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Grasp LED light intensity curves for grow light apps

Posted: 03 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:light intensity curves? LEDs? Lambertian? optics?

The light intensity curves for LEDs, regardless of whether they are collimated with a lens or reflectors, conform to a bell shaped curve (often referred to as a 'Lambertian' curve). The 50% intensity point for an LED with no optic (bare LED) is virtually always at approximately 120 degrees (60 degrees in each direction).

The 50% point (known as the "beam angle" or "viewing angle") is a lesser number of degrees depending on the collimating specification used. The beam angle is the total angle in both plus and minus directions.

What is not commonly known by those not experienced in the physics of LED light emission and optics is that the Lambertian LED light intensity curves can be very misleading in terms of how much light is actually arriving at the receiving end.

The emitted light travels in a straight line whose length varies as the angle increases. A beam of LED light traveling at an angle of 45 degrees from straight ahead is at a diagonal, which is the hypotenuse of that 45-degree angle. That is, the light must travel 1.414 farther to reach its destination. For angles greater or less than 45 degrees, that diagonal distance is more or less.

Furthermore, it should be noted that light intensity (just like radio and sound waves) decreases inversely as the square of the distance. This characteristic is known as the "inverse square law." That means that the light traveling diagonally, per a 45 degree shift to the left or right decreases by 1.0/1.4 x 1.4 or 1/1.96 = 1/2 if rounded off. The result is that the intensity of the LED beam of light, traveling at a 45-degree angle towards a wall, is reduced by 50% from what it would be if just traveling straight ahead at zero degrees.

Consequently, when one looks at a Lambertian LED intensity curve, or a "polar intensity plot" one must divide any value by the correction factor in order to know how much the light level is being attenuated before arriving at the target end.

Figure 1: This table shows the correction factor for any given angle from zero and graphically indicates how received light drops off much more at increasing angles than suggested by simple interpretation LED or LED-optic beam intensity curves.

Graphically, one can see from figure 2 that the light level at the receiving end, because of the "Inverse Square Law," is actually dropping off much faster than the light intensity curve.

Figure 2: Received light level versus emitted light for various beam angles.

Figure 3 indicates another way of looking at it. The received light is dropping off at a faster rate and ends up exhibiting a much narrower beam angle than the emitted light curve would suggest. For example, at 40 degrees in one direction (corresponding to a "full" beam angle of 80 degrees, the received light has dropped down to about 40% while the emitted light has only dropped down to about 70%.

Figure 3: Relative light level versus beam angle (in degrees).

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