What does "illumination" mean?
How to keep excellent eyesight to an advanced age? This issue is very relevant for the modern man, as we all have to more and more use a variety of electronic devices that create a significant load for the eyes. To save acute sight, a whole complex of measures is necessary, and one of them is providing good lighting in the home and in the workplace.
How to do this? Simply putting light bulbs more powerful is not the best solution, considering the ever increasing cost of electricity. The most rational way is to calculate the illumination of the room. 
First let's figure out what kind of indicator this "illumination" is, and why it should be calculated. We all know that each lighting device has a certain power. But how, let's say, compare how the incandescent lamp and the fluorescent lamp illuminate the room? For this, there is such a value as the light flux measured in lumens. This indicator is indicated on the packaging of any light bulb.
Suppose we know the light flux of a lamp. For example, 1200 lumens. Is this enough to make it light at the desk? It turns out that the figure itself says nothing about anything. The lamp installed on the table itself will illuminate it much more intensively than the same lamp hanging from the ceiling. 
In order to take into account this important point, the illumination calculation was introduced. To measure this indicator, the value lux is used, meaning that a luminous flux of 1 lumen falls per square meter of the surface.
How to calculate?
So, how to calculate the illumination of a room, armed with the above theory? If we are talking about an approximate calculation of the power of lighting devices, then a simple formula will help us to calculate. To do this, take the area of the room in square meters, multiply by the specific power of lighting per square meter and divide by the number of light bulbs in the room. As a result, we learn the illumination of the room.
Here the question naturally arises, but how to know this specific power? Recommended indicators for each type of premises and lamps can be found in the reference table. 
The luminosity can be calculated more precisely by a special instrument - a luximeter. For an apartment to use it, maybe it does not make sense, but for a large office the use of such a device will be superfluous.
Given that for the room the recommended illumination should be 200 lux, and for the workplace( for example, directly on the surface of the desk) - 500 lux, we can roughly determine how many devices and what power we need.
Accurate calculation: additional factors
It should be noted that the above method is rather approximate. For a precise calculation, it is necessary to take into account a number of factors:
- height of the ceiling;
- presence or absence of mirrors;
- color of wallpaper and furniture;
- texture of finishing materials, etc.
The result is a more complex formula that gives an accurate calculation of room lighting. For example, the standards of illumination for a room with a ceiling height of up to 3 m are as follows: 
Whether you have enough indicative indicators or not is an individual question. In any case, you can take into consideration a few tips:
- For a room with a light finish range, the amount of illumination can be half as much as for dark coatings.
- Matt surfaces of materials absorb light, and glossy reflect.
- For the bedroom it is appropriate to have muffled light, for the working area - bright, for the living room - a combined option.
In general, the correct organization of lighting will not only save the health of the eyes, but also help create a more comfortable indoor microclimate.