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# Kinds and characteristics of the light

## Intro

Light manipulation can be a really useful skill for every electrician. From lighting a visible to controlling an invisible spectrum. In this tutorial, we'll study the basic features and individual spectra which we use in other tutorials and can be applied to specific projects.

## Wavelength

The basic, and key characteristic of light beams are wavelengths. Light travels by air as a wave, and the distance between the two adjacent wave peaks is the wavelength (λ - Greek lambda). In short, the wavelength is what determines the color of the light beam.

The light beam also acts as a flow of particles, photons. Like everything in physics, this isn't easy as well. (who wants to know more, check the link). We will keep on the basic principles and concepts that will be more than enough for us to apply in praxis.

## Intensity

The second feature of the light beam is intensity. The SI unit measurement system is Watt per Steradian (W / sr). sr - square radian.

According to the formula of the picture, the area doubly distant from the source will give half of the light intensity. As we intend to express the intensity of the light in some other units of measurement, we will study lux, lumen and candela individually.

## Candela, lumen, lux

Candela (symbol: cd) is the original unit light intensity of "standard candle", which is during past included in the SI (International System of Units) and renamed from candle to Candela. It represents a quantity of light which is emitted in the specific three-dimensional range of apex angle. A range of that angle is expressed in Steradian, it has no measuring unit (such as radian for the two-dimensional planes). One steradian on a sphere with the radius of one meter gives a surface of one square meter (pictured above). The full sphere measures the 4PI radians.
Lumen (symbol: lm) is excerpted SI unit for total amount of visible light given by a source. The flow of a lumen is defined by a sensitivity of a human eye toward different wavelengths, while the watt per steradian (W / sr) indicates the total power of electromagnetic waves emitted. To sum up, the lumen is dependent on the ability of the human eye to perceive light.
Lux (symbol: lx) is one lumen per square meter, while the candela (symbol: cd) refers to the lumen by steradian (cd = lm/sr). Difference between lumen and lux is that lux adds an area to account and light is spread through it (picture above). 900 lumen lux, concentrated on one square meter area, brightens that area with 900 lux. Same, 900 lux, spread across nine square meters area, gives the light of only 100 lux. Therefore, 1 lx = 1 lm/m2. In September, 2010., the European Union adopted a directive according to which the intensity of all lighting devices must be expressed in lumens. According to the same, the minimum utility of the lighting devices is determined. For instance,  a 60W light bulb must at least give 700-750 lumen of light, which prohibits the production of an incandescent bulb. More in the section on light intensity.

## Apex angle

Considering that lumen and candela are measuring units dependent on the angle of view (apex angle), it is helpful to know how it is defined. The angle between the focal axis of the light source (the light intensity at this point is 100%) and the axis where the intensity is reduced to 50%.  Apex angle is twice bigger than the mentioned angle. For instance, in a 5mm narrow-angle LED diode, it is 25 degrees, while at 5mm wide-angle LED, it is 120 degrees.

## Light efficiency

It is a measure how good light source produces visible light. Usually, it is defined as the ratio of the power the source consumes (electricity, chemical energy, etc.) and energy of the registered light path by the human eye. Perfect source, theoretically, is monochromatic light on a wavelength of 555 nm (green) which gives 683 lm/W and 100% utility. The efficiency of most commonly used light sources is: candle 0.04% i.e. 0.3lm/W (1W of this source provides a light of 0.3 lumen intensity), plasma display 0.3-1.5% i.e. 2-10lm/W, 100W (220V) fiery thread 2% i.e. 13.8lm/W, 100W (220V) halogen 2.4% i.e. 16.7lm/W, xenon 4.4-7.3% i.e. 30-50lm/W, fluorescent 9W 8% i.e. 46lm/W. LED diodes can have up to 43.9% efficiency (300lm/W).

## Visible light vs. Invisible light

When we talk about light, in general, we think of visible light and cool stuff like a sunset or rainbow. However, light consists of a really wide wavelength range of the electromagnetic spectrum.

Picture source wikipedia.

Picture represents a full spectrum of electromagnetic radiations. At the one end, we have wicked gamma and X-rays which are highly energy-ionized electromagnetic radiation (high frequency is equivalent to high energy per photon)  and as such, they are biologically dangerous. At the other end there are low-frequency radio waves that transmit information at tremendous distances. Visible light represents a very small part of the total spectrum. In this tutorial, we'll cover the spectrum of a visible light and it's closest areas: ultraviolet (UV) and infrared (IR) light. Moving away from the above mentioned, things get strange. They may, perhaps, be explained in some other tutorials. We'll give more attention to infrared spectrum because it is most commonly used in electronics.

## Ultraviolet light

Ultraviolet light's spectrum is between 10nm (nanometers) and 400nm, placed between X-ray and visible light. Potential, it is dangerous for living beings. Most likely, you are already informed about it from adverse, ultraviolet, Sun radiation.

Ultraviolet-A (UVA)

UVA (315nm - 400nm) is energy-wise, weakest UV light barely visible to the human eye. White fluorescent bulbs and white LEDs work by exposing the certain material to UVA light that absorbs UVA photons and emits a visible spectrum that is shown to us as white light. This spectrum is used to detect fake documents or money as well. Mentioned have a watermark which glows under UVA light. Regarding Sun radiation, UVA is least absorbed in ozone layer and least damages our DNA cells. As well, it is responsible for our lovely skin colour in the summer.

