Every diode should have some physical mark to differentiate the anode from the cathode. On most regular LEDs you will work with, the anode “leg” or pin is longer than the cathode. Since you can’t see the pins’ lengths once you plug them in a breadboard or solder them, the cathode side of the case will be slightly cut off. It might be hard to notice at first, but once you find it, you’ll have no trouble seeing it in the future.

When you’re looking at LED’s datasheet , you’ll see three things listed that you’ll find useful. They are the forward current, peak forward current, and suggestion using current. Most LEDs will have the forward current rated at 20 mA. This means for optimal performance, you could run them on 20 mA. This doesn’t mean you have to necessarily run them at that value, just that it’s recommended for most cases. Peak forward current is usually set at 30 mA. You can run them on higher than that, but you’re risking breaking them. Suggestion using current is usually set to 16-18 mA. We would recommend using LEDs at around 10 mA to extend their shelf life, as the brightness difference between 10 and 20 mA is relatively insignificant.

The 14.25° derived in Example 1 (see white box below) can be used to determine the lens that is needed, but the sensor size must also be chosen. As the sensor size is increased or decreased it will change how much of the lens’s image is utilized; this will alter the AFOV of the system and thus the overall FOV. The larger the sensor, the larger the obtainable AFOV for the same focal length. For example, a 25mm lens could be used with a ½” (6.4mm horizontal) sensor or a 35mm lens could be used with a 2/3” (8.8mm horizontal) sensor as they would both approximately produce a 14.5° AFOV on their respective sensors. Alternatively, if the sensor has already been chosen, the focal length can be determined directly from the FOV and WD by substituting Equation 1 in Equation 2, as shown in Equation 3.

We’ve talked about regular, one-color, simple LEDs thus far. But LEDs have come much further than that and there are a bunch of types today. Here are some of the ones we find particularly interesting to work with.

The SMD LEDs are rectangularly shaped and have three cells that contain semiconductor crystals. These crystals produce light when current goes through them. Resin is used for the protection of the SMD cell. A single-color LED will always have one anode and one cathode. The SMD RGB LED will have three anodes and cathodes, one for each color.

Example 2: For an application using a ½” sensor, which has a horizontal sensor size of 6.4mm, a horizontal FOV of 25mm is desired.

Pretty much everywhere you look, you will see an LED in use. It doesn’t matter if you’re looking at your smartphone, TV, car, or coffee machine, there are some likely in them. LEDs are widely used and supported, and come in many colors, shapes, and sizes. The first Arduino project usually being taught is making an LED blink  because of its simplicity and LEDs’ widespread use. Let’s dive into the world of LEDs together and learn a thing or two about them!

Knowledge Center/ Application Notes/ Imaging Application Notes/ Understanding Focal Length and Field of View

LEDsymbol

The light emitted will depend on the current the LED draws. The more light it emits, the more power it uses, thus the more batteries it drains. Diodes in general do not limit current and will destroy themselves if they consume too much of it. Resistors are used to prevent this from happening. They will restrict the electrons’ flow in the circuit and save the LED from drawing too much power. But with so many resistors out there, which one should you use to prevent your LED from burning itself?

The only difference between a diode and an LED in the schematic is the arrows added over the symbol. They represent the light being emitted from the diode.

While most sensors are 4:3, 5:4 and 1:1 are also quite common. This distinction in aspect ratio also leads to varying dimensions of sensors of the same sensor format. All of the equations used in this section can also be used for vertical FOV as long as the sensor’s vertical dimension is substituted in for the horizontal dimension specified in the equations.

LEDanode cathode diagram

Another way to change the FOV of a system is to use either a varifocal lens or a zoom lens; these types of lenses allow for adjustment of their focal lengths and thus have variable AFOV. Varifocal and zoom lenses often have size and cost drawbacks compared to fixed focal length lenses, and often cannot offer the same level of performance as fixed focal length lenses.

We made a tutorial on resistors so if you’re not sure how to calculate its resistance, find out here . For most LEDs, you’ll be fine with using a 330-ohm resistor. That’s the one with two orange and one brown color band. The stronger resistor you use, the less bright LED will be.

