Shedding Light: How to Produce a Professional Lighting Design – monthly column featured in Sparks Electrical News (Crown Publications)

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At the conclusion of my article last month, I said that I would explain how EVE (Equivalent Visual Efficiency) was calculated and that I would also explain more about how this can be included in your calculation manager in Relux Desktop to ensure that your calculated illuminance levels will be correct. Of course, there are a number of other settings within Relux Desktop that must also be correct to be certain that your illuminance results in your lighting design report will be correct.

Before I do that, I need to explain how the EVE factor is calculated. Remember that a photometer, or what some call a lux meter, only measures photopic light or the light for daytime vision. There is, however, also the need to measure scotopic light, which is our night-time vision. The problem is that a standard photometer cannot measure for scotopic light.

A spectrometer is used to measure both photopic and scotopic light. Once the measurements are known for a particular light source, whether it is fluorescent, mercury vapour, LED or any other type, the scotopic/photopic (S/P) ratio can be calculated.

Once that has been calculated, we are able to use tables in the IES TM-24-13 document to determine the EVE, which will also vary according to the Correlated Colour Temperature (CCT). In other words, a warm white or 3000 K LED light will typically have an EVE of 1,1; a 4000 K LED light would have an EVE of 0,88; a 5000 K would be 0,82 and a 6000 K would be 0,75. The values are for high quality LED products and can vary from product to product depending on the quality and the consistency of the CCT measurement.

Once we have determined the correct EVE for the CCT that will be used, it is time to refer to the applicable standard to be used for an office environment activity.

Assuming that the primary task area on the workstations is to be 300l lux, we then refer to the tables in IES TM-24-13, which apply to areas where visually demanding tasks are performed. For example, a 4000 K LED is used and the age of the occupants is between 25 and 65 years, the IES category is Q for the application and the target illuminance value should then be 340 lux. Now, multiply 340 lux by 0,88 (EVE factor) = 299,2 lux, which is bang-on to the illuminance value in the standards.

The working area immediately adjacent to the primary task work plane is referred to as the secondary task area. It should be 60% of the illuminance value of the primary task area, which would be 207 lux and all other areas should be no less than 100 lux. The same EVE factor should be applied to these target illuminance levels as well.

Remember that these illuminance values for the primary and secondary task areas must be applied to every workstation in the office. This has to be applied in the lighting design software being used. Not all lighting design software can be set to perform these calculations. We recommend using the latest version of ReluxDesktop software.

The key is being able to measure the accurate S/P ratio of the LED light source/luminaire being used. A spectrometer is needed to do that. Spectrometers can vary in price from around R15 000 to R80 000. Essentially, to be able to produce professional lighting designs, three critical criteria must be met:

  1. Must have a sound knowledge and understanding of light and lighting, including standards and regulations.
  2. Must have a very high level of proficiency in the use of lighting design software to be able to include the EVE factor in all lighting calculations.
  3. Must have knowledge and understanding of the correct techniques to be used to measure light with a spectrometer. This includes the recording of all measurements as part of the Measurement and Verification process.

Lighting design is challenging and far more complex today than it has ever been in my more than 50 years’ experience.

Next month, I will conclude the series by explaining Circadian Light (CLA) and Circadian Stimulus (CS) and how to measure these including incorporation of these measurements in lighting design software.

I will gladly assist any of the readers who would like more details about this topic and methodology.


BHA SCHOOL OF LIGHTING – 11 October 2021

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BHA School of Lighting

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