The following program is under the GNU General Public License (GPL).

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; Program Written in IDL (Interactive Data Language)
; Free Interpreter availible at www.rsinc.com
; Written by Georg Wiora
; This Program is under the  GNU General Public License (GPL)  
;
; Create a display of interference patterns
; Save this text as file "Interference2.pro" to run it.
 
; Draw a circle
PRO circle,x,y,r,_REF_EXTRA=ex
  n=ULONG(!DPI*r) > 1
  phi = FINDGEN(n)/FLOAT(n)*2*!DPI
  x1 = r*COS(phi)+x
  y1 = r*SIN(phi)+y
  PLOTS,[x1,x1[0]],[y1,y1[0]],_STRICT_EXTRA=ex
END

; Compute the interference
PRO Interference2
   ; Final Image Size
  siz=2000
  ; No display if larger than 800x1600 pixel
  pixmap=SIZ GT 800
  ; Wavelength in Pixel
  lambda=160
  ; Half distance of the sources
  dhalb = lambda

  ; Compute the distance of each point in an array from the
  ; center point: This is the phase of the signal
  d1 = SHIFT(DIST(siz*2,siz),siz,siz/2)
  ; Sinus of distace with 0 LE amplitude LE 2
  f = SIN(d1/lambda*2*!DPI)+1
  ; Compute Interference
  inter = SHIFT(f,-dhalb,0)+SHIFT(f,dhalb,0)
  ; Get maximum and minimum
  ma = MAX(inter,MIN=mi)
  ; Two colors are reserved for marking, the rest is gray scale
  ; Scale interference to byte range [0..253]
  inter = BYTE((TEMPORARY(inter)-mi)/(ma-mi)*253)

  ; Use lookup table to display
  DEVICE,DECOMPOSED=0
  ; Gray scale
  LOADCT,0
  ; Half length of center cross bars in pixel
  cs=30
  ; Half width of center cross bars in pixel
  cb=3
  
  ; Set mark for center
  ; Horizontal left
  inter[siz-dhalb-cs:siz-dhalb+cs, siz/2-cb:siz/2+cb] = 254
  ; Vertical left
  inter[siz-dhalb-cb:siz-dhalb+cb, siz/2-cs:siz/2+cs] = 254
  ; Horizontal right
  inter[siz+dhalb-cs:siz+dhalb+cs, siz/2-cb:siz/2+cb] = 255
  ; Vertical right
  inter[siz+dhalb-cb:siz+dhalb+cb, siz/2-cs:siz/2+cs] = 255

  ; Open a visible or invisible window (depends on pixmap flag)
  WINDOW,XSIZE=2*siz,YSIZE=siz,PIXMAP=pixmap
  ; Display the interference (only gray scale!)
  TV,inter
  ; Draw circles on maximum locations   
  FOR i=0,2.5*siz/lambda DO $
  BEGIN
    ; Maximum location left
    circle,siz-dhalb,siz/2,(i+0.25)*lambda,THICK=6,/DEVICE,COLOR=254
    ; Maximum location right
    circle,siz+dhalb,siz/2,(i+0.25)*lambda,THICK=6,/DEVICE,COLOR=255
  ENDFOR

  ; Colortable 0-253 as gray scake
  r = FINDGEN(256)/253*255
  g=r
  b=r
  ; 254 is green
  r[254]=0
  b[254]=0
  g[254]=255
  ; 255 is red
  r[255]=255
  b[255]=0
  g[255]=0

  ; Read image from display
  im=TVRD()
  ; Crop invalid borders
  im = im[siz/2:3*siz/2-1,*]

  ; Save as PNG with colortable
  WRITE_PNG,'Interference2.png',im,r,g,b
  ; Convert to truecolor and write JPEG
  imt = [[[r[im]]],[[g[im]]],[[b[im]]]]
  ; smooth image a little bit (saves bandwith in jpeg and is OK
  ; with high resolution)
  imt[*,*,0] = SMOOTH(imt[*,*,0],3,/EDGE_TRUNCATE)
  imt[*,*,1] = SMOOTH(imt[*,*,1],3,/EDGE_TRUNCATE)
  imt[*,*,2] = SMOOTH(imt[*,*,2],3,/EDGE_TRUNCATE)
  ; Write as jpeg
  WRITE_JPEG,'Interference2.jpg',TEMPORARY(imt),/PROGRESSIVE,$
             QUALITY=90, TRUE=3 
END