Artículo Científico Traducción, Sensores 3 En 1

Trade-offs: 2-in-1 or 3-in-1 ambient light and proximity sensor
(Cambios: el modelo 2 en 1 o el modelo 3 en 1, sensor de proximidad y luz ambiente)
Tamara Schmitz, Intersil Corporation
10/18/2012 12:52 PM EDT
As sensors become more popular and systems become more complex, the idea of a plug-in module is gaining popularity.  In terms of light sensors, these modules are referred to in terms ofthe number of functions incorporated in one package.  The first devices are simple ambient, or visible, light sensors.  (I guess these would be called 1-in-1.)  When proximity sensors were invented, they existed as a solo product but also soon joined the ambient light sensor, or ALS.  This is called a 2-in-1.  
Please note that a proximity sensor needs an accompanying IR LED to generate theinfrared signal it uses to bounce off of a nearby object and detect its presence.  See Figure 1.  Therefore the next generation, the 3-in-1, is a device where the IR LED is co packaged with the ALS and the proximity sensor. While this seems like a natural and easy progression, let me assure you that the optics and mechanics involved in this process are quite challenging.  Of course, challenge is whatmakes engineering fun, usually…  And remember that customers pay in proportion to the value the device has to them, not in proportion to the amount of effort that might have been exerted to create it.   All of that said, let’s examine the trade-offs associated with using a 2-in-1 device versus a 3-in-1 device.

Figure 1. A proximity sensor needs an accompanying IR LED to generate the infraredsignal it uses to bounce off of a nearby object and detect its presence.
For this discussion, the 2-in-1 device will be the ISL29038.  Like the circuit on its evaluation board, it will be paired with an SFH4650 IR LED.  (Figure 2)  For the 3-in-1 device, I have chosen the ISL29044, shown in Figure 3. 

Figure 2.  Evaluation board of the ISL29038, a 2-in-1.  The LED is larger than the sensor inthis solution.

Figure 3.  Evaluation board of the ISL29044, a 3-in-1.
Let’s start with mechanical considerations.  Most obviously, since the 3-in-1 device is a single package, it has the smallest bill of materials (BOM) and smallest footprint as seen in Figure 3.  This saves the manufacturer from both having to choose an IR LED and from deciding the best distance that should separate the IR LEDand the sensor.  On the other hand, the 2-in-1 sensor forces the user to choose an IR LED.  The advantage of making this choice is that there are a wide range of IR LEDs available.  The two most relevant issues are package size and half angle.  Both will be dictated by the desired application.  

While package size is somewhat self-explanatory, half angle may be a foreign concept.  Some LEDs emitlight with a narrow “beam” (like a good flashlight), while others have a wide “beam” (more like a ceiling light).  The narrow “beam” is similar to a small half angle and the wide “beam” is like a large half angle. See Figure 4 for clarification.  Choosing an LED with a narrow half-angle, like 15 degrees, will increase the proximity distance that the system can be measured. A wider-angle LED willspread the energy over a wider area, but not reach as far from the IR LED and sensor. 

Figure 4.  The half angle of the LED is a measurement of half of the arc of the spreading of the beam of light it emits.
 
Not only can the IR LED be chosen for optimum half angle, it can be placed at the optimum distance from the sensor.  This is more than a subtle effect.  If the IR LED and the sensor aretoo close together  or placed too far below the bottom surface of the glass with improper isolation, internal reflections can saturate the system. It’ll become useless.  Similarly, if the IR LED and the sensor are too far apart – or if they are placed too close to the bottom surface of the cover glass -- there can be a “dead zone” near the surface of the glass. Systems cannot sense the presence...