A few months ago, I received an unsolicited request to write a post on indirect lighting. This sounded like a great idea to me. I immediately dropped a note into my “reminder file” figuring inspiration would quickly burrow itself into my brain and text would come flowing through my fingers into a MS Word document. Alas, that has not occurred.
Indirect light is a remarkable addition to a room. When my wife and I renovated our second home, we added perimeter lighting to a basement rec-room, tucked behind a ceiling mounted valance. Despite the other layers of light I included in the space, it became the first and typically only light we used. It was comfortable, provided no glare and seemed so natural, despite the fact that it was linear fluorescent, in those pre-LED years. In our third home, I included LED linear over-cabinet and toekick lighting in our kitchen remodel. Again, we use them much more that the four other switches in the room. Clearly, we prefer the output provided by the indirect source.
But, what science did I use when I specified it? Surely there was a method to my madness. Actually, I had no method, just a gut response to the need. In the basement installation, I used the available T8 fluorescent product sizes in double-tube, offset models, because I hated the “dark gap” left when non-offset product was used in long linear lines. I had specific lengths, so I used a combination of 48”, 36” and 24” units that would give me a full and even delivery of illumination.
In the kitchen, it was even less “scientific.” I made a decision to install an “All LED” lit room. At the time, it was quite a challenge. My electrician was VERY excited, knowing he could take the experience with him to his next job. I was a Product Manager at the time and my kitchen along with the kitchens of the Engineers involved in the design became “linear lighting guinea pigs.” We learned about output, how color temperature interacted with room settings and how lumen output informed products. (There were limited LED options at the time.) Like Henry Ford’s famous quote, “You can have any color as long as it is black.” I had one option. I made installation videos while I installed the light that I hoped could be used to help others and in the process. I learn of some issues that would inevitably arise, so I was preemptively prepared for Sales Rep questions. What I did not do and could not do was consider the options, because, they simply did not exist.
Since my early interaction with indirect light, I have tried to quantify my preferences. I’ve tried to digest the WIDE assortment of linear lighting now on the market. I’ve also attempted to integrate the needs of more light required for senior eyes and their preference for indirect light. Couple all of that with the variety of room reflectance and ranges quickly become important.
With that in mind, let’s revisit my previous installations. The double T8 installations hidden behind a valance were producing about 1400 lumens per foot. (Florescent T8 lamping produced between 650 and 750 lumens per foot and there were two, side by side in each fixture.) The valances ran the length of the room on both sides. While I no longer have the details, I believe there was about 25’-0” of light, amounting to 35,000 lumens in a 15’-0” x 28’-0” (420 sq. ft.) room. This was enough to illuminate a room comfortably with no-glare. That may seem like a huge amount of light, but we must remember, because it was indirect, a reasonable proportion is lost in absorption and reflectance.
In my second installation, using early LED linear lighting, I know the output was much lower, approximately 50 lumens per foot. With a linear length of 20’-0” in a 9’-0” x 13’-0” room, that delivered 1000 lumens of light. The toekick at 16’-0” of linear length produced 800 lumens. When used together, 1800 lumens of indirect light was easy to like.
My non-exacting use ranged from 50 to 1400 lumens per foot. Looking at the wide variety of products now on the market, we can easily specify eleven different static white options from 112 lumens per foot to 1163 lumens per foot, and that is from just one company! By developing comfort with this “lumens per foot” metric, it is easy to apply the correct amount to each application.
Today, indirect light is typically produced by linear LED. I wrote two companion blog posts on this topic (https://lightingbyjeffrey.com/2024/01/29/how-to-effectively-select-led-tape-part-one/ https://lightingbyjeffrey.com/2024/02/12/how-to-effectively-select-led-tape-part-two/ ) There is lots included there which can be reused here because most of the content referenced indirect illumination. I’ve reprinted this table, because it relates to applications. Use the range to deal with reflectance variations.
| Application | Lumens per Foot Range |
| Mood lighting / Light Used as a Background | 100 to 300 |
| Accent lighting / For Added Aesthetics | 150 to 500 |
| Task Lighting – Close | 275 to 500 |
| Task Lighting – Far Away (light location) | 350 to 700 |
| Indirect Lighting | 375 to 575 |
| Cove Lighting | 180 to 500 |
| Principle Lighting in a Room | 400 to 1000 |
| As a Replacement for Linear Fluorescent Lamping | 500 to 950 |
| Kitchen Under-Cabinet Lighting | 175 to 550* |
Now, let’s understand the relationship between room size and the indirect light’s lumens per foot delivery. If we consider my original rec-room with the old fluorescent, 420 sq. ft. multiplied by the desired 20Fc illuminance level from the chart below, 8400 lumens is needed.
The indirect light in the kitchen is intentionally less functional, so the 117 sq. ft. against a desired illuminance level of 5Fc would require 585 lumens of light.
| Area / Task | Desired Illuminance Level in Footcandles (Fc) |
| Hallway/Passageway | 5-10 |
| Conversation Area / Entertaining | 5-20 |
| Dining | 10-20 |
| Reading (General) | 20-50 |
| Bathroom / Grooming | 20-50 |
| Laundry / Ironing | 20-50 |
| Kitchen (General) | 20-50 |
| Kitchen (Work Areas) | 50-100 |
| Reading (difficult) Study / Hobby / Music | 50-100 |
| Hand Sewing / Detail Hobby | 100-200 |
The 35,000 lumens I liked is four times the anticipated need. The 1800 lumens in the kitchen is closer to three times the need. Because indirect light takes a circuitous path from the light source to the user, much is absorbed in the reflecting surfaces.
Think about this, a mirror only reflects back about 92% of the original. If I measure my above cabinet light directly overhead (12”,) I record 20.7 Fc at the ceiling. My toekick lighting measures 130.8 Fc on the floor below (Very close at 4” to the floor.) Standing in the middle of the room with both systems engaged, I have a mere 3.5 Fc of usable illuminance from 3600 lumens. Obviously, a lot of that light is lost in surface absorption and distance.
To achieve usable levels of indirect light, a 3:1 to 4:1 ratio makes sense. If you want to reach the levels of usable light the second chart suggests, you need to use products that create three to four times that number.
There are more complicated calculations that could be employed to insure exacting levels of light. Many commercial projects first construct models to understand the end result of the proposed light. Both are impractical for the residential space and occasional lighting designers. At the risk of suggesting my hunch, rather than proven data is the way to go, I’ll leave you with a 3:1 to 4:1 ratio.
If anyone else has a better suggestion, or an easy calculation, let me know. Next time, I may be less accommodating when someone suggests a blog post topic!
Lighting by Jeffrey 