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Think back to the time you were last walking in the dark using a flashlight—or torch, if you happened to prefer British English. As you aimed the light forward, the brightest concentration of light was probably at the center of the flashlight’s beam, with the brightness diminishing the further you got from the center. Or you might have been using a flashlight with an adjustable lens that allows you to switch between a tight beam that illuminates greater distances while leaving the edges quite dark, or a wide beam that doesn’t penetrate the dark forward but does a nice job of illuminating the near path and the peripherals.
The Angle of Half Intensity
So, you already intuitively know about viewing angles. This is the term generally used for visible light. For infrared emitters we like to use the term “angle of half intensity”. The angle of half intensity is the angle where the radiated intensity will be half as intense as it would be at 0 degrees, or pointing straight ahead. Why half intensity instead of some other fraction? The human eye can typically only perceive a doubling or halving of intensity, so that’s probably why 50 percent was chosen. Factors that contribute to the angle of half intensity include the amount of diffusing material in the epoxy, the shape of the reflector cup which surrounds the LED chip, the shape of the LED lens, the distance from the LED to the tip of the lens, and the type of emitter chip. Of course, light is still emitted beyond the half-intensity angle; it just gradually gets weaker and fainter. Optical engineers measure the intensity of an emitter chip or package and create a graph called the Relative Radiant Intensity versus Angular Displacement to show this relationship.
How To Read the Graph
Each infrared emitter datasheet includes a graph showing the relative radiant intensity versus the angular displacement (Figure 1). To read the curve in Figure 1, start at the top where the intensity is equal to 100 % of its maximum. Moving clockwise, you’ll see that at ± 30° the intensity is at 90 % of its maximum, and at ± 50° the intensity of the emitter has dropped to 70 % of its’ maximum. Finally, we come to ± 60° where the emitter intensity is 50 % of its maximum. This is the point at which the measured curve for the emitter crosses the 50 % relative radiant intensity line and thus defines the angle of half intensity.
Chips and Packaged Emitters
The analogy with a standard flashlight doesn’t apply to most emitter chips. A standard GaAs emitter chip radiates light in every direction, Figure 2. Light is not just coming out the top but out to the side as well. Most applications require this light to be directed so we put the chip into a package. The most common and familiar package is a standard 5 mm, T1¾ leaded package shown in Figure 3. There are two factors mentioned earlier that affect the angle of half intensity shown in this illustration. First, the wire lead-frame includes a reflective cup which redirects any light emitted sideways from the chip out of the package. Second, the 5-mm emitter includes a lens which focuses the light. This chain of optics redirects the light emitted to a designated direction.
Infrared emitters from Vishay are available in a broad variety of angles of half intensity. They can be classified into narrow, medium, and wide. A narrow angle of half intensity would be from ± 5° to ± 20°. A medium angle of half intensity would be from ± 21° to ± 40°. A wide angle of half intensity would be greater than ± 40°. A common wide angle is ± 60°. Figure 4 provides two graphical representations of the relative radiant intensity versus angular displacement for the VSMF2890RGX01, an infrared emitter with an angle of half intensity of ± 12°. You may notice that there is a small dimple in the top of the radiant intensity graph in the illustration in Figure 4. The emitter chip has a bond pad in the top-center of it where light is not emitted. While light from the rest of the chip’s surfaces fills this area, it does not completely compensate for the bond pad. You may also notice that beyond around ±7°, the intensity drops off dramatically. This is a very focused light source. In contrast, the intensity of the emitter in Figure 1 uniformly rolls off from the peak. It is common to use a cone to represent the light emitted but perhaps a cylinder might have been better for the VSMF2890RGX01.
I don’t want to get all Robert Frost on you but when was the last time you went walking through the night shrouded woods using a flashlight?