Using a Quantitative Measure to Achieve a Qualitative Lighting Design Effect! Introduction What is Luminance?

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1 Using a Quantitative Measure to Achieve a Qualitative Lighting Design Effect! Introduction What is Luminance? Luminance in a simple language is the measurement unit of brightness. In realty brightness can never be measured as it is subjective form situation to another and perhaps form on person to another as well. If you are coming from a dark area to another brighter area then you would see it not bright no matter how powerful the luminance is. Take example as the head light of the car where during night time appears bright, however in daytime is less bright even though the luminance of the headlight is in the same value. The Headlights of the two photos for the car has same luminance value but the brightness of the one at night is much higher. 1

2 Because brightness cannot be measured then we use a different term, however a brightness is used some time to make this term easy to understand for end user of some products like electronic display (TV, Laptop, mobile devices...etc.) The Unit of Luminance Luminance can be measured cd/m 2, and to have a feeling of this metric, different values in the following tables are mentioned. 2

3 Luminance for Typical Natural Light Sources Image Source Luminance values Sun at noon (not be seen by 1.6 x 10 9 cd/m² human eye directly) Solar disk at horizon 600,000 cd/m² Average clear sky 8000 cd/m² Full Moon surface 2500 cd/m² Average cloudy sky 2000 cd/m² Darkest sky cd/m² 3

4 Typical Values of Luminance for Artificial Light Sources Image Source Luminance values OLED 4,000 cd/m² LED Traffic Signal Red 5,000 cd/m² Green 10,000 cd/m2 T8 cool white fluorescent T5 cool white fluorescent High Efficiency HE T5 cool white fluorescent High Output HO Compact Fluorescent lamp 11,000 cd/m² 17,400 cd/m² 26,726 cd/m² 30,000 cd/m² Frosted bulb 60 W 120,000 cd/m² Tungsten-halogen lamp 39,000,000 4

5 Typical Values of Luminance for Smartphone and Displays Image Source Luminance values IPhone cd/m 2 Laptop and TV LCD Display cd/m 2 5

6 Luminance vs. Brightness How luminance could be useful for architectural lighting Luminance if very useful, as a matter of fact I find it much more usful for architectural lighting than Illuminance (lux) I use luminance to evaluate two values 1. Luminance of Luminaires (how bright luminaires in the space) 2. Luminance of architectural elements (Walls floor and ceiling) and interior elements like furniture and objects. Luminance for Luminaires and light fitting Luminaires could have high or low luminance, depends on the design of the luminaire. Usually the lower the luminance the less efficient the luminaire is, but absolutely it has a higher visual comfort You could see that the above right picture has a high luminance glary luminaires while the lower one has a very low luminance hence very good visual comfort. 6

7 How to evaluate the luminance of the luminaire To evaluate the luminance of luminaires, we have to understand the normal visual field of a human in a space. A standard human will look in an angle up to 30 o when seating and up to 45 o when standing. Normally we don t look straight in ceiling in most of our task. As long as the task doesn t require looking into ceiling like swimming at back or sleeping on a bed then this measure is useful When playing a basketball a human will have to look up in unusual visual field to most application and the lighting in this case has to have a luminance suitable for this application 7

8 Zone of Glare in the luminaire The angle between 45 o and 85 o is considered a critical zone where light has to be limited for many applications; however in reality liming the light too mush could end up using many luminaires. So the standard is usual considering an angle of 65 o and above. In some critical application sometimes this is taken at 55 o The nadir is zero which is perpendicular to floor. Luminance form 65 o and above should be limited for most application where the ceiling height is equal or less than 3 meter, otherwise in higher ceiling, it is good to consider 55 o and above. 8

9 Luminaire Luminance Classification Luminance Average class luminance Considerations: A < 1000 cd/m² The luminaire has a low average luminance and can be used in rooms with high antiglare requirements e.g. offices with ordinary monitor work. B cd/m² The luminaire has relatively low average luminance and can generally be used in most situations. Against a light background the risk of glare is small. C cd/m² The luminaire has a relatively high average luminance and should therefore be used in bright surroundings to avoid glare. D > 5000 cd/m² The luminaire has a high average luminance. The risk of glare can be large even if the luminaire is used in very light surroundings. The luminaire should therefore be avoided in rooms within the normal field of vision. 9

10 How to find out the luminance of a luminaire if you have a photometric data like ies ldt or ULD then you can use it in lighting software like DIALux evo You can easily, find out in the documentation a luminance diagram which shows you the values at three standard angles 65 o,75 o,85 o The diagram presented in red in the side image represents the 65 o and shows a value less than 100 cd/m 2 which considered really low. The diagram is circle which means the luminance from all angle of luminaire has the same values. Like a downlight Usually for an office application this should be less than 1000 cd/m 2 depends on the situation you can check the local standard By the way in DIALux you can add other angle like 55 o form the page setting of the luminaire data When a luminaire has a different distribution at c angles then the values is different like the side image. To better understand this diagram it would be good to review the C Gamma angles of luminaires 10

11 The C angles represents the different plane around the luminaire while gamma represents the light disruption in each plane Luminance of surfaces Remember what we see is luminance cd/m 2+ So for a qualitative measure this is the metric of light to be evaluated When 500 is reflected by a white surface this will give a luminance of around 120 cd/m 2 If 500 is reflected by a black surface will give around 10 cd/m 2 So we see the light much brighter on a white surface. The formula The lux value in the next image is the same but we see the ball brighter because the luminance is higher. 11

12 Lux could be misleading, to explain this let s take a corridor like in the side image. The floor is dark while the walls have a white color. When we calculate this room in DIALux using a double washlight to wash both walls of the corridor. We see following values Illuminance values (LUX) If we look at the false color we see that the floor has lux of around 400 Lux while the walls have around 250 Lux. But are we really going to see the floor brighter! Of Corse not! Luminance values cd/m 2 12

13 Luminance vs. Brightness By looking at the luminance we see the wall have around 80 cd/m2 while the floor will have only 30 cd/m2 so this is really mush better metric to understand brightness In DIALux you have the option to switch between the two values. Remember luminance is what we see and that is why I find it more important than lux My recommendation use both in your evaluation of lighting design and planning. Switch to luminance DIALux evo for better evaluation! 13

14 If you are not familiar with DIALux evo, then I would recommend you to start with the following tutorials DIALux evo Tutorial-4 Creating a complete project from A to Z with DIALux evo DIALux evo Tutorial-5 Creating a complete interior and exterior Project 14