How to calculate luminous intensity [cd] in Dialux Evo

ILP Guide - light pollution - obtrusive lighting

Whoever tried to comply with ILP guide - table 2 knows that several different calculations needs to be undertaken. Even though many of them are pretty straightforward I personally struggled with one specific - Maximum intensity of a luminaire in a designated direction [I].

The project is located in the City of London - next to the Southwark bridge and opposite of listed building. Planning authority had a concern that our external lighting will affect the listed building and we needed to comply with ILP obtrusive light guide.

The complication is that we didn't have any downlights, all we had was coffer ceiling - indirect lighting - so manufactures luminance diagram wasn't very helpful.

Luminance diagram

Luminance diagram

In the ILP guide luminare intensity is explained as follows:

Luminaire Intensity
— This applies to each luminaire in the potentially obtrusive direction, outside of the area being lit.

Lumen vs Candela

Measuring light aim

Measuring light aim

A light source, like the filament of an incandescent bulb, emits light in all directions. Effectively, it sits at the center of a sphere of radiated light. The total energy of all the light given off is termed the “luminous flux” or lumen.

But when you consider a projector, it is clear that it lights only in one direction: forward. That is why we need to know how the luminous flux is distributed into every space dimension, using the definition of light intensity.

The fundamental unit of light is the candela, nominally the light given off by one candle. Light intensity (l) is defined as the “luminous flux that is emitted per unit of solid angle (steradian) into a specific direction”.

Lux vs candela per square metre

The number of lumens falling on a surface is expressed as lux. One lux is one lumen per square metre, this relating brightness to distance from the source.

The amount of light falling on a surface is illuminance (lux), and the amount of light reflected off a surface is luminance (cd/m²).

I think Ezzat Baroudi explained about this subject perfectly so please check it out.

Dialux Evo Measurements

So how many candelas my coffer ceiling have from the typical viewpoint at the sensitive area (in our case, someone is walking by the listed building)?

Following the inverse-square law we can turn lux values to candle values if the distance is known.

Calculation point

Calculation point

We placed across the street calculation point angled at 75 degree, 1.7m from the ground. Other calculation surfaces are for other segments of the ILP guide table 2 so let's ignore them for now.

Create small cube 1.7m from the ground.

Under calculation objects tab choose calculation point -> select custom calculation direction -> place calculation object on the cube.

postupak1.jpg

Once you placed calculation point you can rotate it in desired direction.

postupak2.jpg

We achieved 1.72lx. Then the distance is measured from the side view:

distance.jpg

The distance is 16,81m. All we have to do now is to apply formula:

I=E⋅D² 

I [cd] - light intensity in candelas

E [lux] - illuminance in lux

D [m] - distance in metres

I = 1.72 ⋅ 16.81² = 486cd

Other calculations

The number that we got is in compliance during both curfew and post-curfew hours. Let me know in the comments (or on facebook) if you want a tutorial for other segments of ILP table 2.

I hope this was helpful :)