When shopping light bulbs, it is necessary to take into consideration also its illuminance. Illuminance is mostly expressed by the luminous flux in lumens (lm). We can notice this unit on a light bulb packaging. The amount of this unit tells us, how a particular light source will shine. The tungsten bulbs mostly have luminous flux about 13,5 lm/W. LED light bulbs mostly have luminous flux in the range between 30-110 lm / W. Yet the figure in Watts is not directly proportional to the illuminance of the LED bulb, that is why shoppers should not act upon it in order to determine the brightness of a LED bulb, as it was possible when choosing a standard bulb. The input power of a LED bulb depends on the number of diodes and their illuminance as well as on color temperature.
Luminous flux
Luminous flux is an important quantity, which we should pay close attention when choosing a light. It indicates lighting energy, which is emitted by a light source in one second, therefore it is an expression of light output.
Standard bulb |
Halogen bulb |
Energy-saving bulb |
LED bulb |
Luminous flux |
25 W |
25 W |
5 W |
3 W |
210 - 204 lm |
40 W |
40 W |
9 W |
5 W |
400 - 450 lm |
60 W |
60 W |
13 W |
9 W |
700 - 740 lm |
100 W |
100 W |
22 W |
15 W |
1300 - 1500 lm |
It is not new information, that as the light bulb gets older also its luminous flux decreases. The luminous flux decreases at almost all light source technologies. Throughout the whole lifetime of compact light bulbs, the luminous flux drops to mere 30 % of its original value. At LED light bulbs luminous flux drops to 70 % of the original value at the end of its lifetime. The LED bulb should not heat up. The temperatures above 70° lower a lifetime of a LED bulb. The higher temperature might be caused by a poor structure of a bulb, but also by unsuitable placement. For example, don’t use a combination of a LED bulb and another light bulb under one shade.
It is also good to find out, whether the stated luminous flux is available instantly or it lights up gradually. LED light bulbs provide the full light output immediately. Contrary to compact fluorescent bulbs, which need a few minutes to provide full light output. That is why compact fluorescent bulbs are not suitable for rooms, where you switch on and off very often. Frequent switching on/off also significantly decreases the lifetime of these light bulbs.
Illuminance intensity
Illuminance intensity is the part of luminous flux falling on a surface, per unit area and depends on luminous flux of a light source, on beam angle as well as on the distance of the measured area from the light source.
It is a very important unit and can be easily measured by a lux meter. Values of illuminance intensity for particular rooms are listed in the following table:
Type of room, job or activity |
Illuminance (lx) |
Staircase |
30 |
Hallway |
70 |
Bathroom, toilet |
150 |
Kitchen, living room and dining room |
250 - 350 |
Lighting for reading, writing, small jobs |
500 |
The unit of lighting is lux (lx), which is lighting produced by luminous flux 1 lm falling on an area of 1m². However,
the light intensity is also determined by the distance of the light source from the illuminated surface (the longer the distance, the more lumens are needed) and by the angle/direction of falling light rays (the lower the angle of falling light rays, the weaker lighting). When purchasing a light source, it is advisable to find out the number of lumens (lm), which is stated on a light bulb box. The number of lumens is a unit of luminous flux, which expresses the total amount of light, which is emitted by the light source in all directions in one second.