Ultraviolet-B (UVB)

UVB (280nm - 315nm) has bigger energy level than UVA. A source of UVB rays is, for example, a welding machine. Exposure to the same, even at a greater distance, may cause serious eye damage. Sun's rays, also, contain UVB. Besides they cause skin burns, they are in charge, for well-known, photochemical reaction that produces ozone-layer and vitamin D synthesis inside the human body. However, most of the UVB radiation (around 90%) is absorbed in the ozone layer. It is interesting that we can protect (partially) from UVB radiation with a basic glass (that's we get Sun burns only at a part of the arm which we exposed out of a car window). A very interesting experiment was regarding this topic was made by Richard Feynman (bongo musician and Nobel prize winner in a field of physics). Feynman participated in the development of an atomic bomb during WWII. For the purpose of an experiment, he protected himself with a glass window of his pickup from the ultraviolet radiation caused by a nuclear blast.

Ultraviolet-C (UVC)

UVC (100nm - 280nm) is, luckily, the least represented and accordingly to us the least interesting UV light. Almost nothing of the sun's UVC radiation reaches the Earth's surface. uring bad old days, even before EEPROM and flash memory, EPROM memory was used. Once writing to EPROM memory was done, the only way to delete it was to expose the memory to a powerful source of UVC light for 20-30 minutes. It was a solid amount of time just to find out whether our code change has corrected the bug.

## Visible light

Visible light's range is from 380 nm to 740nm. That rank may vary, some people can detect a wider spectrum, but generally speaking, the human eye is sensitive to this area.

There are two specificities in the perception of light with the human eye: sensitivity to different wavelengths in different quantities, i.e. color and amount of light. To measure above mentioned, Candela was introduced. It's unit which measures light intensity according to it's colour. A human eye can register one candela of light with same brightness like any other, one-candela source of light, no matter what their wavelengths are. A brightness of LEDs is usually shown in milicandelas (mcd), so for red is 900mcd, 4000mcd for green and 800cmd for blue. The wavelengths for the above mentioned are 625nm red, 520nm blue and 467.5nm green.

The picture above represents a photonic curve of the Commission Internationale de l '... clairage (CIE). The horizontal line represents the wavelength in nanometers, while vertical is a non-dimensional standard light function. With it, we define the relationship between the registered colors and the light intensity that they give, expressed in lumens. More on Wikipedia. The colors are created by mixing three basic shades (red, green and blue). Mixing these three colors in different intensities gives us different colors, at least if our eyes are asked.

A small color mixing experiment can be done using our Croduino and RGB LED diode. This diode has 4 pins: one represents anode/cathode, while other three represent individual colours (red, green, blue). If we connect those to PWM pins of Croduino, using analogWrite() function we can define intensity of each separate colour. Using colour and intensity we can get desired colour. So, besides colors, the human eye perceives the intensity of light, differently during "day" and uring "night". We are accustomed to say that one source of light, between the two we perceive, is twice as intense as the other. This constellation is almost certainly not accurate as well as our linear understanding of intensity levels. According to the CIE human eye registers the growth of intensity by the curve of the third degree. To test this constellation, we can use the small Croduino test again. Connect components as shown in the picture below, use the code and test your ability to observe.

```const int tipkalo = 9;     // pin of the push button
const int led =  11;       // LED pin (use the PWM pin)
const int interval = 10;   // speed of the LED's light change

boolean boolTipka = true;

void setup() {
pinMode(led, OUTPUT);
pinMode(tipkalo, INPUT);

Serial.begin(9600);
}

void loop() {

while(boolTipka == true)
{
for( int i = 0; i < 256; i++ )
{
analogWrite(led, i);
delay(interval);
{
boolTipka = false;
int p = i*100/255; // percentage
Serial.print("LED stopped at : ");
Serial.print(p);
Serial.print("% power");
break;
}
}
}
}
```

When booting the code LED lights up with the initial intensity, after a certain time this intensity gradually increases. Our task is to press the button at a moment when we consider the current intensity is 50%. After that, we can read the percentage of intensity that we thought was 50% intensity in the Serial Monitor.

## Infrared light

A wavelength of infrared light is from 700nm to 1mm (10e6 nm). Around 55% of sunlight which reaches Earth's surface is infrared.

According to CIE, we divide infrared radiation into 3 areas IC-A, IC-B and IC-C. I consider that the IC-B radiation is currently not overly interesting, so we will only process the remaining two.

IC-A is a close infrared area and is very interesting in the field of electrical engineering. This is the level of radiation used by remote controls, motion sensors and similar sensors in short distances. It is located just above the visible 700nm light, and it reaches up to 1400nm. The transmitters are typically IR LEDs that emit 850nm or 950nm waves, and they are easily controllable. A problem can occur, considering that there is a lot of infrared radiation around us, in the sense of mixing existing radiation with the controlled one. Most IR systems solve that problem with ray modulation on a certain, unchangeable frequency. You can try this out using a remote IC receiver.

IC-C

Long-term infrared light ranges from 8000nm to 15000nm. This is the area used by thermal cameras. In addition, laser cutters use this spectrum. Most of them are based on CO2 laser tubes that generate a laser beam with a wavelength of 10640nm.