If you take a closer look at an LED, you will see it’s made of several parts. The case or housing of the LED is usually made from epoxy or plastic material. This makes it more durable to fall or similar damage. The inside of the LED consists of two main parts – post and anvil. The post is the positive side of the LED, the anode side, and the anvil is the negative side, the cathode. A die is inside a little divot on the anvil, and the bond wire leaps from anvil to post, connecting the two.

You’ve likely seen this type of LED in real use. A TV remote control is a prime example of an infrared LED. Some older cell phones that still used physical buttons had IR LEDs to transfer data to other devices.

LEDplus minus

The focal length of a lens is a fundamental parameter that describes how strongly it focuses or diverges light. A large focal length indicates that light is bent gradually while a short focal length indicates that the light is bent at sharp angles. In general, lenses with positive focal lengths converge light while lenses with negative focal lengths cause light to diverge, although there are some exceptions based on the distance from the lens to the object being imaged.

How to identify positive and negative terminal ofLED

If the red anode is provided voltage, the red LED will light up. Conversely, if the green anode is provided voltage, the green LED will light up. In this system, only one LED can light up at a time.

Note: Horizontal FOV is typically used in discussions of FOV as a matter of convenience, but the sensor aspect ratio (ratio of a sensor’s width to its height) must be taken into account to ensure that the entire object fits into the image where the aspect ratio is used as a fraction (e.g. 4:3 = 4/3), Equation 7.

The first question that might pop into your mind is, what is exactly an LED? A regular diode is a semiconductor device that works as a one-way switch for electrical current. It allows current in only one direction and will stop it from flowing in the other. An LED works the same way. The only difference is that it emits light when the current passes through, as the name suggests. This similarity is reflected even in the schematic , as seen below.

While it may be convenient to have a very wide AFOV, there are some negatives to consider. First, the level of distortion that is associated with some short focal length lenses can greatly influence the actual AFOV and can cause variations in the angle with respect to WD due to distortion. Next, short focal length lenses generally struggle to obtain the highest level of performance when compared against longer focal length options (see Best Practice #3 in Best Practices for Better Imaging). Additionally, short focal length lenses can have difficulties covering medium to large sensor sizes, which can limit their usability, as discussed in Relative Illumination, Roll-Off, and Vignetting.

LEDsymbol circuit

In electronics, polarity  indicates the symmetry of a component, meaning it will matter how you connect it if it’s polarized. Non-polarized components can be connected either way and they will work properly (e.g. resistors). As mentioned, a diode will allow the current to flow in only one direction. Thus, an LED is polarized and will only emit light if it’s connected correctly. For it to work properly, the current must go from anode to cathode. The anode must be connected to the positive end and the cathode to the negative. If it’s the other way around, the LED will remain off. It won’t break, it just won’t work, and it might stop the whole circuit. On a schematic diagram, the anode should be the line coming to the broad side of the triangle.

Note: As the magnification increases, the size of the FOV will decrease; a magnification that is lower than what is calculated is usually desirable so that the full FOV can be visualized. In the case of Example 2, a 0.25X lens is the closest common option, which yields a 25.6mm FOV on the same sensor.

Field of view describes the viewable area that can be imaged by a lens system. This is the portion of the object that fills the camera’s sensor. This can be described by the physical area which can be imaged, such as a horizontal or vertical field of view in mm, or an angular field of view specified in degrees. The relationships between focal length and field of view are shown below.

Bi-color and tri-color LEDs will light up in a color depending on the current flow direction. Regular bi-color LED has two wires. The LEDs are connected back to back, anode to cathode. The color of the light will depend on the anode which is provided with positive voltage. Let’s say the LEDs in our example have colors green and red.

