function index_nad,wave_um

;-----------------------------------------------------------------
;
; Purpose:
;   Return the refrctive index for crystalline NAD at 184 K,
;   from Toon et al., JGR, 99, pp25,631, 1994>  Data are stored 
;   in routine for simplicity.
;
; Input:
;   wave_um...input wavelength, microns 
;
; Output:
;   result....complex refractive index
;
; Source: Mark Hervig 
;
;-----------------------------------------------------------------

     numw = 133            ; # wavelengths in Toon et al. 
     wave = fltarr(numw)   ; 1/cm
     dxr  = fltarr(numw)   ; real index
     dxi  = fltarr(numw)   ; imaginary index

     waves = $
    '482.000 501.000 517.000 532.000 548.000 559.000 571.000 590.000 602.000 613.000 '+$
    '621.000 636.000 656.000 671.000 679.000 690.000 698.000 706.000 710.000 714.000 '+$
    '729.000 733.000 737.000 744.000 748.000 756.000 760.000 768.000 775.000 787.000 '+$
    '802.000 810.000 822.000 833.000 868.000 914.000 949.000 991.000 1003.000 1011.00 '+$
    '1014.00 1022.00 1026.00 1030.00 1034.00 1041.00 1045.00 1049.00 1053.00 1068.00 '+$
    '1099.00 1138.00 1165.00 1200.00 1219.00 1234.00 1246.00 1261.00 1269.00 1280.00 '+$
    '1296.00 1300.00 1315.00 1334.00 1358.00 1385.00 1408.00 1427.00 1442.00 1450.00 '+$
    '1454.00 1462.00 1469.00 1477.00 1489.00 1504.00 1531.00 1577.00 1601.00 1639.00 '+$
    '1662.00 1682.00 1705.00 1743.00 1766.00 1859.00 1921.00 2021.00 2102.00 2183.00 '+$
    '2225.00 2272.00 2326.00 2411.00 2476.00 2522.00 2561.00 2650.00 2750.00 2800.00 '+$
    '2912.00 3074.00 3120.00 3167.00 3221.00 3263.00 3321.00 3359.00 3417.00 3437.00 '+$
    '3456.00 3479.00 3494.00 3529.00 3541.00 3564.00 3583.00 3602.00 3629.00 3660.00 '+$
    '3726.00 3753.00 3784.00 3930.00 3980.00 4091.00 4219.00 4381.00 4597.00 4883.00 '+$
    '5276.00 5828.00 6981.00'
     reads,waves,wave

     dxrs = $
    '2.03200 1.92700 1.88300 1.88800 1.89700 1.86200 1.81000 1.81500 1.85600 1.90200 '+$
    '1.93000 1.93200 1.92100 1.88100 1.83200 1.82800 1.86100 1.90800 1.93600 1.96400 '+$
    '2.04400 2.07500 2.10200 2.01800 1.89100 1.73600 1.73200 1.76500 1.80400 1.83700 '+$
    '1.78900 1.74000 1.73300 1.76300 1.81600 1.87500 1.93900 2.03300 2.08200 2.12900 '+$
    '2.15100 2.14100 2.05500 1.91000 1.77200 1.69800 1.72400 1.76300 1.79900 1.89100 '+$
    '1.96700 2.00200 2.02400 2.11900 2.21800 2.31900 2.38800 2.23700 1.93200 1.51900 '+$
    '1.23800 1.22100 1.27500 1.38600 1.49900 1.62500 1.72300 1.76800 1.65500 1.47200 '+$
    '1.32600 1.09200 1.03400 1.06200 1.13300 1.22500 1.33400 1.45600 1.50500 1.58600 '+$
    '1.63500 1.64100 1.60800 1.52400 1.48500 1.44000 1.44500 1.48100 1.50200 1.49900 '+$
    '1.47900 1.45200 1.43000 1.43700 1.44600 1.46000 1.46500 1.44500 1.41800 1.40000 '+$
    '1.38000 1.38200 1.38700 1.39300 1.39100 1.35900 1.32400 1.31900 1.32900 1.33600 '+$
    '1.34200 1.34100 1.31700 1.29900 1.30200 1.31200 1.32400 1.33500 1.34900 1.36200 '+$
    '1.38100 1.38700 1.39300 1.41500 1.42000 1.43000 1.44000 1.45000 1.46000 1.47000 '+$
    '1.48000 1.49000 1.50000'
     reads,dxrs,dxr

     dxis = $
   '0.213000 0.264000 0.250000 0.226000 0.252000 0.278000 0.252000 0.160000 0.134000 0.134000 '+$
   '0.155000 0.217000 0.223000 0.284000 0.259000 0.178000 0.143000 0.128000 0.129000 0.139000 '+$
   '0.213000 0.245000 0.312000 0.530000 0.564000 0.413000 0.343000 0.272000 0.253000 0.273000 '+$
   '0.326000 0.299000 0.221000 0.188000 0.162000 0.135000 0.149000 0.197000 0.233000 0.285000 '+$
   '0.334000 0.509000 0.624000 0.673000 0.598000 0.371000 0.298000 0.254000 0.233000 0.228000 '+$
   '0.265000 0.321000 0.346000 0.384000 0.462000 0.598000 0.793000 1.29000 1.42000 1.35000 '+$
   '0.999000 0.873000 0.629000 0.489000 0.411000 0.417000 0.496000 0.669000 0.917000 1.04000 '+$
   '1.05000 0.888000 0.662000 0.508000 0.376000 0.290000 0.232000 0.193000 0.194000 0.208000 '+$
   '0.259000 0.321000 0.378000 0.406000 0.386000 0.307000 0.266000 0.230000 0.243000 0.270000 '+$
   '0.281000 0.278000 0.252000 0.219000 0.206000 0.205000 0.218000 0.234000 0.235000 0.226000 '+$
   '0.195000 0.158000 0.152000 0.155000 0.174000 0.188000 0.152000 0.128000 9.64000E-02 9.28000E-02 '+$
   '9.48000E-02 0.111000 0.114000 6.99000E-02 5.73000E-02 3.70000E-02 2.59000E-02 1.88000E-02 1.36000E-02 1.05000E-02 '+$
   '8.43000E-03 8.15000E-03 7.16000E-03 6.59000E-03 6.45000E-03 0.  0.  0.  0.  0. '+$
   '0.  0.  0.'
     reads,dxis,dxi

;- interpolate index to wave_um 

   wave_cm = 10000.0 / wave_um    ; microns to 1/cm

   if (wave_cm ge wave(0) and wave_cm le wave(numw-1) ) then begin

     for i = 0,(numw-2) do begin

       if (wave(i) le wave_cm and wave(i+1) gt wave_cm) then begin

         term   = (wave_cm-wave(i)) /(wave(i+1)-wave(i))
         dr     = term * (dxr(i+1)-dxr(i)) + dxr(i)
         di     = term * (dxi(i+1)-dxi(i)) + dxi(i)
         result = complex(dr,di)
         goto,jump1

       endif
     endfor

     print  ,wave_um,' not in range in index_nad.pro'
     result = complex(0.0,0.0)
   endif

   jump1:
   return,result
   end