lerp( sh, alpha ) {

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

			this.coefficients[ i ].lerp( sh.coefficients[ i ], alpha );

		}

		return this;

	}
Linear interpolates between the given SH and this instance by the given
alpha factor.

@param {SphericalHarmonics3} sh - The SH to interpolate with.
@param {number} alpha - The alpha factor.
@return {SphericalHarmonics3} A reference to this spherical harmonics.