Manual method of calculating the lighting - what are the disadvantages?

This publication was inspired by the article “Calculation of room lighting hand to hand” .
When reading the article, you may feel that computer modeling is unnecessary, since manual calculation takes only 4 minutes and is as accurate as with computer modeling.

Unfortunately, this is not the case, because in the simplified form the method of coefficients used in the article does not take into account the reflection from the surfaces of the walls and ceiling, and in the expanded form the calculation will take much longer than in DIALux.

For details and proofs I ask for cat.

Hand to hand calculation

To begin, let me remind you the source data for the calculation:
• PHILIPS BWG201 lamp (adopted as an analogue of a 60W incandescent lamp);
• room illumination 150lk at a height of 0.8m from the floor;
• reflection coefficients% floor \ ceiling \ walls - 30 \ 70 \ 50;
• The formulas are the same as in the original article.
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In the tables and figures below are the parameters of the premises and the results of measurements of the number of lamps for illuminance 150lk. For the value closest to reality, the result taken is DIALux.

Room 5x4x2.8
Room height	2.8
Room length	five
Room width	four
Light stream of one lamp (lm)	700
Required light (lx)	150
Number of lamps calculated using formulas	9
Number of lamps calculated using DIALux	7

Table 1


Pic1

Table 1 and Figure 1 show data for a room with dimensions of 5x4x2.8m, i.e. the room corresponds to the room from the article. As can be seen, the number of lamps by the formula is more, but not significantly.

Room 10103.2
Room height	3.2
Room length	ten
Room width	ten
Light stream of one lamp (lm)	700
Required light (lx)	150
Number of lamps calculated using formulas	35
Number of lamps calculated using DIALux	25

Table 2


Pic2

Table 2 and Figure 2 show data for a room with dimensions 10x10x3.2. And in this case, the discrepancy is already becoming more significant, but we just increased the room a little.

This discrepancy is caused by the insufficient accuracy of calculations using the “coefficient of utilization method”, since this method neglects the reflection characteristics of the surfaces. This method was also used in conjunction with the point method for manual calculations in order to improve accuracy.

Perhaps, when calculating the lighting, it is necessary to evaluate the quality indicators of the lighting (such as the indicator of glare and discomfort), which in turn will lead to an even greater expenditure of time on the calculation.

Calculation program

If someone wants to check my calculation results in DIALux and get completely different data, then this is most likely due to the operating coefficient * that was specified in the article and was 1. By default, this factor is equal to 0.8 in DIALux and cannot be more than 1. In fact, the operating coefficient is the inverse of the safety factor. In turn, the safety factor ** can take on values ​​between 1.2 and 2.0 ib, based on this, the operating ratio cannot be greater than 0.83.

* the coefficient of exploitation has recently begun to be indicated in Russian norms.
** the safety factor (as well as the operating coefficient) determines the influence of the medium on the bright devices.

But then how, with the same data on the first room, did I get fewer lamps than in the original article? It's all just the fact that the lighting devices in DIALux have such a parameter as a correction factor. It is needed to change the characteristics of the lamp. Unfortunately, there is no indication in the article about this, so the lamp is left unchanged.

Well, back to the question of the shape of the room and the presence of various objects in it. If you even have a rectangular room, but there are objects in it that can affect the calculation (store and storage racks, columns, racks, etc.), then the manual method disappears completely, since It is impossible to correctly calculate the reflection from objects. Accordingly, the more such objects in the room, the more it will be necessary to increase the number of lamps and / or change their location / type. Let me give you a vivid example:


Pic.3

In Figure 3, an empty room with dimensions of 20x15x4m. Illumination 175lk on the working plane 0.8m


Pic.4

In Figure 4, the same room, but with columns and racks, as well as with the ceiling going under a slight slope. The number of fixtures saved intentionally to show the discrepancy in illumination. As you can see, the value is not close to 175k, but it also departs from the previously accepted value of 150k. But if we assume that this is, for example, a store, then different zones should be lit differently and the number of lights will be completely different.

Conclusion

Using the manual method gives a very rough estimate, which in the future may entail undesirable costs for the replacement of lighting equipment. What ultimately will lead to a sad consequence for the designer.

What to see and read

• My channel about lighting design in DIALux www.youtube.com/user/T0L1Kit/videos;
• Reference book on lighting ed. Yu.B. Eisenberg 3rd ed. persrab. and add. M .: Sign. - 972 s: il. expertunion.ru/knigi/spravochnaya-kniga-po-svetotehnike-ayzenberga.html;
• Forum and website for lighting engineering and lighting design expertunion.ru;
• Forum and website with a set of models and plug-ins for DIALux www.dialux-help.ru .

PS And yet the render in DIALux even with standard textures and models has changed for the better.

PPS For those who are interested, you can download the project file and the lamp: www.dropbox.com/s/qvuhmkpv1vwea8f/Projects.zip?dl=0