Risks of LED Lighting – Part III: More on LM Losses

Why Don’t Manufacturers Want to Talk About LM Losses?

We received dozens of comments about our lumen maintenance loss article, often with a recommendation of a useful information source. In more than one instance, readers referenced Philips Greenpower LEDs, and a provided a link to a 39 page brochure with the tagline ‘there’s more to light’.

We searched for lumen maintenance references in the brochure. After all, the readers posted the link as an anecdote to an article on LM losses. Here’s what we found:

The brochure mentions light recipes (13 times), plant growth (12 times), increased results of some type (12 times), light intensity levels (5 times), and long life (3 times).  There was no mention of LM80 testing, Lx ratings, or TM21 estimates of useful life. 

So we checked the Philips Greenpower specification sheet for details of lumen maintenance losses, and there it was. An L90 rating of 25,000 hours. There is no L80 or L90 ratings. There is no TM21. There  is also no indication of the drive current or testing temperature – both of which had a material impact on LM losses. Why not?

We checked with two of Philips closest competitors. Neither publish LM related information on their horticulture light specification sheet. We probed a little deeper with one of the two and found a spec sheet for a non-horticulture light that uses the same basic LED component and configuration. There we found a single LM-related data point – L70 point is greater than 75K hours at 40ºC.

To make comparisons, we thought we would chart the two data points along with TM80 data for a high quality Cree LED component which had the most detailed dataset. Take a look at the graph below and see if you can tease out any meaningful information.

Lumen Maintenance Data

If it seems like an apples-and-oranges comparison, you’re right. Philips provides an L90, but nothing more. The competitor an L70, but no L90 to compare. And perhaps most confusing – the competitor uses a test temperature that is lower than the prescribed minimum of the IES testing standard. This is problematic. Why? Fixture temperatures will vary based on variables outside of the manufacturers control – how closely the fixtures are arranged to each other, airflow around the fixtures, the consistency of cooling the building, etc. So long as both manufacturers use the same standard test temperatures, the local variances will be roughly the same for both fixtures (like a basketball team both playing on 10′ baskets). In this case, however, the competitor is supplying a figure that would have to be considered to be ‘in the best possible circumstances’ (like a basketball team playing on 8′ baskets). You just can’t compare the two.

So what is the takeaway?

  1. Comprehensive, standardized LM data is almost never included in specification sheets, even though that is the purpose of LM80 testing.
  2. Horticulture LED Manufacturers don’t want to talk about LM losses.

So why not? We think the relative differences between manufacturers is large enough to have a greater (negative) impact on total farm production over time than the relative differences in ‘light recipes’. In other words, the buyer should give equal or more weight to LM loss differences than to light recipe differences because the former will have a greater impact on their economics. Unfortunately, you won’t get that information unless you demand the data and do the math yourself.


One thought on “Risks of LED Lighting – Part III: More on LM Losses

  1. First of all, I am pleased to be accepted to the group. Concerning lumen maintenance, I can tell you that there has been far from concensus in the industry on how to specify and test. While I have been “out of the game” for a couple of years, I was previously the Chairman of ANSI C82 and Deputy Technical Advisor to the US national committee to the IEC where lumen maintenance was a “hot topic” while I was there. ANSI followed and even gave some guidance to the IES standards development on L80. Cree was a major contributor. At the IEC though it was a different story. Some companies (including at least one European one) argued strongly for much shorter testing periods and there was also much debate on multiple “L” level specifications as a function of temperature, etc.. I doubt if this has even now been resolved.

    A big issue is that unlike other lightsources (fl, HID, etc.) where there can be a dominant aging factor, LEDs have multiple aging elements – yellowing of bonding materials and lens materials, outgassing of same, and others that can each have a dominant effect depending on the temperature. A plastic may be completely stable at one temperature but age dramatically at a few tens of degrees centigrade higher.

    Horticulture has a much more limited operating temperature range than some. Outdoor and industrial can be -40 to +55C fixture ambient for example compared to something on the order of 10-40C for horticulture. Lightsources in outdoor and industrial can also be enclosed within the luminaire raising temperatures even higher. For these reasons, it may be valid to specify a L90 at a lower temp as horticulture has a more critical lumen depreciation requirement and has a more restricted operating temperature range. General lighting traditionally has used L70 and sometimes even L50 as the human eye can tolerate much larger differences in light than plant growth can

    In any case, testing for LD is at least a six month affair and can take much longer of lifetimes of greater than 25khrs is involved. Often the lifecycle of an LED design passes faster than the lifetest as this is a continually evolving field.

    For those that are really interested, you might want to join the US TAG for the IEC, or IES, or ANSI, to follow the up to date. Membership is about $500.

    Anyway, that’s my 2 cents worth.

    Bob Erhardt


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