All values are automatically clamped in the range [0,128]. Effective light intensity is attenuated by the cosine of the angle between the direction of the light and the direction from the light to the vertex being lighted, raised to the power of the spot exponent. Thus, higher spot exponents result in a more focused light source. The default spot exponent is 0, resulting in uniform light distribution.

Reference

NSpotLightSource Class
NSpotLightSource Members