What is a LUX?
How brightly an object is lit is a measured in Lux . It’s the intensity of illumination:
Imagine we are having difficulty reading a book because the page is not lit brightly enough, we would naturally move it closer to the light. The illumination on the page depends on the distance from the light.
Take a normal household 40 watt light bulb, we know from experience that we will be able to easily read our book if it is installed in a reading lamp on our desk. Put that bulb just 20 feet away, and we will be struggling. It is all a question of light and distance.
SAD therapy treatment lights should provide illumination levels, stated in LUX at a given distance. For optimum 10000 lux treatment you must use it at, or around the distance stated by the manufacturer. A LUX number is meaningless if a distance is not specified.
For light therapy to be successful your retinas that need receive the illumination So you need to know not just that the light is bright enough, and just as importantly that your eyes are close enough to the light source to receive its maximum therapeutic value.
A Bit More Confusion
Lumens are another form of measurement It’s is the total amount of visible light to the eye from a light source. The higher the Lumen rating the brighter the lamp will appear.
Watts and Lumens are often confused. Linking power consumption (Watts) to light output (Lumens) is incorrect. We have all bought 40W or 60W bulbs expecting a certain level of brightness, only to be disappointed.
In a simple approximation, light spreads out according to an inverse square law. If you are twice as far away, the illumination on your book is only one-fourth as great.
Get your Distances Right
Sad lamp users should be very aware of these perhaps surprising numbers.
If a light box has a rating of 10,000 lux at 12-14 inches from the light source, as we distance ourselves from the source the lux value decreases. Here are some sample values:
12 inches = 10,000 lux
24 inches = 2,500 lux
48 inches = 625 lux
This is common to all SAD lamps, so many SAD sufferers have been disappointed with the results of light therapy. They have been unaware of the need to be close to the light source. It is just not going to be as effective from the wrong distance.