Office Lighting – Part I: Goals

This is the first of a two-part series on office lighting. Part I will be Goals, Part II will be How We Reach the Goals. This post applies to both new buildings and to existing buildings. It will focus on artificial lighting, not daylighting, which will be covered separately in the future. This pair of blog posts seeks to minimize artificial lighting, while keeping good and clear lighting for our work in offices.

And before we start, a short detour. I believe strongly that to save energy in buildings we need to move beyond a mistake we have been making for too long, the mistake of incrementalism, of saving too little energy. Our approach to saving energy has been like “Hey, let’s try save 5%, or let’s try save 10%, or let’s be even be real bold and try save 15%.”  Incrementalism, by definition, is small. When we incrementally save energy, we do so without pushing ourselves to save more. And, all too frequently, options for saving more energy are staring right at us. We can frequently save 50% or 70% or 90%, or more, of the energy of a particular building system. But we settle for less.  In the process, we are self-limiting the amount of energy that we save.  We are giving up energy savings. While saving a little energy, we are not saving a lot of energy.  We pat ourselves on the back for saving a little energy, but we are dabbling in greenwashing, rather than doing serious energy conservation. It’s a subtle but important issue, and is particularly relevant for office lighting. We are conveying to building owners that we can only save a small amount of energy, and that if they save this small amount of energy that they have done their bit – there is no more energy to save. And so we are doing a disservice to buildings. Incrementalism is often the foundation of federal, state and utility energy programs. Incrementalism is the foundation of LEED’s energy credits. The entire metrics, goals, program design, approaches, and technologies of these programs are structured around incremental energy savings, in other words small energy savings. Incremental energy improvements are also the foundation of the ENERGY STAR program. For many years and for many products, ENERGY STAR has been defined as delivering 15% energy savings, relative to current technology. It has been incremental by definition. So, as we start to discuss office lighting, one of the largest energy users in buildings, let’s keep in mind this issue of incrementalism, versus what we might call a transformational approach to saving energy, where we try to think outside the box and deliver savings of 50% or 70% or 90% or more.  It turns out the box is often not very large, and so it’s actually not hard to think outside of it.

Traditionally, we find offices with far too much artificial lighting, way overlit.  This is likely because lighting design has too often been done by rule-of-thumb (“hey, a small office, that gets two light fixtures”), and recommendations for light levels (illuminance) are inconsistent and have been far too high. Our over-illumination is also likely the result of having started way too high, with some published speculation that this was the result of lobbying by lighting manufacturers and utility companies in the U.S. In the early 1980’s, the recommended illuminance for offices was 70-90 foot candles (FC), almost twice as high as required in Europe at that time.  Even today, a major lighting manufacturer is still actively lobbying to increase required illuminance level in offices, even as emerging evidence and standards show that our spaces are artificially overlit. (A foot candle is the English-system unit of illuminance. If you take a candle into a dark room, and measure the light it gives off one foot away, you will measure approximately one foot candle. It’s cool. I’ve done it. Geek ecstasy. The SI unit is the lux, and there are about 11 luxes to a foot candle.)

The most authoritative source in the U.S. for recommended illuminance is the Illuminating Engineering Society (IES) handbook, which conveniently costs about $600, and which changes every few years. So…  in my experience, few building design professionals ever buy it.  IES recommendations are the ones that have historically been way high, 70-90 FC for office spaces in the 1980’s, dropping to about 50 FC in the 1990’s, and recently having dropped further. The current handbook (10th edition) has somewhat complex requirements for office lighting, that depend on the age of the occupants and other factors. I summarize these toward the end of this blog post.  My own reading of the requirements suggest that 30 FC is the new target illuminance for most offices, with even lower illuminance recommended for many types of office spaces.

