LdG-4 DDR and Beam angle of light

I am introducing the DDR (Diameter to Distance Ratio) as a quick guide in this LdG-4 (Lighting Design Guide-4)


A lot of beam angles for luminaire/lamps are available for architectural lighting, we can find a lot the terms Narrow Spot spot , super spot , flood , wide flood ,narrow horizontal flood, vertical flood …etc but how can easily use the right beam angle ?

lets find out more about the proposed DDR and how helpful could be for selecting quickly the right beam angle.

Beam Angle

Before looking at the application of the beam angle, let’s see what dose beam angle mean? and how it is calculation

The beam angle is the degree of width that light emits from a light source. In other terms this is the angle between the two points on the beam axis where the intensity drops to 50% of its maximum.

Beam angle
Beam Angle is the angle of the light between two point of 50% of Maximum intensity

but why the beam angle is using the angle of light where intensity of light drops to 50% only.

Because after Imax  the light will fade and will not be part of the highlight, the light after the Imax is spill light. and the human eye will see mainly the beam angle.

Beam Angle DDR

The beam angle is main highlight that seen by eye, spill light can be calculated or measured by not part of the main highlight

Beam Angles

there are many different beam angles with different names, the most common angles are


while some manufacturer call the 5° Narrow spot some other call the 15° Narrow spot, as there is no standard for the name I am going to use the ERCO beam angles name in this tutorial which are as follows

Narrow Spot 7°

Spot 14°

Flood 28°

wide flood >45°

this system is easy to remember as it start form the 7° then 2×7=14° , then 2×14 =28°

what is good about this system that is unique in ERCO range indoor or outdoor, if you select any lumianire in Narrow Spot it will be 7°±2,Spot will always be 14°±2 and flood will be 28°±2

The Cone Diagram

the cone Diagram is very helpful and provide useful data for designers

Spot Cone Diagram LdG_4
The Cone Diagram shows the diameter of light at a certain height with the lux value at that height, in this cone you can see the diameter of this 16.2 degree (spot beam)  is 0.85 meter at 3 meter height and lux in the center is 585 while lux at edge of beam is 286

the cone diagram is provided by the manufacture of generated by lighting design software like DIALux , relux , AGI32 …etc when you use the photometric file e.g IES or Eulumdat

some manufacture provide table not the cone like the one bellow but it has similar information

Table cone dia ERCO LdG-4.png
the table shows height diameter of light and lux @3 meter the diameter is 0.84 m

The relation between Diameter , Angle and Distance.

The closer the beam angle to a surface the smaller the diameter is and the higher the lux is.

NS FC Ldg-4 Narrow spot rendering.jpg
Using same spotlight at different distance , the diameter get bigger when the spot is moved away from the surface.


NS FC Ldg-4 Narrow spot_bar
Also the lux will be higher for the closer spot to the surface

if two spotlight with same lumen but with different beam angle are used at the same distance to the surface then the bigger angle will have bigger diameter and less lux.

NS FC Ldg-4 Narrow spot rendering spot vs narrow spot2.jpg
two spot with same lumen but different beam angle

the cone diagram is a good tool to see the relation between beam angle , diameter of light and distance, however during the design to avoid looking at many data there should be an quicker way.

The Magical Number 0.018

to know the diameter of light without getting back to the cone diagram there is a quick formula

Diameter of light = 0.018 × beam angle × the distance

for example if you need the diameter of light for a spotlight of 14° @ 3 meter distance then


this formula is correct for beam angle up to 50°

DDR(Diameter to Distance Ratio)

the 0.018 is quite handy but I am using more handy way which I would like to share with you.

I am proposing the  DDR (Diameter to Distance Ratio)

this DDR simple relation between different angle and diameter of light for famous angles


from the above chart if you want to use narrow spot beam with 6° @ 7 meter the diameter will be =0.1×7 = 0.7 meter.