We use LEDs pretty much daily in our work. We even use them when tinkering with our projects at home. Take a look at our tutorial pages  to find some projects using them. With so many varieties available and so many applications LEDs have, we’re sure you thought of some Arduino projects on your own. For which project are you planning to use an LED? Let us know! 🙂

SMDLEDpolarity marking on PCB

Note: Fixed focal length lenses should not be confused with fixed focus lenses. Fixed focal length lenses can be focused for different distances; fixed focus lenses are intended for use at a single, specific WD. Examples of fixed focus lenses are many telecentric lenses and microscope objectives.

\begin{align}\text{AFOV} & = 2 \times \tan^{-1} \left( {\frac{50 \text{mm}}{2 \times 200 \text{mm}}} \right)  \\ \text{AFOV} & = 14.25° \end{align}

A term you’ll likely see often when working with LEDs is the forward voltage. It is a number that will help you with the voltage your circuit needs to supply to LEDs. This number becomes especially important if you have more than one LED connected to one power source. If you provide 5 V to your circuit, your components put together shouldn’t exceed that number.

There are a couple of SMD LED variants, such as with diffuse or water-clear lens, and three chips. The three chips can have the same or three different colors. If it has different colors, they are red, green, and blue. Thus, the LED can produce virtually any color wanted.

Generally, lenses that have fixed magnifications have fixed or limited WD ranges. While using a telecentric or other fixed magnification lens can be more constraining, as they do not allow for different FOVs by varying the WD, the calculations for them are very direct, as shown in Equation 4.

If you don’t want to calculate the resistance, you can always try plugging in different resistors to see what happens. Resistors let go of extra power as heat so if it’s getting warm, you’ll likely want a smaller one. If it’s too small, however, you might burn out your LED. When you plug in your resistor and LED on the circuit, check if the LED quickly blinked on and off. If that’s the case, you burned your LED and should use the stronger resistor. If the LED is on, but it’s not as bright as you hoped for, try using a different resistor. Experiment this way until you get wanted LED brightness. Keep in mind that this is not the recommended way of doing things, and we strongly encourage you to learn how to read resistor values.

If the required magnification is already known and the WD is constrained, Equation 3 can be rearranged (replacing $ \small{ \tfrac{H}{\text{FOV}}} $ with magnification) and used to determine an appropriate fixed focal length lens, as shown in Equation 6.

As with everything we use, LEDs have some pros and cons. Compared to incandescent light sources, LEDs have a longer lifespan and need less power. That is why they are becoming a norm in light bulbs, among other places. Here’s a list of why you should consider using LED is in your projects, and why you might want to seek an alternative.

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The red, green, and blue wires are all connected to the external anode or cathode wire (the second leg). Depending on whether you have anode or cathode RGB LED, you’ll supply power differently. If you have an anode one, you’ll connect the anode wire on the power supply to the positive terminal. A low signal will need to be applied to red, green, and blue wires. However, if you have a cathode one, you’ll connect the cathode wire on the power supply to the negative terminal. A high signal will need to be applied to the red, green, and blue wires.

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The focal length of a lens defines the AFOV. For a given sensor size, the shorter the focal length, the wider the AFOV. Additionally, the shorter the focal length of the lens, the shorter the distance needed to obtain the same FOV compared to a longer focal length lens. For a simple, thin convex lens, the focal length is the distance from the back surface of the lens to the plane of the image formed of an object placed infinitely far in front of the lens. From this definition, it can be shown that the AFOV of a lens is related to the focal length (Equation 1), where $ \small{f} $ is the focal length and $ \small{H} $ is the sensor size (Figure 1).

Tri-color LEDs are a bit different. They have three wires, one for each anode and the middle one for the cathode. Like with the bi-color LED, depending on the anode provided with a positive voltage, that color will light up. Here’s a catch. Because the two anodes are wired separately, you can apply a positive voltage to both wires. If you do, the LED will light up in a third color, which is a mixture of both.

OLED stands for organic light-emitting diode. Unlike other LEDs we’ve talked about so far, OLED is a conductive sheet of organic compounds that emits light when an electric current passes through. This layer, grouped with some others, is placed between two electrodes. The “organic” compound means that it contains carbon-hydrogen bonds, and doesn’t refer to materials harvested from nature.

OLEDs are massively used in screens, from smartphones and TVs to Arduino projects . Because of how it’s made, each pixel on the OLED display is controlled individually and emits its light. In LCDs, for example, the light comes from a backlighting unit. That is why OLEDs offer better image quality in screens, display bright colors better, and show “real” black.