Other organizations have illuminance recommendations that are all over the map.  The federal government’s construction arm, the GSA, requires 46 foot candles (500 lux) in its 2003 standards, last updated and unchanged in 2016, likely kept from the IES recommendation in the 1990’s.  Lighting manufacturers commonly provide illuminance recommendations, almost all of which are way too high.  Examples are 50-100 FC (Bristolite) and “over 50 FC” (Lithonia).  Makes sense, right?  More footcandles means… more lighting sales.  Best practice #1:  NEVER use illuminance recommendations from lighting manufacturers, distributors, or contractors. Their tables often date back over 30 years, when recommended artificial light levels were MUCH higher. OSHA, the federal agency that oversees worker safety, requires a light levels of 30 FC in offices.

With widely varying recommendations for illuminance, that have changed over time, and with lighting design so often done by rule of thumb (a nice way of saying “seat of our pants”), I believe the average office has an illuminance over 80 FC, ALMOST THREE TIMES BRIGHTER THAN CURRENT STANDARDS. If you want to check the illuminance in your office, let me know and I’ll let you borrow our lightmeter. And lightmeters are cheap, it’s almost worth getting your own.

Another thing to keep in mind is that the work we do has changed entirely in the last 30 years. Today, most of us doing office work are looking at lit computer screens. Not only do they provide their own lighting, which is in fact adjustable, but too much light is a problem, causing glare, reflection, contrast, and more.

I did a small survey of people’s happiness with light levels in a small professional office building. I varied the light levels, and asked people to identify light levels with which they were clearly comfortable, light levels with which they were clearly not comfortable (inadequate), and light levels that were too high.  11 out of 11 people in the study found a light level of 10 FC as being clearly adequate for them.  I’ll say that again:  100% of the people in my (as usual, admittedly small) study found 10 FC clearly adequate. That is close to 1/10 the illuminance that is found in most offices.  One tenth! 10%!  Two out of the 11 found a light level of only 2 FC (!) as being clearly adequate for them. That’s maybe 2% the illuminance found in most offices! Four out of the 11 people found a light level of 20 FC as being possibly too high. Recognizing that the need for light varies with age, the randomly-selected participants’ ages varied widely, from 25 to 70 years old, with an average of about 50, and a median of 58.  In other words, this was not a young crowd with unusually strong eyesight. The bottom line is that, based on my small survey, people may well be most comfortable with illuminance levels (10-15 FC) that are FAR lower than historic standards, and FAR FAR lower than what is actually typically put in offices.

It probably goes without saying, but the difference in energy required for different levels of illuminance is huge.  I modeled various scenarios for a 12 foot by 12 foot office with a 9 foot high ceiling:

Scenario Illuminance (FC) Energy Use (watts)
Typical lighting manufacturer recommendation, fluorescent 80 232
Rule of Thumb, using two 3-Lamp Fluorescent Fixtures 56 176
Energy Code (1.11 w/SF) NA 160
Current standards, using LED lighting 30 78
Acceptable to most people, using LED lighting 15 39

 

If we follow typical recommendations from lighting manufacturers, designing let’s say to 80 FC, we don’t even meet the energy code.  We have broken the law. Way too much energy use. If we use a typical rule of thumb, and throw two 3-lamp fluorescent fixtures in the office, we come close to meeting the energy code, but still exceed it. We are still breaking the law. Furthermore, the energy code is really still based on fluorescent lighting, so it itself is too high in energy use. If we design to 30 FC, using LED lighting, we save over 50% relative to the energy code. And if we design to 15 FC with LED lighting, a light level that I believe is acceptable to people and is in fact preferable to many people, we save 75% relative to the energy code. Remember incremental savings versus transformational savings?

So, let’s choose a target illuminance as a goal for our own work.  I recommend 30 FC.  This meets OSHA, would satisfy 100% of the people in the office study I did, and most importantly meets IES (in most cases – see below for more details).  Forget 40 FC, forget 50 FC, forget 50-100 FC.  All way too high.

