public void ModifyLightDistributionStyle(Document familyDoc)
{
    // Get the light family from the static method.
    LightFamily lightFamily = LightFamily.GetLightFamily(familyDoc);

    // Set the light distribution style to PhotometricWeb
    lightFamily.SetLightDistributionStyle(LightDistributionStyle.PhotometricWeb);

    // After light shape style set to PhotometricWeb, each tyoe returns a CircleLightShape instance,
    for (int index = 0; index < lightFamily.GetNumberOfLightTypes(); index++)
    {
        LightType lightData = lightFamily.GetLightType(index);
        PhotometricWebLightDistribution lightDistribution = 
            lightData.GetLightDistribution() as PhotometricWebLightDistribution;
        lightDistribution.PhotometricWebFile = @"C:\IES\1x4 2Lamp.ies"; // input a full file path here.
        lightDistribution.TiltAngle = Math.PI / 6;    // use radian value to set
        lightData.SetLightDistribution(lightDistribution);  // set back
    }
}

Public Sub ModifyLightDistributionStyle(familyDoc As Document)
    ' Get the light family from the static method.
    Dim lightFamily__1 As LightFamily = LightFamily.GetLightFamily(familyDoc)

    ' Set the light distribution style to PhotometricWeb
    lightFamily__1.SetLightDistributionStyle(LightDistributionStyle.PhotometricWeb)

    ' After light shape style set to PhotometricWeb, each tyoe returns a CircleLightShape instance,
    For index As Integer = 0 To lightFamily__1.GetNumberOfLightTypes() - 1
        Dim lightData As LightType = lightFamily__1.GetLightType(index)
        Dim lightDistribution As PhotometricWebLightDistribution = TryCast(lightData.GetLightDistribution(), PhotometricWebLightDistribution)
        lightDistribution.PhotometricWebFile = "C:\IES\1x4 2Lamp.ies"
        ' input a full file path here.
        lightDistribution.TiltAngle = Math.PI / 6
        ' use radian value to set
        ' set back
        lightData.SetLightDistribution(lightDistribution)
    Next
End Sub