The uniformity of artificial lighting in residential and public buildings is determined by measuring its level at several points on the test surface. Lighting is considered uniform if the ratio of the minimum illumination, taken per unit, to the maximum for 0.75 meters of the test surface is not lower than 0.5 (1 : 2), and for 5 meters – not lower than 0.3 (1 : 3). Note . Under industrial conditions, the uniformity of artificial lighting is estimated by the coefficient of unevenness, which is the ratio of the maximum illumination in the room to the minimum, taking into account the discharge accuracy of the work performed. When working with high precision using fluorescent lamps, it should not exceed 1.3; with other light sources – 1.5; during medium and low precision operations, the value of this coefficient is 1.5 and 2.0, respectively. The unevenness of natural light in a work environment should not exceed a 3: 1 ratio.
The calculation method for determining artificial illumination by the “watt” method is based on calculating the total power of all lamps in the room and determining their specific power. Specific power is the amount of energy, expressed in watts, per unit area, that is, the ratio of the total power of the lamps to the floor area is W / m 2 . This value is multiplied by the coefficient “e”, which shows what illumination (in lux) gives the specific power equal to 1 W / m 2 . The value of “e” for rooms with an area of not more than 50 m 2 at a voltage of 220 V for incandescent lamps with a power of less than 100 W is 2.0; for lamps of 100 W and more – 2.5; for fluorescent lamps – 12.5.
An example . Classroom area of 40 m 2 is 10 for luminaires, each of which consists of two luminescent lamps at 40 watts.
Specific power: 40 W • 2 lamps • 10 lamps = 800 W: 40 m 2 = 20 W / m 2 .
Illumination: 20 W / m 2 • 12.5 (lux / W / m 2 ) = 250 lux.
Note . When calculating the illumination created by fluorescent lamps, it is tentatively considered that the specific power of 10 W / m 2 corresponds to 100 lux.
Using the tables of specific power (table. 5), you can determine the required number of fixtures to create a given illumination. Specific power tables are compiled for various types of luminaires taking into account the color of the interior color of the room, therefore, they indicate the name of the luminaire and the reflection coefficients of the ceiling, walls, floor (Рп, Рс, Рр).
The magnitude of the specific power depends on the suspension height of the lamp, the area of the room and the illumination that must be created in this room. It is found at the intersection of a horizontal line corresponding to the area and height of the suspension of the lamp and a vertical line corresponding to a given level of illumination.