A Java program that converts a wavelength of light in nanometers to an RGB colour.

Wavelength2RGB.java

/**
 * A Java program that converts a wavelength of light in nanometers to an RGB colour. See the main method for example
 * usage.
 *
 * It uses linear interpolation over the "CIE 1964 standard observer" table and sRGB matrix + gamma correction
 * to do the conversion.
 *
 */
public class Wavelength2RGB {
    static int
            LEN_MIN = 380,
            LEN_MAX = 780,
            LEN_STEP = 5;

    static double[]
            X = {
            0.000160, 0.000662, 0.002362, 0.007242, 0.019110, 0.043400, 0.084736, 0.140638, 0.204492, 0.264737,
            0.314679, 0.357719, 0.383734, 0.386726, 0.370702, 0.342957, 0.302273, 0.254085, 0.195618, 0.132349,
            0.080507, 0.041072, 0.016172, 0.005132, 0.003816, 0.015444, 0.037465, 0.071358, 0.117749, 0.172953,
            0.236491, 0.304213, 0.376772, 0.451584, 0.529826, 0.616053, 0.705224, 0.793832, 0.878655, 0.951162,
            1.014160, 1.074300, 1.118520, 1.134300, 1.123990, 1.089100, 1.030480, 0.950740, 0.856297, 0.754930,
            0.647467, 0.535110, 0.431567, 0.343690, 0.268329, 0.204300, 0.152568, 0.112210, 0.081261, 0.057930,
            0.040851, 0.028623, 0.019941, 0.013842, 0.009577, 0.006605, 0.004553, 0.003145, 0.002175, 0.001506,
            0.001045, 0.000727, 0.000508, 0.000356, 0.000251, 0.000178, 0.000126, 0.000090, 0.000065, 0.000046,
            0.000033
    },

    Y = {
            0.000017, 0.000072, 0.000253, 0.000769, 0.002004, 0.004509, 0.008756, 0.014456, 0.021391, 0.029497,
            0.038676, 0.049602, 0.062077, 0.074704, 0.089456, 0.106256, 0.128201, 0.152761, 0.185190, 0.219940,
            0.253589, 0.297665, 0.339133, 0.395379, 0.460777, 0.531360, 0.606741, 0.685660, 0.761757, 0.823330,
            0.875211, 0.923810, 0.961988, 0.982200, 0.991761, 0.999110, 0.997340, 0.982380, 0.955552, 0.915175,
            0.868934, 0.825623, 0.777405, 0.720353, 0.658341, 0.593878, 0.527963, 0.461834, 0.398057, 0.339554,
            0.283493, 0.228254, 0.179828, 0.140211, 0.107633, 0.081187, 0.060281, 0.044096, 0.031800, 0.022602,
            0.015905, 0.011130, 0.007749, 0.005375, 0.003718, 0.002565, 0.001768, 0.001222, 0.000846, 0.000586,
            0.000407, 0.000284, 0.000199, 0.000140, 0.000098, 0.000070, 0.000050, 0.000036, 0.000025, 0.000018,
            0.000013
    },

    Z = {
            0.000705, 0.002928, 0.010482, 0.032344, 0.086011, 0.197120, 0.389366, 0.656760, 0.972542, 1.282500,
            1.553480, 1.798500, 1.967280, 2.027300, 1.994800, 1.900700, 1.745370, 1.554900, 1.317560, 1.030200,
            0.772125, 0.570060, 0.415254, 0.302356, 0.218502, 0.159249, 0.112044, 0.082248, 0.060709, 0.043050,
            0.030451, 0.020584, 0.013676, 0.007918, 0.003988, 0.001091, 0.000000, 0.000000, 0.000000, 0.000000,
            0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
            0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
            0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
            0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
            0.000000
    };

    static double[]
            MATRIX_SRGB_D65 = {
            3.2404542, -1.5371385, -0.4985314,
            -0.9692660,  1.8760108,  0.0415560,
            0.0556434, -0.2040259,  1.0572252
    };

    public static int[] Calc(double len) {
        if(len < LEN_MIN || len > LEN_MAX)
            return new int[3];

        len -= LEN_MIN;
        int index = (int)Math.floor(len / LEN_STEP);
        double offset = len - LEN_STEP * index;

        double x = Interpolate(X, index, offset);
        double y = Interpolate(Y, index, offset);
        double z = Interpolate(Z, index, offset);

        double[] m = MATRIX_SRGB_D65;

        double r = m[0] * x + m[1] * y + m[2] * z;
        double g = m[3] * x + m[4] * y + m[5] * z;
        double b = m[6] * x + m[7] * y + m[8] * z;

        r = Clip(GammaCorrect_sRGB(r));
        g = Clip(GammaCorrect_sRGB(g));
        b = Clip(GammaCorrect_sRGB(b));

        return new int[] {
                (int)(255 * r),
                    (int)(255 * g),
                    (int)(255 * b)
        };
    }
    static double Interpolate(double[] values, int index, double offset) {
        if(offset == 0)
            return values[index];

        double x0 = index * LEN_STEP;
        double x1 = x0 + LEN_STEP;
        double y0 = values[index];
        double y1 = values[1 + index];

        return y0 + offset * (y1 - y0) / (x1 - x0);
    }

    static double GammaCorrect_sRGB(double c) {
        if(c <= 0.0031308)
            return 12.92 * c;

        double a = 0.055;
        return (1 + a) * Math.pow(c, 1 / 2.4) - a;
    }

    static double Clip(double c) {
        if(c < 0)
            return 0;
        if(c > 1)
            return 1;
        return c;
    }

    public static void main(String[] args) {
        double len = 635; //wavelength of red light in nanometers
        int[] rgb = Calc(len);
        System.out.println("r: "+rgb[0]+ " g: "+rgb[1]+" b: "+rgb[2]);
    }
}