Dear sir,

For one of my project I want average 300 Lux at 2.8 m distance from ceiling for Room of ( 12ft X 12ft ).

I want to use high power LED of below specifications.

current ratings : 350mA constant current
Voltage drop : 3.4 voltage drop.
Viewing angle : 120 degree
lumen per LED : 122 at 25 degree ambient temperature



I have confusion to calculate LED Lux at particular distance .

I have found few methods to calculate Lux but I am not sure which one is right or more accurate.

please check below methods :



Method 1: By considering Viewing angle of LED .




I know that Lux = Lumen/ m2 , but I'm not exactly sure of how that
applies to something that is 2.1 meter away, and would be, say the size
of a small book.


The lux depends upon the size of the spot, and thus the illumination
angle. A wide-angle LED will produce a lower lux figure for the same
lumen.

To determine a rough lux value, divide the lumens figure by the area of
the LED's "spot".
E.g. for even illumination over a 120-degree (+/-60-degree) cone at 2.1 metre,
the spot radius will be 2.1 m*60*pi/180 ~=2.199 m , and the area will be ~15.1930 square metres,
so lux ~= lumen/15.1930= 122/ 15.1930 = 8.030 lux . --- ----- Ans 1

If the "size of a small book" refers to focusing the entire output of
the LED on a small area, then the only factor is the area on which you're
focusing it, not the distance (obviously, you'll need to narrow the angle
as the distance increases to keep the smaller area).

E.g. if you're focusing on a 10cm square, lux = lumen / 0.01, so you
would need 100 lumen to get 10000 lux.






Method 2: By Light Theory Law.




If I have LED with 122 Lumen at 25 degree junction temperature then
luminous flux = 122 lm
Luminous intensity ( I ) = 122 / ( 4 * pi ) = 122 / 12.56 = 9.71 candle
Lux at 2.1 meter distance = I / (d^2) = 9.71 / ( 2.1 ^2) = 9.71 / 4.41= 2.20 lux--------Ans 2






Method 3: By calculator provided in LED rise Website




pls check link http://www.ledrise.com/shop_content.php?coID=19

I get following lux value by calculator for same example as above.

so illuminous = 8.81lux ----------Ans 3




Answer I get in Methods 1 and 3 is somewhat similar but answer in method 2 is different .Now I am confuse Which method is accurate ?




Method 4:


E = N * luminous flux * utility factor (UF) * Maintenance factor ( MF ) / Area of room ( A )




Method 5 : By calculating total Lumen required




Now if I need 300 Lux level for room of (3.6576 m * 3.6576 m ).

here lux 300 (requirement )
Area = 3.6576 * 3.6576 = 13.37803 m^2



so by calculation E (illumination) = Lumen flux / area

total Lumen = E * A

total Lumen = 300 * 13.3780 = 4013.4 lumen

Her for our LED Lumen is 122 each at ambient junction temperature

so total LED required to achieve 4013.4 Lumen = 4013.4 /122
= 32.89 =33 LED








Answer I get in Methods 1 and 3 is somewhat similar but answer in method 2 is different .Now I am confuse Which method is accurate ?





Thanks & Regards

San