7 are combined to output one image.

Since the wavefront measuring apparatus according to Embodiment 3 of the present invention is configured as described above, the following effects can be obtained.
Even in a state where only one image with a brightness value greatly different for each condensing spot can be obtained in one image, by using a plurality of images, it is possible to reduce variation in the brightness value for each condensing spot, and thus high accuracy. Wavefront measurement is possible.
Furthermore, it is possible to reduce the possibility that a desired calculation accuracy cannot be obtained as compared with the first embodiment of the present invention.

1 Wavefront of light, 2 Wavefront detector, 3 Lenslet array, 4 Imaging means, 5 Signal processor, 5A Frame grabber, 6 Image signal, 6A Condensing spot pattern when distortion is large, 6B Collection when distortion is small Light spot pattern, 7 Control signal, 10 Upper limit of gradation of image signal, 11 Lower limit of gradation of image signal, 12 Line profile of image signal 6A, 12A Line profile when using short exposure time, 12B Normal exposure Line profile when using time, 12C Line profile when using long exposure time, 13 Threshold value indicating the lower limit of the peak to compensate for accuracy depending on S / N ratio, 1412A, 12B and 12C , 30 binary pixel, 31 pixel, 32 partial area, 33 smaller than threshold 13 and Large set threshold than Zureberu, 40 exposure time changing means, 41 optical intensity control unit, 50 a plurality data storage unit, 51 image cropping section, 52 image combining unit, 53 wavefront computation section

Claims (2)

A lenslet array in which a plurality of lenses are arranged to collect light; an exposure time control unit that controls an exposure time of the light; an imaging unit that converts light collected by the lenslet array into an image; Signal processing means for outputting the wavefront shape of light based on the image converted by the imaging means,
The imaging means includes
Based on the exposure time controlled by the exposure time control means, the condensed light is converted into a continuous image,
From the converted image, a partial image is generated by extracting pixels belonging to a focused spot whose peak exceeds a threshold and does not exceed the upper limit of gradation ,
A label is given to the partial image based on the connectivity of the pixels,
For the set of pixels having the same label, find a barycentric coordinate weighted with luminance,
Define a partial area of a predetermined size and shape around the barycentric coordinates,
By extracting pixels having a luminance exceeding a threshold smaller than the threshold for pixels in the partial area, pixels whose signal-to-noise ratio does not satisfy a predetermined condition are removed,
In a plurality of frames of the partial image, if the condensing spots overlap, leaving the one with the higher signal-to-noise ratio,
Combining the remaining partial images into one image,
The signal processing means includes
A wavefront measuring apparatus that outputs a wavefront shape of light received by the imaging means using the synthesized image. In the Shack-Hartmann method of wavefront measurement,
A plurality of exposure times of light collected by a lenslet array composed of a plurality of lenses are set, and an image is output using each of the set exposure times, and a peak is generated from each of the output images. A step 2 for generating a partial image by extracting pixels belonging to a condensing spot that exceeds a threshold and does not exceed an upper limit of gradation;
Step 2A for applying a label to the partial image based on the continuity of the pixels;
Determining a barycentric coordinate weighted with luminance for the set of pixels having the same label; and
Defining a partial area of a predetermined size and shape around the barycentric coordinates;
Step 2D for removing pixels whose signal-to-noise ratio does not satisfy a predetermined condition by extracting pixels having luminance exceeding a threshold value smaller than the threshold value for pixels in the partial region;
In a plurality of frames of the partial image, if the condensing spots overlap, step 2E that leaves a higher signal-to-noise ratio;
A wavefront measuring method comprising: step 3 for combining the remaining partial images into one image; and step 4 for calculating a wavefront of light from the combined image.

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