The useful 4 DDR

the one tenth , quarter, half and 1 are very useful to select the right beam angle very quickly.

if you want to highlight an object with onetenth (¹/10) the distance then use

DDR1 =0.1→ beam angle Narrow Spot around 6°

in dont really require the exact beam angle, ±2° should be fine

¹/10 the distance  →Narrow spot 7°±2

¼ the distance →Spot 14°±2

½ the distance →Flood 28°±2

full the distance →wide flood >45°

Beam Angle DDR one tenth.png
The useful 4 DDR : one tenth,quarter  , half and 1 Diameter to Distance Ratio

Indoor Application of the DDR

lets say you want to highlight a flower vase on a table in a room height of 3 meter

the distance between the spotlight on the ceiling and the flower on a table 2.25 meter

DDR 1 narrow spot flower.png
highlighting a flower vase @2.2 meter distance with DDR=0.1

then the 0.1 will give a diameter of 0.22 meter which should be good, of course the lighting will fade out and will not stop at 0.22

¹/10 the distance  →Narrow spot 7°±2 for this flower vase

Another example if there is a niche in the wall with 1 meter and you have small item lets say 30 cm and the light to this item is 0.6 meter then you need diameter that is half of the distance

niche in the wall lighting spotlight beam angle DDR ldg 4.png

because 30 cm is half of the distance then you need ½ the distance beam angle which is flood.

½ the distance →Flood 28°±2

these 4 DDR are useful to know the Diameter of a beam angle @ certain distance in a very quick way .

after selecting the closest beam angle then you need to select the required lumen and intensity of light, for good accent lighting , it would be advisable to use minimum 5 times the general lighting e.g if you have 200 lux as general lighting then 1000 lux or higher would be appropriate to have good contrast between the highlighted object and the background.

ERCO flower
the ratio between general and accent lighting should be not less than 5 times to establish good contrast.

Outdoor Application of the DDR

if a  crown of a palm tree with 10 meter height need to be highlighted by using two uplight , lets say the crown is 2 meter wide then each uplight will have to cover half of the crown which is 1 meter @ a distance of 10 meter then DDR=0.1 would be suitable for this application

Palm tree beam angle uplight 2
Two uplight for a 10 meter palm tree

¹/10 the distance  →Narrow spot 7°±2

anther example if you are highlightng a tree where the part of crown to be highlighted is half of the distance then ½ the distance →Flood 28°±2

DDR flood for tree.jpg
Flood with DDR=0.5 where the part of the crown is half of the distance
Each size and distance require certain DDR


  • The DDR works well when the light is perpendicular to the surface, once there is a tilt then the beam will changed especially when the tilt is @steep angle, but would be useful in general.
Narrow spot tilted
Narrow spot when is tilted at Steep angle becomes more elliptical.
  • It is always good to check the final results with a design software like DIALux.
  • The design examples are not necessarily the best option for design, they are only examples and a designer should not be limited by by ideas.
  • these DDR mainly for accent lighting for other type of lighting like general lighting there is an industry standard which is called the SHR (Space to hight Ratio)
  • these 4 DDR are the most common but there are many other beam like oval beam.

if you want to see more applications of thses beam angles then you would probably be interested if you see the LdG-2

Lighting design LdG + LdC Page

LDG and LDC-2

if you are interested to learn lighting design software DIALux evo then visit DIALux evo Tutorials DLX-T page

DIALux evo Tutorials-DLX-T 

21 thoughts on “LdG-4 DDR and Beam angle of light

  1. شكرا باشمهندس لقد أمتنا كثيرا وربنا يزيد من فضله وعلمه

    Liked by 1 person

  2. Good article Ezzat, as always!

    Liked by 1 person

  3. Very good and helpful explanation, waiting to see more from your side.

    Liked by 1 person

    1. Thanks Khalid


  4. Michael Wenceslao February 28, 2016 — 2:33 pm

    Very informative and useful.

    Liked by 1 person

    1. Thanks Michael


  5. Your job and dedication makes me learn and grow up. Thank you so much fro Guatemala

    Liked by 1 person

  6. Great work, keep it up

    Liked by 1 person

  7. Very informative thanks a lot

    Liked by 1 person

  8. WOW! WOW! WOW! This is exactly the article I have been searching .

    My company is a LED lighting fixture manufacturer selling mostly to interior designers. We do provide complete photometric data and IES file for each of our lighting fixtures. Still, the most common question they have is how to position and space the lights.

    Most small retail and residential interior design projects do not have the budget to hire lighting designers, but the interior designers should at least know how to use DDR cone diagrams.

    With your permission, I like to translate this article into Chinese and use the photos to make a youtube video in Chinese and English.

    Liked by 1 person

  9. thank you so much for your nice Explanation

    Liked by 1 person

  10. I am doing an introductory lighting course, six months by assignment work, this article really helped. Thank you for taking the time to share your knowledge, I love seeing the internet being used in really positive ways.


    1. Where are you doing this course? i am looking for short courses on lighting and i live in India. Thanks.


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