respective cut off point c1 that produces the basic luminosity lav a cut off point c2 that introduces the brightest luminosity lmax and a cut off point c3 that brings the darkest luminosity lmin are determined accordingly these values if not been set by a user will ones preset by the device itself in order to simplify the structure of the control circuit a triac is used as a controlling component in this embodiment of the present invention there are a limited number such as 256 of different cut off points at in each half cycle of the wave of the power source the number of these cut off points is so determined that the change in luminosity between adjacent cut off points is below the consciousness of human eyes and each luminosity will be maintained for a small time interval t that interval t can be set and changed by a user however it is selected to be large enough that a change of the luminosity during the time interval will not exceed what the pupils can accommodate therefore the control of the values of the cut off points is as shown in fig4 to appear as stairs first the luminosity of the modulated lighting facility starts from a basic value that is the cut off point c1 is so selected that it will provide the user a selected basic luminosity lav then the cut off point is changed gradually to higher values till a maximum cut off point c2 is met so that the luminosity is modulated step by step toward brighter luminosities until the maximum luminosity lmax is reached afterwards the cut off points are reduced step by step.