Lumens, lux and all that

[CC45]

Hi all,

 

I've wired upstairs bathrooms (3) with Enlite E5 led lamps - all good except possibly the main bathroom which is on the bright side (dimmer switch will sort that out).  I have 3 rooms downstairs to do now - kitchen, office & utility.  I am keen to inject a bit more science into the calcs here.

 

So I know the lumens & angle of the Enlite E5's and room / desk / worktop heights but what I can't seem to get sorted is how big the splay is on the floor / desk / worktop.  I know its a cone of light of various depths (floor / desk / worktop) - does anyone know what the formula for this is?  I've found one or two sites that work out the splay but they use their led's and I'd like to get it onto a spreadsheet so I can look at various manufacturers alternative bulbs.

 

Thanks for any light you can shed on the required calculations.

 

CC

[Ferdinand]

I assume splay means the dimensions of where the cone of light will ‘land’? 

 

Bit what do you want to know?

 

Working that out should be mainly 1-2-root3 ie 30, 60, 90 degree right angled triangles if you are using lights with a 60 or 120 degree cone, and they are mounted pointed down.

 

So the dimension you need as input is light source to incident surface vertical distance, and you can work out the horizontal distances and where the light falls from there using ratios.

 

I am sure someone can link to a guide .. cannot draw on my iPAD.

 

F

[Ed Davies]

https://www.downlights.co.uk/aurora-enlite-e5-led-downlight.html says beam angle 60°.

 

https://www.electrical101.com/beam-angle.html says beam angle is right across the beam (not from the centreline to the edge, I wasn't sure).

 

So half-width of the beam is 30°. If d is the diameter of the spot and h is the height of the light above the surface then

 

tan 30 = 0.5d / h

 

d = 2 × h × tan 30 = 1.1547 h.

 

In other words, for each metre the surface is away from the light the spot will be just over a metre wide.

[CC45]

Thanks chaps.

 

Finally got it to work.

 

Since lights are 60deg I used the 30-60-90 triangle (thanks @Ferdinand) to work out the spread of light made by each led on the floor. I'd long forgotten all this stuff.

 

Works a treat - I can now work out lux from lumens and see how bright the office & utility lights will be.

 

Off to tart up the spreadsheet.