Reading lights - How to specify in Photometric terms the suggested level of lighting at the work area?

Question

I am designing this space where we want it to be bright enough to read and dim enough for mood lighting or TV watching. This can easily be accomplished via a dimmer. The question is how do I specify to a builder the level of lighting necessary to read by in terms that I can measure. I know the spec will be in lumens or foot-candles but I don't know the level. Also where can I find specifications like this for various applications?

Answer 1

The measurement would be LUX (lumen per square meter).

The sun at high noon putputs about 1000 Lux. So for reading you need something in the region of 300 to 600 Lux. Mood lighting or TV watching would require about 40 Lux. CHeck out the australian standard for Lux requirement:

http://www.ndlight.com.au/lux_levels.htm

Note: 1 footcandle = 10 Lux

Answer 2

Visual illuminance for various tasks is measured in footcandles and the optimal levels have been set by various architectural and engineering organizations. This commercial site is a good quick reference for residential use:
http://www.signaturelighting.net/designing_light/howmuch.htm

Lumens refers to the output of a specific lamp. Footcandles is the measured amount of light that falls on a surface. This can vary with the distance of the surface from the light source so it is more important to specify what footcandles you want on a specific area (that's why you would not use lumens in your sepcification). Most architectural grade light fixtures have in their catalogs what are called "photometrics." These show graphically and with charts the level of footcandles which the fixture will produce at various distances and with different wattage bulbs. But these are confusing to someone not familiar with them.

Bear in mind that it is not simply the level of light that will optimize visual comfort, it is also the quality of the light and the position of the source of the light. You have to take into account glare, shadows, light color and even the atmosphere -- for instance, it is much more comfortable for most people to read with a nearby table or floor lamp to one side of them than with a strong overhead light. Also, most people tend to over-light spaces. Too much light can cause eyestrain and make it hard to see details. In a residential space, you are going to find that the most effective light is NOT built in but comes from portable light fixtures. I would caution against using a lot of recessed downlighting. Most people find they don't care for it once it is installed. The same with track lighting -- both tend to cause glare and hot spots and are useless for most tasks or reading.

For general ambient light in a room that you want to watch TV in, recessed perimeter cove lighting on a dimmer works best (you can get dimmable fluorescent ballasts now and the lamps are available in a range of comfortable color shades -- not the ugly "cool-white" everyone thinks of.)

I've been in the electrical design-build field for 25 years. I would recommend NOT leaving this design factor to the builder. Take your room designs to the lighting showroom of an electrical supply house or large lighting store and ask them for assistance in doing the design and specifications. They are trained in this and will generally do it for free if they have a reasonable expectation that they will be selling you some of the fixtures. Actually, this is what your builder will do anyway so I'd advise doing it yourself so you have more input on the quality (the builder will tend to go with what is easiest to install, not what might be best for you.)

Answer 3

I have a lux meter, and I'm comfortable with 85 to 213 lux, which corresponds to a source having an apparent magnitude of −19 or −20, resulting in a visible light irradiance of 1.02 to 2.55 watts per square meter.

Apparent magnitude is a logarithmic scale of brightness. The linear measure to which it corresponds is radiant flux. The relation between the two is:

m = −2.5 log₁₀ F − 18.982249379206

The illuminance in lux, E, and the apparent visual magnitude, m, are related as follows:

m = −14.18 − 2.5 log E

Or, to make it simple:

F = visible light flux in Wm⁻²
E = illuminance in lux

F = 0.012 E
E = 83.3 F

Answer 4

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