RGB to Visible Spectrum

The spectrum is generated using approximate RGB values for visible wavelengths between 380 nm and 780 nm. The red, green and blue values are assumed to vary linearly with wavelength (for gamma = 1). Below is the original FORTRAN code by Dan Bruton (astro@tamu.edu) converted to JavaScript by me

// given a wavelength 'i', wl(i) returns the constituent r,g,b values
function wl(i) {
    let [r, g, b] = [0, 0, 0];

    const gamma = 0.80;
    const depth = 255;

    if (i >= 380 && i <= 440) {
        r = -1 * (i - 440) / (440 - 380);
        b = 1;
    }
    else if (i >= 440 && i <= 490) {
        g = (i - 440) / (490 - 440);
        b = 1;
    }
    else if (i >= 490 && i <= 510) {
        g = 1;
        b = -1 * (i - 510) / (510 - 490);
    }
    else if (i >= 510 && i <= 580) {
        r = (i - 510) / (580 - 510);
        g = 1;
    }
    else if (i >= 580 && i <= 645) {
        r = 1;
        g = -1 * (i - 645) / (645 - 580);
    }
    else if (i >= 645 && i <= 780) {
        r = 1;
    }

    // LET THE INTENSITY SSS FALL OFF NEAR THE VISION LIMITS
    let sss = 1;
    if (i > 700) {
        sss = 0.3 + 0.7 * (780 - i) / (780 - 700);
    }
    else if (i < 420) {
        sss = 0.3 + 0.7 * (i - 380) / (420 - 380);
    }

    // Gamma adjust and return the r,g,b values
    return [
        Math.floor(depth * ((sss * r) ** gamma)), 
        Math.floor(depth * ((sss * g) ** gamma)), 
        Math.floor(depth * ((sss * b) ** gamma))
    ]
}