- How to choose the wattageof light
①First let’s explain what is luminous flux
Luminous flux (Lx), means the radiant power that the human eye can feel, refer to the standard value of illuminance per level in the figure below:
Place height illuminance value (Lx=LM/m2)
Daily activity place 0.75m 100
Office writing and reading 0.75m 300
Restaurant table surface 0.75m 150
②Reference luminous flux, depending on the size of the room
For example, if the area of the room is 10 square meters, 10 square meters x (100~300)LM/m2 = 1000~3000 LM of luminous flux.
③Decided by the function of the space
Based on the luminous flux and the function of the space, for example, reference to the table above, the daily activity need 100LM/m2, while writing and reading need 300LM/m2, but in the case of a desk in a room, if only one main lamp is installed, it will be too bright for daily activities. then installing a desk lamp or other auxiliary lighting is necessary.
④Wattage and luminous flux
Normally, when people buy a light, they only know the wattage (W), so what is the relationship between wattage and luminous flux? This brings us to another concept – luminous efficiency (lm/W), the efficiency with which electrical energy is converted into light energy.
For instance，a 20W light, the luminous efficiency is 100lm/W (general LED light source luminous efficiency in 80 ~ 120lm/W), then the luminous flux = 20W * 100lm/W = 2000lm.
Because the light effect of each vendor and model of the light are not the same, you can look at the product packing or consult the vendor for relevant information.
⑤Will it be too dark without a main light?
When we use multiple lamps, the luminous fluxes are generally added together.
For example, a 10 m² room requires 1000~3000 LM of luminous flux. If the luminous flux of one down light is 200LM, then theoretically 5 to 15 down lights installed evenly in the space will be sufficient.
Of course, under actual conditions, it is impossible to install so many downlights in a 10 square meter space. So, you can install a 300LM desk lamp in the corner of the desk on the basis of six 200LM down lights to ensure that daily activities are bright enough. The area where the desk needs to be illuminated is also about 1 square meters, which is enough.
This kind of lighting is contrary to the design using the main light, which will make the whole space brighter but the middle area darker.
2.Factors Affecting the Perception of Spatial Brightness
It is possible to use luminous flux as the light bulb choice, but in terms of the overall lighting experience, the factors to consider are:
①Space tones, e.g. dark spaces are darker and lighter spaces are brighter.
②Furniture size. If there are some larger size furniture in the space that will block the light, you should also consider separating the light reasonably to make it more comfortable.
③Personal feeling, this is the most important thing because everyone has a different sensitivity to light. I met one client who wanted every corner of his home to be ‘very bright’ because he liked it that way.
Let’s summarize the concepts mentioned in the article:
① Luminous flux (unit: lm), refers to the radiant power that the human eye can feel;
② Illumination standard value (unit: LM/㎡), normal space reference value of 100~300LM/㎡ according to demand;
③ Watt (unit: W), power;
④Luminous efficacy (unit: lm/W), the efficiency of converting electrical energy into light energy, general LED light efficiency is 80 ~ 130 lm/W.
So, if a led light, it’s about a 1-3 watt for one square meter
However, in reality, it is impossible for the whole space to be in the reading and writing space, only the small corners, so usually 10 square meters of room, about 10W LED main lights, and then the reading and writing area, and then a separate table lamp can be.
In addition to brightness, you should also consider the color temperature of the light fixture, whether it is cool or warm, which is also an important factor in creating an atmosphere in your place.