Converting Wavelength to RGB

PowerShell is a generic script language so you can do all kinds of stuff with it. Below is a function that takes a light wavelength and converts it to the appropriate RGB color value:

function Convert-WavelengthToRgb
{
  # derived from http://www.physics.sfasu.edu/astro/color/spectra.html

  param
  (
    [Parameter(Mandatory,ValueFromPipeline,ValueFromPipelineByPropertyName)]
    [ValidateRange(380,780)]
    [int]
    $Wavelength,
        
    [double]
    $Gamma = 0.8,
        
    [int]
    [ValidateRange(1,255)]
    $Intensity = 255,
        
    [switch]
    $AdjustHumanSensitivity
  )

  process
  {
    #from: https://www.codedrome.com/exploring-the-visible-spectrum-in-python/
    $factor = 0

    $result = [PSCustomObject]@{
      Wavelength = $Wavelength
      R = 0
      G = 0
      B = 0
    }

    switch($Wavelength)
    {

      { $_ -lt 420 }  {   
        $result.R = -($_ - 440) / 60
        $result.B = 1
        $factor = 0.3 + 0.7 * ($_ - 380) / 40
        break
      }

      { $_ -lt 440 }  { 
        $result.R = -($_ - 440) / 60
        $result.B = 1
        $factor = 1
        break
      }

      { $_ -lt 490 }  { 
        $result.G = ($_ - 440) / 50
        $result.B = 1
        $factor = 1
        break
      }

      { $_ -lt 510 }  { 
        $result.B = -($_ - 510) / 20
        $result.G = 1
        $factor = 1
        break
      }

      { $_ -lt 580 }  { 
        $result.R = ($_ - 510) / 70
        $result.G = 1
        $factor = 1
        break
      }

      { $_ -lt 645 }  { 
        $result.G = -($_ - 645) / 65
        $result.R = 1
        $factor = 1
        break
      }
      { $_ -le 700    }  { 
        $result.R = 1
        $factor = 1
        break
      }


      { $_ -le 780 }  { 
        $result.R = 1
        $factor = 0.3 + 0.7 * (780 - $_) / 80
        break
      }
    }
    
    if ($AdjustHumanSensitivity.IsPresent -eq $false) { $factor = 1 }
        
    $result.R = [Math]::Pow( ($result.R * $factor), $gamma)    * $Intensity -as [int]
    $result.G = [Math]::Pow( ($result.G * $factor), $gamma)    * $Intensity -as [int]
    $result.B = [Math]::Pow( ($result.B * $factor), $gamma)    * $Intensity -as [int]

    return $result
  }
}

Now it’s simple to convert the entire visible light spectrum to the corresponding RGB values:

# Calculate RGB values for the visible light spectrum (wavelengths)

380..780 | 
  Convert-WavelengthToRgb | 
  Out-GridView -Title 'Without Correction'

Since the human eye sensitivity is different for different colors, the function can even take into account this sensitivity and automatically apply a correction:

380..780 | 
  Convert-WavelengthToRgb -AdjustHumanSensitivity | 
  Out-GridView -Title 'With Correction'

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