Estimating Watts/cm^2 from LED Specs

GregGreg

Newly Enlightened
Joined
Dec 15, 2015
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I'm trying to get a rough estimate as to the power in watts/cm^2 that an LED imparts to a surface a fixed distance away. For example, when I look at the specs of the BIVAR UV3TZ-405-30 LED, PURPLE, T-1 (3MM), 405NM, it lists 10mW for the typical Emitting Power. I've found various on-line calculators for converting candela to lumens, and lux to watts but I don't seem to have the right data and this is all very new to me. I get that the LED is projecting a cone-shaped beam of light that strikes a flat surface in a circular pattern. This size of the circle is based upon the viewing angle and distance from the surface. However, I get lost when reading about luminous intensity versus luminous flux and so on :shakehead

You guys sound like you really know what you're talking about so I'm hoping someone will take the time to help me out? I don't need an exact value. Anything close is good enough. I just want to know if there is a relatively easy way to either use the 10mW Emitting Power and the viewing angle, or to pick off a value for the Relative Luminous Intensity for a given amperage from the specification chart, to come up with a rough estimate for the real-life watts/cm^2 of power an LED imparts to the circular area defined by the viewing angle at a given distance away – like for example 5 centimeter away. I understand there are all sorts of tolerances and factors involved but I'm hoping there is a way to roughly estimate the actual power applied in a way that even I can understand?

LED: http://www.farnell.com/datasheets/1760165.pdf
Calculator: http://www.rapidtables.com/calc/light/lux-to-watt-calculator.htm

Thank you very much!
 

GregGreg

Newly Enlightened
Joined
Dec 15, 2015
Messages
2
Well, so far no one has ventured an answer. I'm guessing it's because either I haven't given enough information (the question is too vague) or it's just really difficult to estimate?

If I stand back and look at what I know, it's simple geometry to calculate the size of the circle of light that is projected onto a flat surface (the base of a cone) when the light is a given distance away with a given Viewing Angle. The formula is roughly:

Circular Light Area = [Distance from Surface * Tangent(Viewing Angle/2)]^2 * pi
For example, if the light is 0.5cm away and has a 30-deg viewing angle, then the circular area of light is
Area at 0.5cm = [0.5 * tan(15)] * 3.14= 0.056cm^2.

If I'm correct that the 10mW specification represents the total output of the LED, then there must be some percentage of this power that is contained within the viewing angle – some light projects outside of the viewing angle. It's a totally wild guess but maybe 75% isn't lost in factors like light outside the viewing angle, temperature, and what not. As such, the watts/cm^2 for this example would be
Power at 0.5cm = (.75 * 10)/0.056 = 134 watts/cm^2

This seems like a rather high value. Any suggestions on how I can improve this estimate or what I need to read to teach myself how to answer this question?

Thanks.
 
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