How to measure light output – the question

The question seemed so simple… How to measure light output from an LED grow lamp?

A lamp radiates energy, hopefully more visible light than anything else. For example not heat – compare: incandescent bulbs convert approximately 2.2%, at best 5%, of energy into light – en.wikipedia.org/wiki/Incandescent_light_bulb – versus LEDs with an approximate efficiency of 30%. And how does an LED lamp compare to a High Pressure Sodium (HPS) lamp for example? Time for a journey down the rabbit hole…

This will be a multi-part post because there’s a lot of ground to cover. I have a couple of years of university undergraduate level physics under the “belt”, will certainly admit to not being a lighting engineer, and yet I really don’t recall covering any of this material during said physics lectures… So this is my best interpretation of the theory and practice. Thus to start at some basics.

“Visible” light has a wavelength between 400 and 700 nm, give or take a bit. Ultimately it’s all electromagnetic radiation, for example with radiated heat having a longer wavelength, in the infra-red (IR) range – beyond 700 nm. Ultraviolet light (UV) – the kind that can cause sunburn – has a shorter wavelength being less than 400 nm. (So an incandescent bulb is simply a good IR radiator, but not so good for visible light.) The sensitivity to light for the “average” human eye is below, per the CIE 1931 Luminous Efficiency function:

At the low end, the shorter wavelengths, there are the blues. At the high end, the longer wavelengths, there are the reds. Ultraviolet colours are off the shorter wavelength end and infrared colours are off the longer wavelength end. Basically humans are very good at perceiving mainly green’ish colours, though certainly not limited to such.

In comparison, an HPS bulb produces the following spectrum. And hopefully it is somewhat obvious why such lamps produce an “orange” glow.

What about LEDs? Specifically the “full spectrum” types, for which these posts will focus on. It depends on their temperature, being a measure in Kelvin (K), representing what a black body radiator of a certain temperature would produce. For more information on black body radiation/radiators see en.wikipedia.org/wiki/Black-body_radiation. Below are spectrum outputs for three different temperatures: 3000 K, 5000 K and 6500 K. These are taken from the datasheet for the Cree MHD-E series LEDs.

3000 K:

5000 K:

6500 K:

So far this doesn’t address how much light but rather what type of light. Both are/can be relevant and depending on the exact need. Until the next post.

By | 2017-10-18T10:12:22+00:00 Sunday, 11 June 2017|Lighting|0 Comments

About the Author:

Colin is the chief designer at lagrangianpoint. His interests are in (too) many areas - to name a few: the Arduino platform, LED lighting, Software Defined Radio, statistics, trying to learn French, cooking and wine. Unfortunately lagrangianpoint isn't his day job just yet - but it's the plan. In the meantime he is Clinical Operations Manager for Cochlear EMEA. (Cochlear specialises in implantable hearing solutions for adults and children who are affected by deafness or hearing loss - check out www.cochlear.com)

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