A number of assumptions have been made in the xonal cavity method for determining a CU value. The accuracy of the results is therefore limited. Also, itmust be remembered that an average uniform horizontal illuminance is a calculated value. Use of the lumen method should be limited to noncritical, uniform (nontask-oriented) lighting and for making a preliminary determination of the approximate number of luminaires needed where visibility of critical tasts and nonuniform lighting layout or task-oriented lighting is important , more sophisticated techniques (see Chapter 12 ) should be used. The reader is referred to referred 8 for other techniques for determining the level of illuminance.