The beneficial trend to design lighting with focus on human nature and how natural light behaves has lead to the wide adoption of lines of light as a principal way to illuminate interiors.
A line of light can be obtained via a LED strip hidden from view inside a cove in the wall or ceiling that illuminates an adjacent surface. Light is reflected from this surface into the space that has to be illuminated. That is why lines of light are commonly known as cove or indirect lighting.
The allure is the similarity with natural light. With the proper light source used we could imagine that the cove is actually a hidden window to the outside from where sunlight flows in.
This particular way to illuminate raises a particular set of challenges for the light fixtures used.
1. Energy efficiency.
As much light as possible has to reach the reflective surface, a wall or ceiling, that directs it in the interior space. Wasting energy to illuminate the insides of the cove should be prevented. LED strips with 120-180° are recommended, placed, when possible, at an angle:
Surfaces that are more absorptive than reflective, like dark paint or wood should be avoided unless the design is primordial to energy efficiency. White ceilings produce the best results.
2. Light source output
Cove lighting can serve a variety of purposes. It can guide people, highlight architectural elements or evoke a certain design trend while providing ambient, general or task lighting.
It is highly recommended that the purpose and role played in the illumination of a space decide the type of light source to be used. The following luminous flux per meter value can serve as a useful guide:
ambient light or highlight of objects or furniture elements: up to 500 lumens per meter
highlight architectural elements: up to 800 lumens per meter (many other light sources exist in the space)
general illumination via lines of light and task lighting via spot lights: up to 2400 lumen per meter, depending on the cove design and materials used. In many cases up to 1400 lumen per meter is sufficient
3. Installation ease and redundancy of systems.
The light sources have to be easily spread out over long distances and use as less auxiliary items as possible, for example wires, connectors, screws or transformers.
4. Light quality, spectrum and color rendering index.
If a cove light is to be an imitation of daylight then it should be in almost all aspects.
From the available light sources today fluorescent and LED can be used for cove lighting. We should compare the spectrum of the source used to that of daylight at similar color temperature.
Below you find an example of light spectrum comparison. For LED and Fluorescent (CFL) the spectrum is dependent on the product and usually can be found in the its datasheet.
A good spectrum will also influence CRI:
5. Lifetime, color shift and lumen maintenance.
Long lasting light sources with minimal color shift in time are recommended, especially for installations with considerable lengths. If the brightness of the light source fades too soon or the color temperature changes the total replacement of the installation could be the only solution for uniform lighting.
For example, if "LEDs 1" are used in the installation and the light changes as in the image on the right, replacing only some sections with "LEDs 2" will exacerbate the lack of uniformity, as in the image below.
A "rainbow" of shades of white is generally not desirable. Replacing parts of an installation can lead to such an outcome.
Additional technologies should be explored, for example:
- Dimming, manual or part of a intelligent building system
- Multi color lighting
- Tunable White for Human Centric Lighting that mimics daylight at various time of the day.