So what does the new (10th) edition of the IES handbook really require?  I wish I could just reproduce their tables, but I don’t want to get into trouble. If you need details and can’t spring $600 for the book, give me a call. But in summary….  For offices, IES now breaks out requirements for different ages of occupants (younger than 25, 25-65 years old, and older than 65), and also for horizontal versus vertical illuminance. And there is a long list of different types of office work, each requiring different illuminance, from different types of computer work to reading electronic readers (think Kindle) to faxing and more. As I peruse the LONG list of activities, I think the ones that are most meaningful are those for people 25-65 years old, doing work at a computer screen, or reading printed material at 8-10 point font.  And for ALL of these activities, the requirement is 300 lux or less, equal to 28 FC or less.  For some applications, the recommended office lighting is as low as 75 lux, barely 7 FC! I specifically am looking at horizontal illuminance requirements, because that is what is given by most computer design programs for illuminance. Vertical requirements are generally lower, and I presume are generally met by meeting the horizontal illuminance requirements. In my next blog post, I’ll get into the details of design. In many cases, the requirements are for less then 300 lux, and this is consistent with the small survey I did.  So I think that 30 FC is really a fine goal for design today.  Just be aware that if designing for an elderly population, or for very fine work (reading extremely small fonts, for example) the illuminance goal should be higher.

For energy design, we keep in mind the energy code requirements of 1.11 w/sf for closed offices and 0.98 for open plan offices.  It is critical to note that these are way too high, for LED lighting, and will likely be reduced in the future. If your lighting design is more than 0.75 w/SF, you are missing an opportunity to save energy, and in the second post in this series, I will discuss going way lower than 0.75 w/SF.

In my view, as important as designing to 30 FC is an associated goal to allow illuminance to be reduced below 30 FC, through controls, either multiple light fixtures and associated switches, or dimming capability.  This allows adjusting illuminance to levels that are likely to be preferable AND to save even more energy.

How is this all important? For new buildings, if we don’t design to IES but rather use rules of thumb, we likely will come close to meeting the energy code, but our offices will be way overlit, and we will use probably twice as much energy than necessary. If we design to 30 FC, with the ability to dim to 15 FC, we will likely save 75% or more in energy, relative to the energy code. And that’s before some tricks I’ll discuss next time, to take savings closer to 90%.  Remember that discussion of incrementalism?  No incrementalism here. And those 75%+ savings will ALSO reduce the energy use and cost of air conditioning. And the story for existing buildings is even bigger. In existing buildings, if we simply replace existing fluorescent lighting with LED lighting, we will save about 30-40% in energy use. The bigger story is that we are leaving another 40-50% savings on the table!  If we are serious about lighting energy conservation in existing buildings, we must look at “right-lighting”. We must look at reducing the overlighting. That typically means reducing the number of light fixtures. If we only replace lamps, one for one, we are robbing the building of potential energy savings for decades to come. If we reconfigure the lighting to deliver the 30 FC, we will… save 80% or more in energy… save even more energy in reduced air conditioning… make lighting more comfortable for people (remember those people in my survey who found 20 FC too bright?)… and reduce future maintenance (lamp replacements). What a great gift to a building!

OK, that’s it for goals. Next time, we’ll discuss how to meet the goals, delivering GREAT lighting while saving a TON of energy.

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2 thoughts on “Office Lighting – Part I: Goals

  1. Having retrofit about 4 million commercial square feet to 30 fc, I couldn’t agree more. Anything else is overlit. Something less is possible for many (maybe even most) office workers but you may get a few complaints about dimness. The very rare comment we got about “not enough light” at 30 fc, we gave them a task lamp. The secret was doing the relamping over the weekend when folks were gone for two days so they didn’t mentally compare the before and after brightness.

    One thing that’s important to mention is that its 30 fc at desk level. Illuminance, as you know, reduces as the square of distance so it might be much less at the floor. If you’re relamping an office fitout, don’t put your illuminance meter on the floor and get 30 fc and think you’re doing it right! You’ll have much too much light at desk height. Unless you have employees lying on the floor to do their work, you’ll be overlit.

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  2. Great point about the desk height measurement (and associated design). I’ll add that in the second post. Good to hear about your success at 30 FC, a nice affirmation. Ian

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