When you look at an Arduino board, you will notice a few SMDs (surface-mount-devices). Like other components, LEDs can also be surface mounted. The RX and TX LEDs on Arduino are, as you might have guessed, examples of SMD LEDs. These types of LEDs don’t require any wiring and are soldered directly onto a circuit board.

For the light to be emitted, the diode has to be made from special material. The semiconductor die, the thing that makes LED emit light, is made from gallium arsenide (GaAs), gallium phosphide (GaP), gallium arsenide phosphide (GaAsP), silicon carbide (SiC), gallium indium nitride (GaInN), aluminum gallium indium phosphide (AlGaInP), and similar chemical compounds. The color of the LED will depend on the compound used.

Once the required AFOV has been determined, the focal length can be approximated using Equation 1 and the proper lens can be chosen from a lens specification table or datasheet by finding the closest available focal length with the necessary AFOV for the sensor being used.

Be aware that Equation 6 is an approximation and will rapidly deteriorate for magnifications greater than 0.1 or for short WDs. For magnifications beyond 0.1, either a fixed magnification lens or computer simulations (e.g. Zemax) with the appropriate lens model should be used. For the same reasons, lens calculators commonly found on the internet should only be used for reference. When in doubt, consult a lens specification table.

LEDpolarity marking on PCB

As previously stated, some amount of flexibility to the system’s WD should be factored in, as the above examples are only first-order approximations and they also do not take distortion into account.

An infrared LED, or IR LED, looks like a normal LED at first and can be hard to distinguish. The main difference is that it emits light in the infrared range. This is outside of the normal visible spectrum so you won’t see this type of LED emit light. They allow for the cheap production of infrared light and enable wireless communication between devices and sensors. That is why they are common in machine-to-machine environments, as well as Internet of Things applications.

In many applications, the required distance from an object and the desired FOV (typically the size of the object with additional buffer space) are known quantities. This information can be used to directly determine the required AFOV via Equation 2. Equation 2 is the equivalent of finding the vertex angle of a triangle with its height equal to the WD and its base equal to the horizontal FOV, or HFOV, as shown in Figure 2. Note: In practice, the vertex of this triangle is rarely located at the mechanical front of the lens, from which WD is measured, and is only to be used as an approximation unless the entrance pupil location is known.

A fixed focal length lens, also known as a conventional or entocentric lens, is a lens with a fixed angular field of view (AFOV). By focusing the lens for different working distances (WDs), differently sized field of view (FOV) can be obtained, though the viewing angle is constant. AFOV is typically specified as the full angle (in degrees) associated with the horizontal dimension (width) of the sensor that the lens is to be used with.

LEDpositive negative symbol

In general, however, the focal length is measured from the rear principal plane, rarely located at the mechanical back of an imaging lens; this is one of the reasons why WDs calculated using paraxial equations are only approximations and the mechanical design of a system should only be laid out using data produced by computer simulation or data taken from lens specification tables. Paraxial calculations, as from lens calculators, are a good starting point to speed the lens selection process, but the numerical values produced should be used with caution.

When using fixed focal length lenses, there are three ways to change the FOV of the system (camera and lens). The first and often easiest option is to change the WD from the lens to the object; moving the lens farther away from the object plane increases the FOV. The second option is to swap out the lens with one of a different focal length. The third option is to change the size of the sensor; a larger sensor will yield a larger FOV for the same WD, as defined in Equation 1.

LEDs are really simple to use and very cheap to produce. They see use virtually everywhere because of this. You’re likely using items with LEDs without even realizing it. Our smartphones and smartwatches are using OLEDs. Cars and other vehicles come with LED lights more often than with incandescent as they used to before. Camera flashes, traffic lights, alarm systems, flat panel displays in public places… The list goes on.

RGB is short for red, blue, and green. RGB LED is essentially LEDs in those colors put together. Combining those colors, RGB LED can produce almost any color, but it struggles with shades of pink and brown. With legs for each of the three colors, an RGB LED will have a fourth leg, for anode or cathode. When looking at an RGB LED, you should face it so the second leg from the left is the longest. They should then be in the following order: red, anode or cathode, green, and blue.