LdG-5 Applying Luminance in Lighting Design

LdG-5 Applying Luminance in Lighting Design

Luminance vs. Brightness

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. 

 Luminance of car headlights brightness_cd

The Headlights of the two photos for the car has same luminance value but the brightness of the one at night is much higher.

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 users of some products like electronic display (TV, Laptop, mobile devices…etc.)

 Luminance of  smartphone_luminance cd

The Unit of Luminance

Luminance can be measured in cd/m2

to have a feeling of this metric, different values in the following tables are mentioned.

 

Typical  Values of Luminance for Natural Light Sources

Image

Source

Luminance values

 Luminance Sun at noon cd m2

Sun at noon (not be seen by human eye directly)

1.6 x 109 cd/m²

 Solar disk at horizon luminance

Solar disk at horizon

600,000 cd/m²

 Average clear sky luminance

Average clear sky

8000 cd/m²

 Full Moon surface luminance

Full Moon surface

2500 cd/m²

 Average cloudy sky Luminance

Average cloudy sky

2000 cd/m²

 Darkest sky Luminance

Darkest sky

0.0004 cd/m²

 

Typical  Values of Luminance  for  Artificial Light Sources

Image

Source  

Luminance values

OLED Luminance

OLED

4,000 cd/m²

LED Traffic Signal Luminance

LED Traffic Signal

Red       5,000 cd/m²
Green 10,000 cd/m2

Fluorescent T8 T5 HE T5 HO  Luminance

T8 cool white fluorescent

11,000 cd/m²

T5 cool white fluorescent High Efficiency HE

17,400 cd/m²

T5 cool white fluorescent High Output HO

26,726  cd/m²

 Compact Fluorescent lamp Luminance

Compact Fluorescent lamp

30,000 cd/m²

Frosted bulb 60 W Luminance

Frosted bulb 60 W

120,000 cd/m²

Tungsten-halogen lamp Luminance

 Tungsten-halogen lamp

39,000,000

 

Typical  Values of Luminance for Smartphone and Displays

Image

Source  

Luminance values

 Iphone smartphone luminance brightness

IPhone 5

Max value 500 cd/m2

 Laptop and TV LCD Display luminance brightness

Laptop and TV LCD Display

250-400cd/m2

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.

 High luminance Luminaire
 Low luminance Luminaire
 

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 30o when seating and up to 45o when standing.

Normally we don’t look straight in ceiling in most of our task.

 normal visual field of a human in a space_seating

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

 normal visual field of a human in a space_Standing

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

 visual field of a human basket ball luminance

Zone of Glare in the luminaire

The angle between 45o and 85o 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 65o and above.

In some critical application sometimes this is taken at 55o

 Zone of Glare in the luminaire

The nadir is zero which is perpendicular to floor.

Luminance form 65o 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 55o and above.

 Zone of Glare in the luminaire_02

 

Luminaire Luminance Classification  

Luminance class

Average luminance

Considerations:

 Dark light reflector luminance luminaire

A

< 1000 cd/m²

The luminaire has a low average luminance and can be used in rooms with high anti-glare requirements e.g. offices with ordinary monitor work.

 cross baffle reflector luminance luminaire

B

1000–3500 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.

 Opal downlight luminaire luminance

C

3500–5000 cd/m²

The luminaire has a relatively high average luminance and should therefore be used in bright surroundings to avoid glare.

 chandelier lumiance

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.

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 65o,75o,85o

The diagram presented in red in the side image represents the 65o and shows a value less than 100 cd/m2 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/m2 depends on the situation you can check the local standard

recetllay standard accept a luminance up to 1500 cd/m2

 Luminance diagram

 Luminace values 65 55 75 85

 Wide beam

By the way in DIALux you can add other angle like 55o form the page setting of the luminaire data

 DIALux evo page setting

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

 Luminance diagram oval flood

 oval flood

The C angles represents the different plane around the luminaire while gamma represents the light disruption in each plane

 c gamma angles of luminaire

Luminance of surfaces

Remember what we see is luminance cd/m2+

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/m2

If 500 is reflected by a black surface will give around 10 cd/m2

So we see the light much brighter on a white surface.

The formula

 Luminace of Surfaces

 luminance lux formula

The lux value in the next image is the same but we see the ball brighter because the luminance is higher.

 Color luminance

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

 luminance wall

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!

 false color for lux

 lux preso false color rendering values

Luminance values cd/m2

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.

 false color for luminance cd m2 presdo

 luminance  presdo false color rendering 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.

 Wallwash luminance

Switch to luminance in DIALux evo for better evaluation!

 DIALux evo luminance

 switch to luminance dialux evo

Lighting design LdG + LdC Page

LDG and LDC-2

if you are intersected  in  learning lighting design software then you can visit this  page

DIALux evo Tutorials

11 Comments

  1. many thanks for sending mee thi valubale information, i’m an electrical engineer from northern Iraq   let be in touch  thanks again

    Kamaran Aziz Ali Mobile Phone 009647701574928

    ________________________________

    Like

  2. Dear Mr Ezzat,
    At the outset, thank you for the valuable information…on the topic – people usually take it very lightly….
    pl send more details / ppts / information on LIGHTING to my email id …
    I wish to know your personal opinion on LED Lighting…
    Thanking you once again
    yateen mujumdar

    Like

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