add( sh ) {

		for ( let i = 0; i < 9; i ++ ) {

			this.coefficients[ i ].add( sh.coefficients[ i ] );

		}

		return this;

	}
Adds the given SH to this instance.

@param {SphericalHarmonics3} sh - The SH to add.
@return {SphericalHarmonics3} A reference to this spherical harmonics.