Obtain an RGB color based on wavelength

wavelength_to_rgb.rb

# credits: http://www.physics.sfasu.edu/astro/color/spectra.html
# Calculates an RGB color based on factor betwen 0 (380nm) and 1 (780nm)
def color_by_factor(f)
  f = f.to_f
  max = 255.0
  wl = 380.0 + (780.0 - 380.0) * f
  if wl >= 380 and wl <= 440
    r = -1.0 * (wl - 440.0)/(440.0 - 380.0)
    g = 0.0
    b = 1.0
  elsif wl > 440 and wl <= 490
    r = 0.0
    g = (wl - 440.0)/(490.0 - 440.0)
    b = 1.0
  elsif wl > 490 and wl <= 510
    r = 0.0
    g = 1.0
    b = -1.0 * (wl - 510.0)/(510.0 - 490.0)
  elsif wl > 510 and wl <= 580
    r = (wl - 510.0)/(580.0 - 510.0)
    g = 1.0
    b = 0.0
  elsif wl > 580 and wl <= 645
    r = 1.0
    g = -1.0 * (wl - 645.0)/(645.0 - 580.0)
    b = 0.0
  elsif wl > 645 and wl <= 780
    r = 1.0
    g = 0.0
    b = 0.0
  end
  if wl > 700
    sss = 0.3 + 0.7 * (780.0 - wl)/(780.0 - 700.0)
  elsif wl < 420
    sss = 0.3 + 0.7 * (wl - 380.0)/(420.0 - 380.0)
  else
    sss = 1.0
  end
  '#' + [r, g, b].map{|c| (max * c * sss) }.map(&:to_i).map{|c| c.to_s(16).upcase.rjust(2, '0') }.join
end